[Tutor] Permutations?

[Tim Peters]

[Hee-Seng Kye, playing with permutations]
> ...
> p.s. By the way, does anyone have any idea if it would drive my
> computer (667 MHz and 512MB RAM) nuts to perm(range(12))?

Yes, that can't work.  There are n! permutations of a collection with
n elements, and 12! = 479001600.  You'll run out of memory long before
materializing a list of nearly half a billion 12-element lists.

You can use generators, though, to produce the permutations one at a
time.  The memory burden is trivial then, although it will still take
a long time to generate half a billion results.

For example,

def perm(k):
    if k:
        for i, ith in enumerate(k):
            for x in perm(k[:i] + k[i+1:]):
                x.insert(0, ith)
                yield x
    else:
        yield []

>>> for p in perm([1, 2, 3]):
...     print p

[1, 2, 3]
[1, 3, 2]
[2, 1, 3]
[2, 3, 1]
[3, 1, 2]
[3, 2, 1]
>>>