Python advanced list slicing

Question

I want to reduce the size of an image using Python

    import numpy as np
    from skimage import io

    img = io.imread('img.jpg')
    smallImg = img[::2, ::2]

gives me image which 50 percent of the original because the step size of the slice is 2. How can i make it to be let's say 90 percent of original one?

Regular python slice did not help me. It seems that i don't know how to slice a list, so it will return me for example, 2nd, 3rd, 5th, 7th etc. elements. Let's say i have something like:

    arr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10])
    arr[::2]

Running the code above gives me:

   array([1, 3, 5, 7, 9])

However, i want opposite to this result:

   array([2, 3, 5, 6, 8, 9])

Possible duplicate of [Skip every nth index of numpy array](https://stackoverflow.com/questions/40929560/skip-every-nth-index-of-numpy-array) — help-ukraine-now, Aug 10 '19 at 10:04
`arr[np.mod(np.arange(arr.size),3)!=0]` would give you the desired output — help-ukraine-now, Aug 10 '19 at 10:05
Possible duplicate of [Understanding slice notation](https://stackoverflow.com/questions/509211/understanding-slice-notation) — MisterMiyagi, Aug 10 '19 at 10:07
Considering what you want to use the slicing for: for image resizing, just skipping pixels seems inappropriate. Better look for suitable interpolation methods... — FObersteiner, Aug 10 '19 at 10:44
[What should I do when someone answers my question?](https://stackoverflow.com/help/someone-answers) — 0xdb, Oct 16 '20 at 13:46

score 2 · Answer 1 · answered Jun 19 '20 at 12:30

2

we can easily achieve that specifiying correct index values:

res = arr[np.arange(len(arr)) % 3 != 0]

result:

In [59]: res
Out[59]: array([2, 3, 5, 6, 8, 9])

answered Jun 19 '20 at 12:30

MaxU - stand with Ukraine

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score 0 · Answer 2 · edited Sep 22 '19 at 08:03

0

Use this slicing method

arr[1:3,4:6,7:9]...

Or you can slice the list 2 values at a time, store it in different variable, and append or extend the list together.

edited Sep 22 '19 at 08:03

Giulio Caccin

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answered Sep 22 '19 at 05:03

seunmi

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Rami Isam · Answer 3 · 2019-08-10T10:43:05.900

-2

To get array([2, 3, 5, 6, 8, 9]) You have to alter the code to: arr = np.array([1, 2, 3, 4, 5, 6, 7, 8, 9, 10]) arr[1:-1:2] i forgot the step earlier

Negative indexes start form the last item So the item indexed -1 is the last (10) And the item indexed - 2 is before the last (9) and so on

edited Aug 10 '19 at 10:43

answered Aug 10 '19 at 10:36

Rami Isam

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This does not give array([2, 3, 5, 6, 8, 9]). It gives array([2, 3, 4, 5, 6, 7, 8, 9]) instead. – Vladimir Aug 10 '19 at 10:42
I have edited it try it again I'm far from my pc right now – Rami Isam Aug 10 '19 at 10:45

Python advanced list slicing

3 Answers3