Get some elements from numpy array in reverse order

Question

I need to generate numpy arrays getting elements in reverse order from another array.

Toy example code

Lets say I use following toy example code:

import numpy as np
a = np.array([1,2,3,5,8,13])
n = len(a)

for i in range(n):
    print a[n:n-i-2:-1]

I would expect that last print is a [13 8 5 3 2 1], however I get an empty array [] as seen below:

>>> 
[13]
[13  8]
[13  8  5]
[13  8  5  3]
[13  8  5  3  2]
[]

Fix

So I had to create below fix to my code within the for loop to get what I expect.

for i in range(n):
    print a[n-i-1:n+1][::-1]

Which means selecting the arrays from the original array and then reversing it.

Questions

Why when I try a[6:0:-1] I get [13, 8, 5, 3, 2] but once I try a[6:-1:-1] I get an emtpy array []? I would expect to get the whole array reversed as when you try a[::-1].
Is the fix I implemented the way to go or there is something I'm missing here?

Edit

The post Understanding slice notation answers my first question but not the second one.

You might find this helpful stackoverflow.com/questions/509211/understanding-slice-notation — yatu
– yatu, Commented Jun 18, 2019 at 13:02
And -1 is a "shorthand" for the index of the last element (in this case, 5), so it should (and does) do the same. — Scott Hunter
– Scott Hunter, Commented Jun 18, 2019 at 13:26

Shovalt · Accepted Answer · 2022-11-18 20:45:19Z

3

Here is a slightly more elegant fix.

for i in range(n):
    print(a[n:-i-2:-1])

or even more elegant

for i in range(n):
    print(a[:-i-2:-1])

both print

[13]
[13  8]
[13  8  5]
[13  8  5  3]
[13  8  5  3  2]
[13  8  5  3  2  1]

It also demonstrates the rule that negative indices count from the top, which is why your original loop switches behavior when the stop index gets to -1.

edited Nov 18, 2022 at 20:45

Shovalt

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answered Jun 18, 2019 at 13:28

Paul Panzer

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1 Comment

Cedric Zoppolo Over a year ago

This makes sense now.

Cedric Zoppolo · Accepted Answer · 2019-06-18 13:31:37Z

1

Another option would be to use below code:

import numpy as np
a = np.array([1,2,3,5,8,13])
n = len(a)

l = range(n)
l[n-1] = None

for i in range(n):
    print a[n:l[n-i-2]:-1]

As performing a[n:None:-1] is same as a[::-1]

answered Jun 18, 2019 at 13:31

Cedric Zoppolo

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Comments

seralouk · Accepted Answer · 2019-06-18 13:39:00Z

1

Use list comprehension and get the results in one line:

import numpy as np

a = np.array([1,2,3,5,8,13])
n = len(a)

# use list comprehension here
[list(a[n:-i-2:-1]) for i in range(n)]

# [[13],
#  [13, 8],
#  [13, 8, 5],
#  [13, 8, 5, 3],
#  [13, 8, 5, 3, 2],
#  [13, 8, 5, 3, 2, 1]]

In case you really need the exclicit for loop use this:

for i in range(n):
    print(a[n:-i-2:-1])

edited Jun 18, 2019 at 13:39

answered Jun 18, 2019 at 13:33

seralouk

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