Write a program to determine if a positive integer, n, is prime.

// program in C++.

#include <iostream>

using namespace std;

// main function

int main()

{

   // variables

 int N;

 bool flag = true;

 cout << "Enter a positive number: ";

 // read number

 cin >> N;

 // check number is positive or not

 while(N<0)

 {

     cout<<"Number is not positive!! Enter again:";

     // read again if input is negative

     cin>>N;

 }

 // check prime or not

 for(int a = 2; a <= N / 2; a++)

 {

     if(N % a == 0)

     {

         flag = false;

         break;

     }

 }

 // if flag is true then prime

 if (flag)

     cout << "Input number is prime.";

 else

     cout << "Input number is not prime.";

 return 0;

}

Explanation:

Read a number from user and assign it to variable "N".Check number is positive  or not.If input is negative then ask again to enter a positive number until user enters a positive number.If input is divisible by any number from  2 to n/2 set flag equal to "false".After the loop, if flag is "true" then print number is prime else number is not prime.

Output:

Enter a positive number: -12                                                                                              

Number is not positive!! Enter again:-5                                                                                    

Number is not positive!! Enter again:23                                                                                    

Input number is prime.