Factorial function only counts to 12

user_12642588 0
Viewed 301

This factorial function starts giving wrong results with 13 and above. I have no idea why.

#include <stdio.h>

int fatorial (int p);

int main() {
    int x = 13;
    int test = fatorial(x);
    printf("%d", test);
}  

int fatorial (int p) {
    if (p <= 0)
        return 1;
    else
        return p*fatorial(p-1);
}

for x = 0, 1, 2 ...12 it prints the right result, but for 13! it prints 1932053504 which is not correct. For x=20 it prints -210213273 for example.

I know that this is not the best way to do a factorial. Its my homework tho, it HAS to be this way.

factorial integer-overflow c algorithm
4 Answers

If you try this you will get the maximum value that int can hold:

#include <stdio.h>
#include <limits.h>
int main(void) 
{
    printf("%d\n", INT_MAX);
}

Your code causes overflow.

You could get a few more numbers if you use a bigger type, but not by very much. You could use this:

unsigned long long fatorial (unsigned long long p) {
    if (p <= 0)
        return 1;
    else
        return p*fatorial(p-1);
}

It won't get you far though. If you want bigger than that you need to find a library for bigger integers or create some custom solution. One such library is https://gmplib.org/ but that is likely out of scope for your homework.

And btw, a condition like p <= 0 is not good. It indicates that the factorial of a negative number is always one, which is false.

user_66994330

It is because after 12, the result of factorial of any number exceeds the size of int.

you can try the following code:

#include<stdio.h>
int main()
{
    int a[100],n,counter,temp,i;
    a[0]=1;
    counter=0;
    printf("Enter the number: ");
    scanf("%d",&n);
    for(; n>=2; n--)
    {
        temp=0;
        for(i=0; i<=counter; i++)
        {
            temp=(a[i]*n)+temp;
            a[i]=temp%10;
            temp=temp/10;
        }
        while(temp>0)
        {
            a[++counter]=temp%10;
            temp=temp/10;
        }
    }
    for(i=counter; i>=0; i--)
        printf("%d",a[i]);
    return 0;
}
user_141700090

The result of the function is too big. I think big int would work better for your purposes. Big int allows you to have bigger numbers. Also, this is what I would do.

int x = the number you want to factorialize
int ans = 1;
(Then instead of all of those functions)
for(var i = x; i > 0; i--) {
ans = ans*i;
}

System.out.println(ans);

Javascript link: https://jsfiddle.net/8gxyj913/

user_142819330

I need to get to 100!

100! is about 9.332622e+157. Simply using standard integer types is insufficient. 32-bit int is good to 12!. With 64-bit integer math, code could get to about 21!

Could use floating point math and give up precision.

Instead consider a string approach.

user_24103590
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