On Sun, 15 Apr 2007 13:58:38 -0700, Mark Dickinson wrote: > ... for large n and k it's difficult to imagine any real-world > applications that wouldn't be better served by using the lngamma > function instead. That is, the natural log of n choose k can be > computed much more quickly as > > lngamma(n+1) - lngamma(k+1) - lngamma(n-k+1)
Yes, but if you actually want the number of combinations, not the log of the number of combinations, you then have to raise e to that power to get the answer you want -- and that takes time. Is it still faster? How about round-off error due to floating point? Should you be truncating or rounding? What about overflow error for large enough n, k? What exactly are we doing with a five hundred digit long integer anyway? Exact (long) integer maths still is useful. > I assume that lngamma is implemented somewhere in either numpy or > scipy, but right now I can't find it... You know what they say about what happens when you ASS_U_ME :-) -- Steven. -- http://mail.python.org/mailman/listinfo/python-list
