Slicing an array with arrays

Question

I know this has been answered many times and I went through every SO question on this topic, but none of them seemed to tackle my problem.

This code yields an exception:

TypeError: only integer scalar arrays can be converted to a scalar index

a = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9])

sindex = np.array([0, 3, 4])
eindex = np.array([2, 5, 6])

r = a[sindex: eindex]

I have an array with start indexes and another one with end indexes and I simply wanted to extract whatever is in between them. Notice the difference between sindex and eindex is constant, for example 2. So eindex is always what ever is in sindex + 2.

So the expected result should be:

[1, 2, 4, 5, 5, 6]

Is there a way to do this without a for loop?

If the start index and end index have a difference of 1 in their corresponding elements, just use `a[sindex]`. — Mohit Motwani, May 05 '19 at 12:45
They don't unfortunately, in reality the difference is 15. Thank you. — Melron, May 05 '19 at 12:46
You should change your example, to avoid any confusion. Also add more explanation, for example, do you want the elements from first index from sindex to first index of eindex and continue similarly for second index of sindex? — Mohit Motwani, May 05 '19 at 12:49
What'd happen for `eindex=[1, 5, 6]` - what'd be your expected output? — Jon Clements, May 05 '19 at 12:51
That cannot happen, the difference between every element of ```sindex``` and ```eindex``` is 2. So ```eindex``` must be at least 2 to begin with. — Melron, May 05 '19 at 13:09

score 2 · Answer 1 · answered May 05 '19 at 13:20

For a constant interval difference, we can setup sliding windows and simply index with the starting indices array. Thus, we can use broadcasting_app or strided_app from this post -

d = 2  # interval difference

out = broadcasting_app(a, L = d, S = 1)[sindex].ravel()

out = strided_app(a, L = d, S = 1)[sindex].ravel()

Or use scikit-image's built-in view_as_windows -

from skimage.util.shape import view_as_windows

out = view_as_windows(a,d)[sindex].ravel()

To set d, we can use -

d = eindex[0] - sindex[0]

hpaulj · Accepted Answer · 2019-05-05T16:48:23.487

You can't tell compiled numpy to take multiple slices directly. The alternatives to joining multiple slices involve some sort of advanced indexing.

 In [509]: a = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9]) 
 ...:  
 ...: sindex = np.array([0, 3, 4]) 
 ...: eindex = np.array([2, 5, 6])

The most obvious loop:

In [511]: np.hstack([a[i:j] for i,j in zip(sindex, eindex)])                         
Out[511]: array([1, 2, 4, 5, 5, 6])

A variation that uses the loop to construct indices first:

In [516]: a[np.hstack([np.arange(i,j) for i,j in zip(sindex, eindex)])]              
Out[516]: array([1, 2, 4, 5, 5, 6])

Since the slice size is all the same, we can generate one arange and step that with sindex:

In [521]: a[np.arange(eindex[0]-sindex[0]) + sindex[:,None]]                           
Out[521]: 
array([[1, 2],
       [4, 5],
       [5, 6]])

and then ravel. This is a more direct expression of @Divakar'sbroadcasting_app`.

With this small example, timings are similar.

In [532]: timeit np.hstack([a[i:j] for i,j in zip(sindex, eindex)])                  
13.4 µs ± 257 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [533]: timeit a[np.hstack([np.arange(i,j) for i,j in zip(sindex, eindex)])]       
21.2 µs ± 362 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

In [534]: timeit a[np.arange(eindex[0]-sindex[0])+sindex[:,None]].ravel()            
10.1 µs ± 48.3 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)

In [535]: timeit strided_app(a, L=2, S=1)[sindex].ravel()                            
21.8 µs ± 207 ns per loop (mean ± std. dev. of 7 runs, 10000 loops each)

strided_app and view_as_windows use striding tricks to view the array as an array of size d windows, and use sindex to select a subset of them.

In larger cases, relative timings may vary with the size of the slices versus the number of slices.

score 0 · Answer 3 · answered May 05 '19 at 12:49

0

You can just use sindex. Refer the following image

answered May 05 '19 at 12:49

bigbounty

16,526
5
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Sorry, in reality the difference between sindex and eindex isn't only 1. That was just for simplicity but I have edited the question. – Melron May 05 '19 at 12:55

Slicing an array with arrays

3 Answers3