a good algo to do this

[Gerard Flanagan]

eight02645999 at yahoo.com wrote:
> hi
> i need to do something like this
> eg given a number (as a string) = "123"
> there are a few combination i want to find with this string, ie
> "132","321","231","312" and "213". so there are 6 combinations
> altogether. i remember there's a formula for this, but forgot. Does
> python have any modules/functions to do this?
>
> thanks

factorial = [ 1,
              1,
              2,
              6,
              24,
              120,
              720,
              5040,
              40320,
              362880,
              3628800,
              39916800,
              479001600,
              6227020800,
              87178291200,
              1307674368000,
              20922789888000,
              355687428096000,
              6402373705728000 ]

def permutation( iterable ):
    length = len(iterable)
    count = factorial[length]
    for i in range(count):
        sequence = list(iterable[:])
        index = i
        N = count
        result = []
        for j in range( length, 0, -1):
            N = N / j
            choice, index = index // N, index % N
            result += [ sequence.pop(choice) ]
        yield result

a = [ ''.join(perm) for perm in permutation('123')]

print a    

['123', '132', '213', '231', '312', '321']


Gerard