How to shuffle an array python

What's the easiest way to shuffle an array with python?

Machavity♦

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asked Jan 23, 2009 at 18:34

3

import random
random.shuffle(array)

answered Jan 23, 2009 at 18:37

David ZDavid Z

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9

import random
random.shuffle(array)

answered Jan 23, 2009 at 18:38

Douglas LeederDouglas Leeder

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3

Alternative way to do this using sklearn

from sklearn.utils import shuffle
X=[1,2,3]
y = ['one', 'two', 'three']
X, y = shuffle(X, y, random_state=0)
print(X)
print(y)

Output:

[2, 1, 3]
['two', 'one', 'three']

Advantage: You can random multiple arrays simultaneously without disrupting the mapping. And 'random_state' can control the shuffling for reproducible behavior.

answered Jul 24, 2017 at 3:30

Qy ZuoQy Zuo

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The other answers are the easiest, however it's a bit annoying that the random.shuffle method doesn't actually return anything - it just sorts the given list. If you want to chain calls or just be able to declare a shuffled array in one line you can do:

    import random
    def my_shuffle(array):
        random.shuffle(array)
        return array

Then you can do lines like:

    for suit in my_shuffle(['hearts', 'spades', 'clubs', 'diamonds']):

answered Dec 20, 2011 at 22:05

Mark RhodesMark Rhodes

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Just in case you want a new array you can use sample:

import random
new_array = random.sample( array, len(array) )

answered Mar 29, 2017 at 18:37

Charlie ParkerCharlie Parker

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When dealing with regular Python lists, random.shuffle() will do the job just as the previous answers show.

But when it come to ndarray(numpy.array), random.shuffle seems to break the original ndarray. Here is an example:

import random
import numpy as np
import numpy.random

a = np.array([1,2,3,4,5,6])
a.shape = (3,2)
print a
random.shuffle(a) # a will definitely be destroyed
print a

Just use: np.random.shuffle(a)

Like random.shuffle, np.random.shuffle shuffles the array in-place.

dbliss

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answered Oct 28, 2013 at 9:23

Shuai ZhangShuai Zhang

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2

You can sort your array with random key

sorted(array, key = lambda x: random.random())

key only be read once so comparing item during sort still efficient.

but look like random.shuffle(array) will be faster since it written in C

this is O(log(N)) btw

answered Sep 21, 2018 at 18:37

JamesJames

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In addition to the previous replies, I would like to introduce another function.

numpy.random.shuffle as well as random.shuffle perform in-place shuffling. However, if you want to return a shuffled array numpy.random.permutation is the function to use.

answered Nov 18, 2016 at 9:55

SaberSaber

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I don't know I used random.shuffle() but it return 'None' to me, so I wrote this, might helpful to someone

def shuffle(arr):
    for n in range(len(arr) - 1):
        rnd = random.randint(0, (len(arr) - 1))
        val1 = arr[rnd]
        val2 = arr[rnd - 1]

        arr[rnd - 1] = val1
        arr[rnd] = val2

    return arr

answered Jan 17, 2017 at 11:09

JeevaJeeva

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1

# arr = numpy array to shuffle

def shuffle(arr):
    a = numpy.arange(len(arr))
    b = numpy.empty(1)
    for i in range(len(arr)):
        sel = numpy.random.random_integers(0, high=len(a)-1, size=1)
        b = numpy.append(b, a[sel])
        a = numpy.delete(a, sel)
    b = b[1:].astype(int)
    return arr[b]

MBT

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answered Nov 14, 2018 at 15:41

Be aware that random.shuffle() should not be used on multi-dimensional arrays as it causes repetitions.

Imagine you want to shuffle an array along its first dimension, we can create the following test example,

import numpy as np
x = np.zeros((10, 2, 3))

for i in range(10):
   x[i, ...] = i*np.ones((2,3))

so that along the first axis, the i-th element corresponds to a 2x3 matrix where all the elements are equal to i.

If we use the correct shuffle function for multi-dimensional arrays, i.e. np.random.shuffle(x), the array will be shuffled along the first axis as desired. However, using random.shuffle(x) will cause repetitions. You can check this by running len(np.unique(x)) after shuffling which gives you 10 (as expected) with np.random.shuffle() but only around 5 when using random.shuffle().

answered Feb 21, 2020 at 14:01

Wise CloudWise Cloud

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