Find out the factorial of positive number

For any positive number n, its factorial is given by:

factorial = 1*2*3*4....n

If a number is negative, factorial does not exist and factorial of 0 is 1.

This program takes an integer from a user. If user enters negative integer, this program will display error message and if user enters non-negative integer, this program will display the factorial of that number.

Source Code to Find Factorial of a Number

/* C program to display factorial of an integer if user enters non-negative integer. */

#include <stdio.h>

#include <conio.h>

void main()

{

    int n, count;

    clrscr();

    unsigned long long int factorial=1;        

    printf("Enter an integer: ");

    scanf("%d",&n);

    if ( n< 0)

        printf("Error!!! Factorial of negative number doesn't exist.");

    else

    {

       for(count=1;count<=n;++count)    /* for loop terminates if count>n */

       {

          factorial*=count;              /* factorial=factorial*count */

       }

    printf("Factorial = %lu",factorial);

    }

    getch();

}

Output 1

Enter an integer: -5

Error!!! Factorial of negative number doesn't exist.

Output 2

Enter an integer: 10

Factorial = 3628800

Here the type of factorial variable is declared as: unsigned long long. It is because, the factorial is always positive, so unsigned keyword is used and the factorial of a number can be pretty large. For example: the factorial of 10 is 3628800 thus, long long keyword is used.