random.shuffle?

[Erik Max Francis]

François Pinard wrote:

> def shuffle(items):
>     """Shuffle ITEMS in place.  ITEMS should normally be a list."""
>     for top in range(len(items)-1, 0, -1):
>         index = whrandom.randint(0, top)
>         items[index], items[top] = items[top], items[index]

This is a biased shuffling algorithm.  When swapping cards, you'd want
to pick the other card from the whole of the deck, not the deck that
you've touched so far.  If you start with a sorted deck, for instance,
it's easy to see that cards can't end up in every place; see Knuth.

Better would be this:

    def shuffle(list):
        count = len(list)
        for i in range(count):
            j = whrandom.randint(count)
            list[i], list[j] = list[j], list[i]

Simpler, too.

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