CODEWITHSUNDEEP
pythonpython-3.xnumpynumpy-slicing
Pickniclas
Pickniclas

Reputation: 359

Backwards Slicing

I have a numpy array from 0 to 999 and I would like to make a slice that runs from the last element in the list (999) to the one in the middle (500).

test[500:][::-1] works but if I have a two dimensional array and I only want to do so along the zeroth axis it doesn't work because it also reverses the second axis.

import numpy as np
test = np.arange(1000)
test[500:][::-1]

Upvotes: 1

Views: 1103

Answers (2)

Mark
Mark

Reputation: 92440

You can slice from -1 to your stop index with a step of -1:

> import numpy as np

> n = np.arange(20)
> n[-1:10:-1]
array([19, 18, 17, 16, 15, 14, 13, 12, 11])

> # or (thanks iz_)
> n[:10:-1]
array([19, 18, 17, 16, 15, 14, 13, 12, 11])

Upvotes: 3

Sayandip Dutta
Sayandip Dutta

Reputation: 15872

You can use np.flip()

>>> x = np.arange(20)
>>> x
array([ 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16,
       17, 18, 19])
>>> np.flip(x) 
array([19, 18, 17, 16, 15, 14, 13, 12, 11, 10,  9,  8,  7,  6,  5,  4,  3,
        2,  1,  0])

Upvotes: 0

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