subset permutations

[Anton Vredegoor]

Steven D'Aprano wrote:

> On Fri, 09 Dec 2005 16:03:46 +1100, Ben Finney wrote:
>
> >> Do you want the result to be:
> >> AB, AC, AD, BC, BD, CD
> >
> > That is the complete set of combinations of the letters.
> >
> >> Or, do you want AB,BA,AC,CA,AD,DA,BC,CB,BD,DB,CD,DB ?
> >
> > That is the complete set of permutations of the letters.
>
>
> Only if you are choosing without replacement. If you are choosing with
> replacement, you also get AA, BB, CC, DD.
>
> Unfortunately, there is ambiguity in the terms "permutation" and
> "combination", not just between English common usage and mathematics, but
> even in mathematics.

Why is that unfortunate? Now we can all post our favorite scripts and
let them be severely criticized :-)

Anton

def ncycle(seq,n):
    while True:
        for x in seq:
            i = 0
            while i < n:
                yield x
                i += 1

def cross(*args):
    p = 1
    R = []
    for seq in reversed(args):
        R.append(ncycle(seq,p))
        p *= len(seq)
    R.reverse()
    i = 0
    while i < p:
        yield tuple(x.next() for x in R)
        i += 1

def test():
    s1='a1','a2','a3','a4'
    s2='b1','b2'
    s3='c1','c2','c3'
    for x in cross(s1,s2,s3):
        print x

if __name__=='__main__':
    test()