primality_by_trial.cpp

#include <iostream>
#include <math.h>

using namespace std;

int isPrime(int n)
{
    if (n==1) return -1; //The number ain't prime or composite if it's 1
    for (int i=2; i < sqrt(n) + 1; i++) //It's sufficient to check all factors between 2 to sqrt(n)
    {
        if  (not (n%i) ) //This is the same thing as writing n%i==0
        return 0;
    }
    return 1; //Note that we reach here iff the program has passed through the above loop
}

int main()
{
    /* This part should be clear to you */
    int n = 1;
    while (n != 0){
        cout << "Enter a number(or 0 to quit): ";
        cin >> n;
        switch(isPrime(n)){
            case 1: cout << "The number is prime. \n"; break;
            case 0: cout << "The number is composite. \n"; break;
            case -1: cout << "The number ain't prime or composite. \n"; break;
        }}
    return 0;
}