Thread: factorial for values greater than 16

I have the below program to calculate factorial. It is working good for values upto 16. But when I give a value greater than 16, like 17 the output is in negative. e.g for 17 i get -288522240. Why is this?

Code:

#include<stdlib.h>
#include<stdio.h>

int factorial(int);

int main(){
        int i,j;
        printf("Enter a number to calculate factorial:\n");
        scanf("%d",&i);
        j=factorial(i);
        printf("Factorial is %d\n",j);
        return(0);
}

int factorial(int x){
        int i, j=1;

        for ( i = x; i > 1; i--)
        {
                j = j*i;
        }
        return(j);
}

Arithmetic overflow perhaps?
Factorial grows very quickly.

Originally Posted by Salem

Arithmetic overflow perhaps?
Factorial grows very quickly.

Yes, you are very correct. it is Integer overflow. I tried to use long type instead of int in hope that i may be able to increase this limit from 16 to a little more. But did not work.
Anybody please tell me is it possible to make a program that overcomes this restriction?

Two possibilities: If you are using gcc, you could try "long long", which will give you a bit more digits. Use "%lld" to print it on a Linux system - I think "%I64d" is the right format on Windows, but it's been a while since I used that on Windows.

Alternatively, and definitely more portable/flexible, is to use a "double", which allows a range of 10 to the power of 308 or some such - suffice to say, LARGE numbers. factorial of a more than a hundred should be OK here.

--
Mats

Use "%lld" to print it on a Linux system - I think "%I64d" is the right format on Windows, but it's been a while since I used that on Windows.

It should be %lld too since this is standard in C99.

There is another option: use an arbitrary precision arithmetic library like the GMP.