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.

Asked By: Cedric Zoppolo

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Source

Answer 1

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.

Answered By: Paul Panzer

Answer 2

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 By: Cedric Zoppolo

Answer 3

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])

Answered By: seralouk

Get some elements from numpy array in reverse order

Question:

Toy example code

Fix

Questions

Edit

Answers: