Swap two values randomly in list

Question

I have the following list:

a = [1, 2, 5, 4, 3, 6]

And I want to know how I can swap any two values at a time randomly within a list regardless of position within the list. Below are a few example outputs on what I'm thinking about:

[1, 4, 5, 2, 3, 6]
[6, 2, 3, 4, 5, 1]
[1, 6, 5, 4, 3, 2]
[1, 3, 5, 4, 2, 6]

Is there a way to do this in Python 2.7? My current code is like this:

import random
n = len(a)
if n:
    i = random.randint(0,n-1)
    j = random.randint(0,n-1)
    a[i] += a[j]
    a[j] = a[i] - a[j]
    a[i] -= a[j]

The issue with the code I currently have, however, is that it starts setting all values to zero given enough swaps and iterations, which I do not want; I want the values to stay the same in the array, but do something like 2opt and only switch around two with each swap.

can't you use extra variable `temp = a[i] ; a[i] = a[j] ; a[j] = temp` ? — furas, Dec 09 '17 at 00:30
@furas Not necessary in Python, `a[i], a[j] = a[j], a[i]` will do. — mkrieger1, Dec 09 '17 at 00:30
See: https://stackoverflow.com/questions/2493920/how-to-switch-position-of-two-items-in-a-python-list and https://stackoverflow.com/questions/6494508/how-do-you-pick-x-number-of-unique-numbers-from-a-list-in-python — mkrieger1, Dec 09 '17 at 00:32
@mkrieger1 yes I know :) version with `temp` should be more obvious for beginner so I started with `temp`. — furas, Dec 09 '17 at 00:33

juanpa.arrivillaga · Accepted Answer · 2017-12-09T00:38:08.703

13

You are over-complicating it, it seems. Just randomly sample two indices from the list, then swap the values at those indicies:

>>> def swap_random(seq):
...     idx = range(len(seq))
...     i1, i2 = random.sample(idx, 2)
...     seq[i1], seq[i2] = seq[i2], seq[i1]
...
>>> a
[1, 2, 5, 4, 3, 6]
>>> swap_random(a)
>>> a
[1, 2, 3, 4, 5, 6]
>>> swap_random(a)
>>> a
[1, 2, 6, 4, 5, 3]
>>> swap_random(a)
>>> a
[1, 2, 6, 5, 4, 3]
>>> swap_random(a)
>>> a
[6, 2, 1, 5, 4, 3]

Note, I used the Python swap idiom, which doesn't require an intermediate variable. It is equivalent to:

temp = seq[i1]
seq[i1] = seq[i2]
seq[i2] = temp

edited Dec 09 '17 at 00:38

answered Dec 09 '17 at 00:32

juanpa.arrivillaga

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Can you explain further what the last two lines of code do here? It looks like it takes two values, but... the next line does the a,b = b,a switch? How does that work? – Leggerless Dec 09 '17 at 00:33
@Leggerless, they swap the two elements in the list. – RoadRunner Dec 09 '17 at 00:35
Does it only swap 2 at a time or is there a chance to swap more than 2 at once with the function? – Leggerless Dec 09 '17 at 00:36
@Leggerless "swapping" inherently works with 2 items. What do you mean by swapping 3 items, for example? – mkrieger1 Dec 09 '17 at 00:37
@Leggerless no, you sample two indices and swap the values at those two indices... – juanpa.arrivillaga Dec 09 '17 at 00:37
Ah, ok. I think I get it. I'll try it out now and see if it works. – Leggerless Dec 09 '17 at 00:37
This appears to work. Since it is a function, however, I had to add "return seq" at the bottom for it to display the result (otherwise it would say "None") – Leggerless Dec 09 '17 at 00:47
@Leggerless that is because this function works in-place, i.e., it doesn't return a *new list* and normally such functions should return `None` – juanpa.arrivillaga Dec 09 '17 at 00:50
Consider https://stackoverflow.com/a/22842411/418413 for picking the indices to swap. – kojiro Dec 09 '17 at 00:59

Swap two values randomly in list

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