combination function in python

[mensanator at aol.com]

On Apr 15, 11:34�am, Anton Vredegoor <anton.vredeg... at gmail.com>
wrote:
> Jussi Piitulainen wrote:
> >> There's probably even a really clever way to avoid that final
> >> division, but I suspect that would cost more in time and memory than
> >> it would save.
>
> We're getting closer and closer to something I already posted a few
> times here. This implementation was unfortunate because I consistently
> used an uncommon name for it so people couldn't easily find it

But then, who's looking for it?

> (mea culpa), and maybe also because it uses the despised reduce builtin.
>
> def noverk(n,k):
>      return reduce(lambda a,b: a*(n-b)/(b+1),xrange(k),1)
>
> BTW I hereby give up the strange name for this and request a better name
> that everybody will use so there will be no confusion anymore. Maybe
> comb(n,k) ?

No, that name's already used by gmpy. And a single
function doesn't replace all of gmpy's other
functionality, many of which are needed in the
same applications where comb() is used, so there's
really no need for your function.

Your time is better spent applying the tools provided
by gmpy rather than trying to re-invent the wheel.

>
> > Here's one non-clever one for integers n, k that uses n^k / k^k
> > (falling powers) with the smaller of k and n - k as lower index:
>
> > def choose(n, k):
> >    if 0 <= k <= n:
> >        ntok = 1
> >        ktok = 1
> >        for t in xrange(1, min(k, n - k) + 1):
> >           ntok *= n
> >           ktok *= t
> >           n -= 1
> >        return ntok // ktok
> >    else:
> >        return 0
>
> Ha, my function uses smaller subproducts :-)
>
> A.