Randomizing an Array (Perl Cookbook, 2nd Edition)

## 4.18. Randomizing an Array

### 4.18.1. Problem

You want to randomly shuffle the elements of an array. The obvious application is writing a card game, where you must shuffle a deck of cards, but it is equally applicable to any situation where you want to treat elements of an array in a random order.

### 4.18.2. Solution

Use the `shuffle` function from the standard List::Util module, which returns the elements of its input list in a random order.

>     use List::Util qw(shuffle);
>     @array = shuffle(@array);

### 4.18.3. Discussion

Shuffling is a surprisingly tricky process. It's easy to write a bad shuffle:

>     sub naive_shuffle {                             # DON'T DO THIS
>         for (my $i = 0; $i < @_; $i++) {
>             my $j = int rand @_;                    # pick random element
>             ($_[$i], $_[$j]) = ($_[$j], $_[$i]);    # swap 'em
>         }
>     }

This algorithm is biased; the list's possible permutations don't all have the same probability of being generated. The proof of this is simple: take the case where we're passed a three-element list. We generate three random numbers, each of which can have three possible values, yielding 27 possible outcomes. There are only six permutations of the three-element list, though. Because 27 isn't evenly divisible by 6, some outcomes are more likely than others.

The List::Util module's `shuffle` function avoids this bias to produce a more randomly shuffled result.

If all you want to do is pick one random element from the array, use:

>     $value = $array[ int(rand(@array)) ];

### 4.18.4. See Also

The `rand` function in perlfunc(1) and Chapter 29 of Programming Perl; for more on random numbers, see Recipe 2.6, Recipe 2.7, and Recipe 2.8; Recipe 4.20 provides another way to select a random permutation

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