A function that checks whether a number is prime or not + Factorial function with recursion.

isPrime.cpp

#include <iostream>
using namespace std;

bool isPrime(int N){
    int divider;

    for(divider=N-1 ; divider>1 && N%divider!=0 ; divider--);
    if(divider==1)return true;
    else return false;

}

int main()
{
    int num;
    cout<<"Enter an integer: ";
    cin>>num;

    if( isPrime(num) )cout<<num<<" is a prime number\n";
    else cout<<num<<" is not a prime number\n";

    return 0;
}

recursionFactorial.cpp

#include <iostream>
using namespace std;

double factorial(int N)
{
    if(N==0 || N==1)
        return 1;
    else
        return N*factorial(N-1);
}

int main()
{
    int num;
    do
    {
        cout<<"Enter a positive number to calculate its factorial: ";
        cin>>num;
    }
    while(num<0);

    cout<<"The factorial of "<<num<<" = " <<factorial(num)<<endl;

    return 0;
}