I am looking for a c++ or c version of a function that will implement N choose M.

i.e. set A= {a,b,c,d)

N=4

M=2

4 choose 2 from set A I get 6 total combos = N!/M!*(N-M)!

ab

ac

ad

bc

bd

cd

I tried to develop it using recursion, but I just don't see it.

Any help or pointers would be very helpful.