>>896
>>901の改良版。
product [(n - (r - 1))..n]はそのままn P rにもなる。

p n r = product [(n - (r - 1))..n]

Haskell

c'' n r = product [(n - (r - 1))..n] `div` product [1..r]

Python

def fact(n):
a = 1
i = 1
while i <= n:
a *= i
i += 1
return a

def p(n,r):
a = 1
i = n - (r - 1)
while i <= n:
a *= i
i += 1
return a

def c(n,r):
return (p(n,r) // fact(r))