Question

For solving project euler problem 20 to find the sum of digits in 100! i am running the following program , it is working for factorial of small numbers but not for 100.which data type should i use or is it necessary to use an array for storing the digits?

int rec(int);

void main()
{   
    int f=1,i=1,z,s=0,r,n;

    while(i<=100)
    {
        f=f*i;
        f=rec(f);
        i++;
    }
    n=f;

    while(n!=0)
    {
        r=n%10;
        n=n/10;
        s=s+r;
    }

    printf("\n%d",s);
}

int rec(int t)
{   
    if(t%10==0)
    {
        t=t/10;
        rec(t);
    }
    return t;
}

Answer 1

You should look for overflow, print the value after each iteration.

Note that rec(t); doesn’t do anything as it doesn’t use the returned value… you want t = rec(t);.

int is definitely too short, try long long… if that’s still overflowing, you need another data structure.. eg: GMP Library.

Note: using some “proper” language for the job might give you some insight to the range you have to support… e.g. with python:

>>> import math
>>> math.factorial(100)
93326215443944152681699238856266700490715968264381621468592963895217599993229915608941463976156518286253697920827223758251185210916864000000000000000000000000L