How would you write a non-recursive algorithm to calculate factorials?

Question

How would you write a non-recursive algorithm to compute n!?

Answer 1

I love the pythonic solution to this:

def fact(n): return (reduce(lambda x, y: x * y, xrange(1, n+1)))

Answer 2

in pseudocode

ans = 1
for i = n down to 2
  ans = ans * i
next

Answer 3

int fact(int n){
    int r = 1;
    for(int i = 1; i <= n; i++) r *= i;
    return r;
}

Answer 4

fac = 1 ; 
for( i = 1 ; i <= n ; i++){
   fac = fac * i ;
}

Answer 5

Iterative:

int answer = 1;
for (int i = 1; i <= n; i++){
    answer *= i;
}

Or... using tail recursion in Haskell:

factorial x =
    tailFact x 1
    where tailFact 0 a = a
        tailFact n a = tailFact (n - 1) (n * a)

What tail recursion does in this case is uses an accumulator to avoid piling on stack calls.

Reference: Tail Recursion in Haskell

Answer 6

Recursively using JavaScript with caching.

var fc = []
function factorial( n ) {
   return fc[ n ] || ( ( n - 1 && n != 0 ) && 
          ( fc[ n ] = n * factorial( n - 1 ) ) ) || 1;
}