Howto compute the factorial of x

Question

how to get the value of an integer x, indicated by x!, it is the product of the numbers 1 to x.

Example: 5! 1x2x3x4x5 = 120.

Answer 1

This is the optimal implementation in size and speed:

int factorial(int x)
{
    static const int f[13] = { 1, 1, 2, 6, 24, 120, /* ... */ };
    if ((unsigned)x < (sizeof f/sizeof f[0])) return f[x];
    else return INT_MAX+1; /* or your favorite undefined behavior */
}

Hint: x! (x factorial) does not fit in an int except for very very small values of x.