Function that searches arrays for the first element that satisfies a condition
I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason. The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array. One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)". A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return value, hence why such a function has not been added. Andrew Rosko
Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return value, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
It's typically called shortcircuiting or quick exit when the target
condition is met.
if you have an array a = np.array([1, 2, 3, 4, ...., 10000]) and you are
looking for a true/false result whether anything is negative or not (a <
0).any() will generate a bool array equal to and then check all entries of
that bool array just to reach the conclusion true which was already true at
the first entry. Instead it spends 10000 units of time for all entries.
We did similar things on SciPy side Cython level, but they are not really
competitive, instead more of a convenience. More general discussion I
opened is in https://github.com/dataapis/arrayapi/issues/675
On Thu, Oct 26, 2023 at 2:52 PM Dom Grigonis
Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: ilhanpolat@gmail.com
If such issue is at numpy level, eg xor, which tests for number truth value is equal to n: xor([1, 1, 0], 2) == True xor([1, 0, 0], 2) == False I try to use builtin iterator functions for efficiency, such as combination of filter + next. If, however, the problem is at numpy level, I find `numba` does a pretty good job. I had a similar issue and I couldn’t beat numba’s performance with Cython. Most likely due to the reason that I don’t know how to use Cython most optimally, but in my experience numba is good enough. import numba as nb import numpy as np @nb.njit def inner(x, func): result = np.full(x.shape[0], 1, dtype=np.int32) for i in range(x.shape[0]): for j in range(x.shape[1]): if func(x[i, j]): result[i] = j break return result def first_true_nb_func(arr, cond): func = nb.njit(cond) return inner(arr, func) @nb.njit def first_true_nb(arr): result = np.full(arr.shape[0], 1, dtype=np.int32) for i in range(arr.shape[0]): for j in range(arr.shape[1]): if arr[i, j] > 4: result[i] = j break return result def first_true(arr, cond): result = np.full(arr.shape[0], 1, dtype=np.int32) for i in range(arr.shape[0]): for j in range(arr.shape[1]): if cond(arr[i, j]): result[i] = j break return result arr = np.array([[1,5],[2,7],[9,10]]) print(first_true_nb_func(arr, lambda x: x > 4)) # [1, 1, 0] 163 ms print(first_true(arr, lambda x: x > 4)) # [1, 1, 0] 4.48 µs print(first_true_nb(arr)) # [1, 1, 0] 1.02 µs # LARGER ARRAY arr = 4 + np.random.normal(0, 1, (100, 5)) print(first_true_nb_func(arr, lambda x: x > 4)) # 152 ms print(first_true(arr, lambda x: x > 4)) # 69.7 µs print(first_true_nb(arr)) # 1.02 µs So numba is a very good option if not needing to source callable. Although I think with certain size numba with callable would outperform purepython solution. Having that said, I completely support the idea that optimised mechanism for such situations was part of numpy. Maybe np.where_first_n(arr, op, value, n=1, axis=None), where op is a selection of standard comparison operators. Args: * Obviously having `cond` to be a callable would be most flexible, but not sure if it was easy to achieve good performance with it. Same as in example above. * `first`, `last` args are not needed as input can be the slice view. * where_last_n is not needed as input can be reversed view. Regards, DG
On 26 Oct 2023, at 16:07, Ilhan Polat
wrote: It's typically called shortcircuiting or quick exit when the target condition is met.
if you have an array a = np.array([1, 2, 3, 4, ...., 10000]) and you are looking for a true/false result whether anything is negative or not (a < 0).any() will generate a bool array equal to and then check all entries of that bool array just to reach the conclusion true which was already true at the first entry. Instead it spends 10000 units of time for all entries.
We did similar things on SciPy side Cython level, but they are not really competitive, instead more of a convenience. More general discussion I opened is in https://github.com/dataapis/arrayapi/issues/675 https://github.com/dataapis/arrayapi/issues/675
On Thu, Oct 26, 2023 at 2:52 PM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
mailto:rosko37@gmail.com> wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: ilhanpolat@gmail.com mailto:ilhanpolat@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
An example with a 1D array (where it is easiest to see what I mean) is the
following. I will follow Dom Grigonis's suggestion that the range not be
provided as a separate argument, as it can be just as easily "folded into"
the array by passing a slice. So it becomes just:
idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function
pointer" (i.e., the name of a function defined elsewhere, turning
first_true into a higherorder function), unless there's some way to pass a
parseable expression for simple cases. A few special cases like the first
zero/nonzero element could be handled with dedicated options (sort of like
matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this:
******************
def cond(x):
return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond))
*******************
This should print 4, because the element of search_arr at index 4 (i.e. the
5th element) is e^4, which is slightly greater than 50 (while e^3 is less
than 50). It should return this *without testing the 6th through 1000th
elements of the array at all to see whether they exceed 50 or not*. This
example is rather contrived, because simply taking the natural log of 50
and rounding up is far superior, not even *evaluating the array of
exponentials *(which my example clearly still doesand in the use cases
I've had for such a function, I can't predict the array elements like
thisthey come from loaded data, the output of a simulation, etc., and are
all already in a numpy array). And in this case, since the values are
strictly increasing, search_sorted() would work as well. But it illustrates
the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com
I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used. first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """ class FirstTrue: CONTEXT = {'np': np} def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name]) def __call__(self, arr, n=1, axis=None): # PREPARE INPUTS in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0)) [[1 0 0 0 0] [2 1 1 1 1]] In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each) Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even evaluating the array of exponentials (which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
mailto:rosko37@gmail.com> wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com mailto:rosko37@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
I've implemented such functions in Cython and packaged them into a library
called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a
first_above(a, v) # returns the index of the first element in a that is
strictly above v
first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have
a significant performance gain if the element to be
found is closer to the beginning of the array. Have roughly the same speed
as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False)
first_above(a, v, sorted=False, missing=1, raises=False)
first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks
because accepting them would nullify any speed gain
one would expect from such a function. A Python callback can be implemented
with Numba, but anyone who can write the callback
in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy
to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here
https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=...
.
Best regards,
Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used.
first_true_str = """def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result"""
class FirstTrue: CONTEXT = {'np': np}
def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
def __call__(self, arr, n=1, axis=None): # PREPARE INPUTS in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res
if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0)1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this *without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not*. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even *evaluating the array of exponentials *(which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com
If you add a layer of indirection with Numba you can get a *very* nice API: @numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred) This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself. On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=....
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used.
first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
*class* *FirstTrue*: CONTEXT = {'np': np}
*def* __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") *exec*(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
*def* __call__(self, arr, n=1, axis=None): *# PREPARE INPUTS* in_1d = False *if* axis *is* None: arr = np.ravel(arr)[:, None] in_1d = True *elif* axis == 0: *if* arr.ndim == 1: in_1d = True arr = arr[:, None] *else*: *raise* *ValueError*('axis ~in (None, 0)') res = self.func_nb(arr, n) *if* in_1d: res = res[:, 0] *return* res
*if* __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') *print*(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this *without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not*. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even *evaluating the array of exponentials *(which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com
NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com
NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com
Could a subpython level spin up extra threads (processes)? Search could benefit easily. I switched back to Java for number crunching because one gets to share memory without using OSsupplied shared memory. Maybe put a JVM behind python, or do python in the JVM? Bill  Phobrain.com On 20231031 16:05, Juan NunezIglesias wrote:
If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated [1]
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here [2].
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used.
first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
class FirstTrue: CONTEXT = {'np': np}
def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
def __call__(self, arr, n=1, axis=None): _# PREPARE INPUTS_ in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res
if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ******************
def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this _without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not_. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even _evaluating the array of exponentials _(which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible ! return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: bross_phobrain@sonic.net Links:  [1] https://pypi.org/project/numpyillustrated/ [2] https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=...
This results in a very slow code. The function calls of _pred = numba.njit(pred) are expensive and this sort of approach will be comparable to pure python functions. This is only recommended for sourcing functions that are not called frequently, but rather have a large computational content within them. In other words not suitable for predicates. Regards, DG
On 1 Nov 2023, at 01:05, Juan NunezIglesias
wrote: If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=....
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used. first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
class FirstTrue: CONTEXT = {'np': np}
def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
def __call__(self, arr, n=1, axis=None): # PREPARE INPUTS in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res
if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
mailto:rosko37@gmail.com> wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even evaluating the array of exponentials (which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
mailto:rosko37@gmail.com> wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com mailto:rosko37@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com mailto:lev.maximov@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com mailto:jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
Have you tried timing things? Thankfully this is easy to test because the Python source of numbajitted functions is available at jitted_func.py_func. In [23]: @numba.njit ...: def _first(arr, pred): ...: for i, elem in enumerate(arr): ...: if pred(elem): ...: return i ...: ...: def first(arr, pred): ...: _pred = numba.njit(pred) ...: return _first(arr, _pred) ...: In [24]: arr = np.random.random(100_000_000) In [25]: %timeit first(arr, lambda x: x > 5) 72 ms ± 1.36 ms per loop (mean ± std. dev. of 7 runs, 1 loop each) In [26]: %timeit arr + 5 90.3 ms ± 762 µs per loop (mean ± std. dev. of 7 runs, 10 loops each) In [27]: %timeit _first.py_func(arr, lambda x: x > 5) 7.8 s ± 46.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each) So numba gives a >100x speedup. It's still not as fast as a NumPy function call that doesn't have an allocation overhead: In [30]: arr2 = np.empty_like(arr, dtype=bool) In [32]: %timeit np.greater(arr, 5, out=arr2) 13.9 ms ± 69.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each) But it's certainly much better than pure Python! And it's not a huge cost for the flexibility. Juan. On Wed, 1 Nov 2023, at 10:42 AM, Dom Grigonis wrote:
This results in a very slow code. The function calls of
_pred = numba.njit(pred)
are expensive and this sort of approach will be comparable to pure python functions.
This is only recommended for sourcing functions that are not called frequently, but rather have a large computational content within them. In other words not suitable for predicates.
Regards, DG
On 1 Nov 2023, at 01:05, Juan NunezIglesias
wrote: If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=....
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used.
first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
*class* *FirstTrue*: CONTEXT = {'np': np}
*def* __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") *exec*(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
*def* __call__(self, arr, n=1, axis=None): *# PREPARE INPUTS* in_1d = False *if* axis *is* None: arr = np.ravel(arr)[:, None] in_1d = True *elif* axis == 0: *if* arr.ndim == 1: in_1d = True arr = arr[:, None] *else*: *raise* *ValueError*('axis ~in (None, 0)') res = self.func_nb(arr, n) *if* in_1d: res = res[:, 0] *return* res
*if* __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') *print*(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this *without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not*. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even *evaluating the array of exponentials *(which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
> On 25 Oct 2023, at 22:59, rosko37
wrote: > > I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason. > > The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array. > > One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)". > > A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added. > > Andrew Rosko > _______________________________________________ > NumPyDiscussion mailing list  numpydiscussion@python.org > To unsubscribe send an email to numpydiscussionleave@python.org > https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ > Member address: dom.grigonis@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com
NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com
NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com
Thanks Juan, this is really great! I plan to make use of this right away.
On Wed, Nov 1, 2023 at 8:13 AM Juan NunezIglesias
Have you tried timing things? Thankfully this is easy to test because the Python source of numbajitted functions is available at jitted_func.py_func.
In [23]: @numba.njit ...: def _first(arr, pred): ...: for i, elem in enumerate(arr): ...: if pred(elem): ...: return i ...: ...: def first(arr, pred): ...: _pred = numba.njit(pred) ...: return _first(arr, _pred) ...:
In [24]: arr = np.random.random(100_000_000)
In [25]: %timeit first(arr, lambda x: x > 5) 72 ms ± 1.36 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [26]: %timeit arr + 5 90.3 ms ± 762 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
In [27]: %timeit _first.py_func(arr, lambda x: x > 5) 7.8 s ± 46.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
So numba gives a >100x speedup. It's still not as fast as a NumPy function call that doesn't have an allocation overhead:
In [30]: arr2 = np.empty_like(arr, dtype=bool)
In [32]: %timeit np.greater(arr, 5, out=arr2) 13.9 ms ± 69.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
But it's certainly much better than pure Python! And it's not a huge cost for the flexibility.
Juan.
On Wed, 1 Nov 2023, at 10:42 AM, Dom Grigonis wrote:
This results in a very slow code. The function calls of
_pred = numba.njit(pred)
are expensive and this sort of approach will be comparable to pure python functions.
This is only recommended for sourcing functions that are not called frequently, but rather have a large computational content within them. In other words not suitable for predicates.
Regards, DG
On 1 Nov 2023, at 01:05, Juan NunezIglesias
wrote: If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=... .
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used.
first_true_str = """def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result"""
*class* *FirstTrue*: CONTEXT = {'np': np}
*def* __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") *exec*(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
*def* __call__(self, arr, n=1, axis=None): *# PREPARE INPUTS* in_1d = False *if* axis *is* None: arr = np.ravel(arr)[:, None] in_1d = True *elif* axis == 0: *if* arr.ndim == 1: in_1d = True arr = arr[:, None] *else*: *raise* *ValueError*('axis ~in (None, 0)') res = self.func_nb(arr, n) *if* in_1d: res = res[:, 0] *return* res
*if* __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') *print*(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0)1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this *without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not*. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even *evaluating the array of exponentials *(which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output.
Regards, DG
On 25 Oct 2023, at 22:59, rosko37
wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: ndbecker2@gmail.com
 *Those who don't understand recursion are doomed to repeat it*
Your comparisons do not paint correct picture. a) Most of time here is spent for array allocation and the actual time of the loop gets swallowed in the noise a) You do not test early stop  your benchmarks simply test full loop as condition is almost never hit  5 standard deviations... Here is comparison with Cython: import npi arr = np.random.random(100_000) %timeit npi.first_above(arr, 5) # 66.7 µs %timeit first(arr, lambda x: x > 5) # 83.8 ms arr[100] += 10 %timeit npi.first_above(arr, 5) # 16.2 µs %timeit first(arr, lambda x: x > 5) # 86.9 ms It is in the magnitude of 1000 x slower. Regards, DG
On 1 Nov 2023, at 14:07, Juan NunezIglesias
wrote: Have you tried timing things? Thankfully this is easy to test because the Python source of numbajitted functions is available at jitted_func.py_func.
In [23]: @numba.njit ...: def _first(arr, pred): ...: for i, elem in enumerate(arr): ...: if pred(elem): ...: return i ...: ...: def first(arr, pred): ...: _pred = numba.njit(pred) ...: return _first(arr, _pred) ...:
In [24]: arr = np.random.random(100_000_000)
In [25]: %timeit first(arr, lambda x: x > 5) 72 ms ± 1.36 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [26]: %timeit arr + 5 90.3 ms ± 762 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
In [27]: %timeit _first.py_func(arr, lambda x: x > 5) 7.8 s ± 46.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
So numba gives a >100x speedup. It's still not as fast as a NumPy function call that doesn't have an allocation overhead:
In [30]: arr2 = np.empty_like(arr, dtype=bool)
In [32]: %timeit np.greater(arr, 5, out=arr2) 13.9 ms ± 69.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
But it's certainly much better than pure Python! And it's not a huge cost for the flexibility.
Juan.
On Wed, 1 Nov 2023, at 10:42 AM, Dom Grigonis wrote:
This results in a very slow code. The function calls of
_pred = numba.njit(pred)
are expensive and this sort of approach will be comparable to pure python functions.
This is only recommended for sourcing functions that are not called frequently, but rather have a large computational content within them. In other words not suitable for predicates.
Regards, DG
On 1 Nov 2023, at 01:05, Juan NunezIglesias
mailto:jni@fastmail.com> wrote: If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=....
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used. first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
class FirstTrue: CONTEXT = {'np': np}
def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
def __call__(self, arr, n=1, axis=None): # PREPARE INPUTS in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res
if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
mailto:rosko37@gmail.com> wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even evaluating the array of exponentials (which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
On 25 Oct 2023, at 22:59, rosko37
mailto:rosko37@gmail.com> wrote: I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason.
The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array.
One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)".
A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added.
Andrew Rosko _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com mailto:rosko37@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com mailto:lev.maximov@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com mailto:jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com mailto:jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
Ok, I think I didn’t do a full justice. So actually, numba is fairly fast, given everything is precompiled. Predicates still make things slower, but not as much as I initially thought. However, the way your code is structured, it has to recompile both predicate and target function (with new predicate) on every call. If you source the same already compiled predicate, it does not need to recompile the target function. See timings below. Full scan results: * Precompiled numba function (hardcoded predicate) is fastest * Precompiled numba function (with flexible predicate) is second in speed and in line with Cython * Pure python function is significantly slower, but 3rd place * Any need to compile numba for oneoff call makes it the slowest option Early stopping results: * Precompiled numba function (hardcoded predicate) is significantly faster than anything else * Precompiled numba function (with flexible predicate), cython and pure python perform roughly the same Conclusions: * Use precompiled hardcoded numba when it is reused many times * Use precompiled numba with predicates when it is reused many times with several variations (so flexibility justifies decrease in performance) * Use Cython for oneoff cases whenever it can handle it * Use pure python for oneoff cases, when Cython isn’t flexible enough. Finally, just use numpy when prototyping and optimise later. These tests are done on smaller arrays (100K size) than you tested. With large arrays such as yours there is an extra dimension to take into account, which I don’t think is relevant for majority of use cases, but obviously needs to be explored when working with arrays of such size. Regards, DG import numpy as np import numba as nb import npi def first_pure_py(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i _first = nb.njit(first_pure_py) def first_nb(arr, pred): return _first(arr, nb.njit(pred)) @nb.njit def first_pure_nb(arr): for i, elem in enumerate(arr): if elem > 5: return i pred = lambda x: x > 5 pred_cmp = nb.njit(pred) # FULL SCAN arr = np.random.random(100_000) TIMER.repeat([ lambda: first_pure_py(arr, pred), # 0.15640 lambda: nb.njit(first_pure_py)(arr, pred_cmp), # 0.54549 cmp target lambda: nb.njit(pred)(1), # 0.32500 cmp predicate lambda: first_nb(arr, pred), # 0.84759 = sum of previous 2 lambda: _first(arr, pred_cmp), # 0.00069 precompiled lambda: first_pure_nb(arr), # 0.00052 lambda: npi.first_above(arr, 5), # 0.00071 ]).print(5, idx=1, t=True) # {'reps': 5, 'n': 10} # EARLY STOPPING arr2 = np.random.random(100_000) arr2[100] += 10 TIMER.repeat([ lambda: first_pure_py(arr2, pred), # 0.00014 lambda: nb.njit(first_pure_py)(arr, pred_cmp), # 0.55114 cmp target lambda: nb.njit(pred)(1), # 0.31801 cmp predicate lambda: first_nb(arr2, pred), # 0.83330 = sum of previous 2 lambda: _first(arr2, pred_cmp), # 0.00013 precompiled lambda: first_pure_nb(arr2), # 0.00001 lambda: npi.first_above(arr2, 5), # 0.00021 ]).print(5, idx=1, t=True) # {'reps': 5, 'n': 10}
On 1 Nov 2023, at 16:19, Dom Grigonis
wrote: Your comparisons do not paint correct picture. a) Most of time here is spent for array allocation and the actual time of the loop gets swallowed in the noise a) You do not test early stop  your benchmarks simply test full loop as condition is almost never hit  5 standard deviations...
Here is comparison with Cython:
import npi arr = np.random.random(100_000) %timeit npi.first_above(arr, 5) # 66.7 µs %timeit first(arr, lambda x: x > 5) # 83.8 ms arr[100] += 10 %timeit npi.first_above(arr, 5) # 16.2 µs %timeit first(arr, lambda x: x > 5) # 86.9 ms
It is in the magnitude of 1000 x slower.
Regards, DG
On 1 Nov 2023, at 14:07, Juan NunezIglesias
mailto:jni@fastmail.com> wrote: Have you tried timing things? Thankfully this is easy to test because the Python source of numbajitted functions is available at jitted_func.py_func.
In [23]: @numba.njit ...: def _first(arr, pred): ...: for i, elem in enumerate(arr): ...: if pred(elem): ...: return i ...: ...: def first(arr, pred): ...: _pred = numba.njit(pred) ...: return _first(arr, _pred) ...:
In [24]: arr = np.random.random(100_000_000)
In [25]: %timeit first(arr, lambda x: x > 5) 72 ms ± 1.36 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
In [26]: %timeit arr + 5 90.3 ms ± 762 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)
In [27]: %timeit _first.py_func(arr, lambda x: x > 5) 7.8 s ± 46.2 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
So numba gives a >100x speedup. It's still not as fast as a NumPy function call that doesn't have an allocation overhead:
In [30]: arr2 = np.empty_like(arr, dtype=bool)
In [32]: %timeit np.greater(arr, 5, out=arr2) 13.9 ms ± 69.6 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
But it's certainly much better than pure Python! And it's not a huge cost for the flexibility.
Juan.
On Wed, 1 Nov 2023, at 10:42 AM, Dom Grigonis wrote:
This results in a very slow code. The function calls of
_pred = numba.njit(pred)
are expensive and this sort of approach will be comparable to pure python functions.
This is only recommended for sourcing functions that are not called frequently, but rather have a large computational content within them. In other words not suitable for predicates.
Regards, DG
On 1 Nov 2023, at 01:05, Juan NunezIglesias
mailto:jni@fastmail.com> wrote: If you add a layer of indirection with Numba you can get a *very* nice API:
@numba.njit def _first(arr, pred): for i, elem in enumerate(arr): if pred(elem): return i
def first(arr, pred): _pred = numba.njit(pred) return _first(arr, _pred)
This even works with lambdas! (TIL, thanks Numba devs!)
> first(np.random.random(10_000_000), lambda x: x > 0.99) 215
Since Numba has ufunc support I don't suppose it would be hard to make it work with an axis= argument, but I've never played with that API myself.
On Tue, 31 Oct 2023, at 6:49 PM, Lev Maximov wrote:
I've implemented such functions in Cython and packaged them into a library called numpy_illustrated https://pypi.org/project/numpyillustrated/
It exposes the following functions:
find(a, v) # returns the index of the first occurrence of v in a first_above(a, v) # returns the index of the first element in a that is strictly above v first_nonzero(a) # returns the index of the first nonzero element
They scan the array and bail out immediately once the match is found. Have a significant performance gain if the element to be found is closer to the beginning of the array. Have roughly the same speed as alternative methods if the value is missing.
The complete signatures of the functions look like this:
find(a, v, rtol=1e05, atol=1e08, sorted=False, default=1, raises=False) first_above(a, v, sorted=False, missing=1, raises=False) first_nonzero(a, missing=1, raises=False)
This covers the most common use cases and does not accept Python callbacks because accepting them would nullify any speed gain one would expect from such a function. A Python callback can be implemented with Numba, but anyone who can write the callback in Numba has no need for a library that wraps it into a dedicated function.
The library has a 100% test coverage. Code style 'black'. It should be easy to add functions like 'first_below' if necessary.
A more detailed description of these functions can be found here https://betterprogramming.pub/thenumpyillustratedlibrary7531a7c43ffb?sk=....
Best regards, Lev Maximov
On Tue, Oct 31, 2023 at 3:50 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: I juggled a bit and found pretty nice solution using numba. Which is probably not very robust, but proves that such thing can be optimised while retaining flexibility. Check if it works for your use cases and let me know if anything fails or if it is slow compared to what you used. first_true_str = """ def first_true(arr, n): result = np.full((n, arr.shape[1]), 1, dtype=np.int32) for j in range(arr.shape[1]): k = 0 for i in range(arr.shape[0]): x = arr[i:i + 1, j] if cond(x): result[k, j] = i k += 1 if k >= n: break return result """
class FirstTrue: CONTEXT = {'np': np}
def __init__(self, expr): self.expr = expr self.expr_ast = ast.parse(expr, mode='exec').body[0].value self.func_ast = ast.parse(first_true_str, mode='exec') self.func_ast.body[0].body[1].body[1].body[1].test = self.expr_ast self.func_cmp = compile(self.func_ast, filename="<ast>", mode="exec") exec(self.func_cmp, self.CONTEXT) self.func_nb = nb.njit(self.CONTEXT[self.func_ast.body[0].name])
def __call__(self, arr, n=1, axis=None): # PREPARE INPUTS in_1d = False if axis is None: arr = np.ravel(arr)[:, None] in_1d = True elif axis == 0: if arr.ndim == 1: in_1d = True arr = arr[:, None] else: raise ValueError('axis ~in (None, 0)') res = self.func_nb(arr, n) if in_1d: res = res[:, 0] return res
if __name__ == '__main__': arr = np.arange(125).reshape((5, 5, 5)) ft = FirstTrue('np.sum(x) > 30') print(ft(arr, n=2, axis=0))
[[1 0 0 0 0] [2 1 1 1 1]]
In [16]: %timeit ft(arr, 2, axis=0) 1.31 µs ± 3.94 ns per loop (mean ± std. dev. of 7 runs, 1,000,000 loops each)
Regards, DG
On 29 Oct 2023, at 23:18, rosko37
mailto:rosko37@gmail.com> wrote: An example with a 1D array (where it is easiest to see what I mean) is the following. I will follow Dom Grigonis's suggestion that the range not be provided as a separate argument, as it can be just as easily "folded into" the array by passing a slice. So it becomes just: idx = first_true(arr, cond)
As Dom also points out, the "cond" would likely need to be a "function pointer" (i.e., the name of a function defined elsewhere, turning first_true into a higherorder function), unless there's some way to pass a parseable expression for simple cases. A few special cases like the first zero/nonzero element could be handled with dedicated options (sort of like matplotlib colors), but for anything beyond that it gets unwieldy fast.
So let's say we have this: ****************** def cond(x): return x>50
search_arr = np.exp(np.arange(0,1000))
print(np.first_true(search_arr, cond)) *******************
This should print 4, because the element of search_arr at index 4 (i.e. the 5th element) is e^4, which is slightly greater than 50 (while e^3 is less than 50). It should return this without testing the 6th through 1000th elements of the array at all to see whether they exceed 50 or not. This example is rather contrived, because simply taking the natural log of 50 and rounding up is far superior, not even evaluating the array of exponentials (which my example clearly still doesand in the use cases I've had for such a function, I can't predict the array elements like thisthey come from loaded data, the output of a simulation, etc., and are all already in a numpy array). And in this case, since the values are strictly increasing, search_sorted() would work as well. But it illustrates the idea.
On Thu, Oct 26, 2023 at 5:54 AM Dom Grigonis
mailto:dom.grigonis@gmail.com> wrote: Could you please give a concise example? I know you have provided one, but it is engrained deep in verbose text and has some typos in it, which makes hard to understand exactly what inputs should result in what output. Regards, DG
> On 25 Oct 2023, at 22:59, rosko37
mailto:rosko37@gmail.com> wrote: > > I know this question has been asked before, both on this list as well as several threads on Stack Overflow, etc. It's a common issue. I'm NOT asking for how to do this using existing Numpy functions (as that information can be found in any of those sources)what I'm asking is whether Numpy would accept inclusion of a function that does this, or whether (possibly more likely) such a proposal has already been considered and rejected for some reason. > > The task is thisthere's a large array and you want to find the next element after some index that satisfies some condition. Such elements are common, and the typical number of elements to be searched through is small relative to the size of the array. Therefore, it would greatly improve performance to avoid testing ALL elements against the conditional once one is found that returns True. However, all builtin functions that I know of test the entire array. > > One can obviously juryrig some ways, like for instance create a "for" loop over nonoverlapping slices of length slice_length and call something like np.where(cond) on eachthat outer "for" loop is much faster than a loop over individual elements, and the inner loop at most will go slice_length1 elements past the first "hit". However, needing to use such a convoluted piece of code for such a simple task seems to go against the Numpy spirit of having one operation being one function of the form func(arr)". > > A proposed function for this, let's call it "np.first_true(arr, start_idx, [stop_idx])" would be best implemented at the C code level, possibly in the same code file that defines np.where. I'm wondering if I, or someone else, were to write such a function, if the Numpy developers would consider merging it as a standard part of the codebase. It's possible that the idea of such a function is bad because it would violate some existing broadcasting or fancy indexing rules. Clearly one could make it possible to pass an "axis" argument to np.first_true() that would select an axis to search over in the case of multidimensional arrays, and then the result would be an array of indices of one fewer dimension than the original array. So np.first_true(np.array([1,5],[2,7],[9,10],cond) would return [1,1,0] for cond(x): x>4. The case where no elements satisfy the condition would need to return a "signal value" like 1. But maybe there are some weird cases where there isn't a sensible return val ue, hence why such a function has not been added. > > Andrew Rosko > _______________________________________________ > NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org > To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org > https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ > Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: rosko37@gmail.com mailto:rosko37@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: lev.maximov@gmail.com mailto:lev.maximov@gmail.com _______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com mailto:jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: jni@fastmail.com mailto:jni@fastmail.com
_______________________________________________ NumPyDiscussion mailing list  numpydiscussion@python.org mailto:numpydiscussion@python.org To unsubscribe send an email to numpydiscussionleave@python.org mailto:numpydiscussionleave@python.org https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ https://mail.python.org/mailman3/lists/numpydiscussion.python.org/ Member address: dom.grigonis@gmail.com mailto:dom.grigonis@gmail.com
participants (7)

Bill Ross

Dom Grigonis

Ilhan Polat

Juan NunezIglesias

Lev Maximov

Neal Becker

rosko37