Question

I have memoized the factorial function in C as follows:

int fact(int n)
{
int temp;
int lookup_table[n];
if(lookup_table[n])
    return lookup_table[n]; 
else{
    if(n == 0 || n == 1)
        return 1;
    else
        temp = n * fact(n-1);
        lookup_table[n] = temp;
        return temp;
    }
}

But then wehn I input n = 5 ,it ouputs
-1! = 134514064
Can someone please explain what is happening?

Answer 1

int lookup_table[n] should be marked static (actually it can’t, you need a constant there, but it doesn’t have to be too big as factorials grow very quickly), but its not really why you are getting a wrong answer. Instead, lookup_table is initialized to indeterminate junk, instead of 0’s.

However, theres no reason to initialize it to zero when you make it static; that will be done automatically.

Oh, and as others have pointed out, you have an out of bounds error because you need to exchange int lookup_table[n] to use a constant rather than n.