Shuffle 2D array

Question

I was trying to shuffle 2D array, and I encountered some stange behavior, that can be resumed with the following code:

import random
import numpy

a = numpy.array([[1,2,3],[4,5,6],[7,8,9]])
random.shuffle(a)
print 'With rand\n', a

a = numpy.array([[1,2,3],[4,5,6],[7,8,9]])
numpy.random.shuffle(a)
print 'With numpy\n', a

Output

With rand
[[1 2 3]
 [1 2 3]
 [1 2 3]]
With numpy
[[4 5 6]
 [7 8 9]
 [1 2 3]]

As you can see, with random library (my first try), it seems to overwrite elements (or something else, I really don't understand what happen here), consequently the shuffling is not performed. However with numpy library, it works perfectly.

Can anyone explain me why? I.e. where does this difference come from? And if possible, what does the random.shuffle function does with 2D array? Thanks,

Possible duplicate of [Shuffle a numpy array](http://stackoverflow.com/questions/22426609/shuffle-a-numpy-array) — zipa, May 09 '17 at 14:53
The question is not really a duplicate but the answer is indeed [there](http://stackoverflow.com/a/35183004/3545273) — Serge Ballesta, May 09 '17 at 15:02
@SergeBallesta: Yes indeed, the answer is in one of the answer, thanks! — Nuageux, May 09 '17 at 15:53

score 2 · Accepted Answer · answered May 09 '17 at 15:09

In a check for random source code..

https://svn.python.org/projects/stackless/trunk/Lib/random.py

def shuffle(self, x, random=None, int=int):
    """x, random=random.random -> shuffle list x in place; return None.

    Optional arg random is a 0-argument function returning a random
    float in [0.0, 1.0); by default, the standard random.random.
    """

    if random is None:
        random = self.random
    for i in reversed(xrange(1, len(x))):
        # pick an element in x[:i+1] with which to exchange x[i]
        j = int(random() * (i+1))
        x[i], x[j] = x[j], x[i]

the last line as you see: make the shuffle fails because numpy somehow perform the last line in parts , but python lists perform it altogether ..

score 0 · Answer 2 · answered May 09 '17 at 15:06

random.shuffle is designed to work with a list and not an array. Basically, you should use random.shuffle whenever you have a list and np.random.shuffle when you are working with an array.

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

b = numpy.array([[1,2,3],[4,5,6],[7,8,9]])
numpy.random.shuffle(b)

Shuffle 2D array

2 Answers2