#include <stdio.h>

int main() {

    int n, i;

    unsigned long long fact = 1;

    printf("Enter an integer: ");

    scanf("%d", &n);




    // shows error if the user enters a negative integer

    if (n < 0)

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

    else {

        for (i = 1; i <= n; ++i) {

            fact *= i;

        }

        printf("Factorial of %d = %llu", n, fact);

    }




    return 0;

}

Optimization problems where the variables are required to take on integer values are called integer programming (IP) problems. If some of the variables are continuous, then we get a mixed variable problem. With all functions as linear, an integer linear programming (ILP) problem is obtained, otherwise it is nonlinear. The ILP problem can be converted to a 0-1 programming problem. Linear problems with discrete variables can also be converted to 0-1 programming problems. Several algorithms are available to solve eg Sysko.