CODEWITHSUNDEEP
c++
Shailav Malik
Shailav Malik

Reputation: 23

Showing wrong value of factorial of a number above 12 if I run a C++ program using recursion

I have the following C++ code to find factorial of a number:

#include <iostream>
using namespace std;
int factorial(int n){
   if (n<=1)
   {return 1;
    
    }
    return n*factorial(n-1);
}
int main()
{
    int a;
    cout<<"Enter a no: ";
    cin>>a;
    cout<<"The value of "<<a<<"! is "<<factorial(a);

return 0;
}

It works fine for values upto 12! But show wrong values above it. e.g. -- 20! is showing -2102132736 ( a negative no)

,for some big values like 50! show 0.

What could be the probable reason ???

Upvotes: 1

Views: 519

Answers (1)

Bathsheba
Bathsheba

Reputation: 234715

Although formally the behaviour on overflowing an int is undefined, modulo arithmetic on a number that's a power of 2 is a common treatment.

A number like 50! has as its many factors a large power of 2. This accounts for the zeros you are getting as output.

The solution is to use an arbitrary precision integer library. There's a good one in Boost.

Upvotes: 1

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