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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