On Thu, Jul 12, 2012 at 2:32 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:
numpy ndarray indexing in the function. Like when I call:
func(a,'1:3,:,2:4'), it knows I want to retrieve a[1:3,:,2:4], and func(a,'1:3,:,4') for a[1:3,:,4] ect.
why do the string packing/unpacking? why not use an interface much like the slice() and range() functions? func(a, ( (start, stop, step),(start, stop, step),(sart, stop, step) )) or, I agree, jsut pass in the sliced array: func( a[start:stop:step, start:stop:step, start_stop:step] ) Will the rank of a always be 3? Do you ned to support "step" that could simplify it a bit. -Chris
I am very close now.
#so this function changes the string to list of slice objects. def convert_string_to_slice(slice_string): """ provide slice_string as '2:3,:', it will return [slice(2, 3, None), slice(None, None, None)] """ slice_list=[] split_slice_string_list=slice_string.split(',') for sub_slice_string in split_slice_string_list: split_sub=sub_slice_string.split(':') if len(split_sub)==1: sub_slice=slice(int(split_sub[0])) else: if split_sub[0]=='': sub1=None else: sub1=int(split_sub[0]) if split_sub[1]=='': sub2=None else: sub2=int(split_sub[1]) sub_slice=slice(sub1,sub2) slice_list.append(sub_slice) return slice_list
In [119]: a=np.arange(3*4*5).reshape(3,4,5)
for this it works fine. In [120]: convert_string_to_slice('1:3,:,2:4') Out[120]: [slice(1, 3, None), slice(None, None, None), slice(2, 4, None)]
In [121]: a[slice(1, 3, None), slice(None, None, None), slice(2, 4, None)]==a[1:3,:,2:4] Out[121]: array([[[ True, True], [ True, True], [ True, True], [ True, True]],
[[ True, True], [ True, True], [ True, True], [ True, True]]], dtype=bool)
And problems happens when I want to retrieve a single number along a given dimension: because it treats 1:3,:,4 as 1:3,:,:4, as shown below:
In [122]: convert_string_to_slice('1:3,:,4') Out[122]: [slice(1, 3, None), slice(None, None, None), slice(None, 4, None)]
In [123]: a[1:3,:,4] Out[123]: array([[24, 29, 34, 39], [44, 49, 54, 59]])
In [124]: a[slice(1, 3, None), slice(None, None, None), slice(None, 4, None)] Out[124]: array([[[20, 21, 22, 23], [25, 26, 27, 28], [30, 31, 32, 33], [35, 36, 37, 38]],
[[40, 41, 42, 43], [45, 46, 47, 48], [50, 51, 52, 53], [55, 56, 57, 58]]])
Then I have a function:
#this function retrieves data from ndarray a by specifying slice_string: def retrieve_data(a,slice_string): slice_list=convert_string_to_slice(slice_string) return a[*slice_list]
In the list line of the fuction "retrieve_data" I have problem, I get an invalid syntax error.
return a[*slice_list] ^ SyntaxError: invalid syntax
I hope it's not too long, please comment as you like. Thanks a lot!!!!
Chao
2012/7/12 Benjamin Root <ben.root@ou.edu>
On Thu, Jul 12, 2012 at 4:46 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:
Hi Ben,
it helps a lot. I am nearly finishing a function in a way I think pythonic. Just one more question, I have:
In [24]: b=np.arange(1,11)
In [25]: b Out[25]: array([ 1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
In [26]: b[slice(1)] Out[26]: array([1])
In [27]: b[slice(4)] Out[27]: array([1, 2, 3, 4])
In [28]: b[slice(None,4)] Out[28]: array([1, 2, 3, 4])
so slice(4) is actually slice(None,4), how can I exactly want retrieve a[4] using slice object?
thanks again!
Chao
Tricky question. Note the difference between
a[4]
and
a[4:5]
The first returns a scalar, while the second returns an array. The first, though, is not a slice, just an integer.
Also, note that the arguments for slice() behaves very similar to the arguments for range() (with some exceptions/differences).
Cheers! Ben Root
2012/7/12 Benjamin Root <ben.root@ou.edu>
On Thu, Jul 12, 2012 at 3:38 PM, Chao YUE <chaoyuejoy@gmail.com> wrote:
Dear all,
I want to create a function and I would like one of the arguments of the function to determine what slicing of numpy array I want to use. a simple example:
a=np.arange(100).reshape(10,10)
suppose I want to have a imaging function to show image of part of this data:
def show_part_of_data(m,n): plt.imshow(a[m,n])
like I can give m=3:5, n=2:7, when I call function show_part_of_data(3:5,2:7), this means I try to do plt.imshow(a[3:5,2:7]). the above example doesn't work in reality. but it illustrates something similar that I desire, that is, I can specify what slicing of number array I want by giving values to function arguments.
thanks a lot,
Chao
What you want to do is create slice objects.
a[3:5]
is equivalent to
sl = slice(3, 5) a[sl]
and
a[3:5, 5:14]
is equivalent to
sl = (slice(3, 5), slice(5, 14)) a[sl]
Furthermore, notation such as "::-1" is equivalent to slice(None, None, -1)
I hope this helps! Ben Root
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