Example: Prime Numbers Between two Intervals

#include <iostream>
using namespace std;

int checkPrimeNumber(int);

int main(){
    int n1, n2;
    bool flag;

    cout << "Enter two positive integers: ";
    cin >> n1 >> n2;

    // swapping n1 and n2 if n1 is greater than n2
    if (n1 > n2) {
      n2 = n1 + n2;
      n1 = n2 - n1;
      n2 = n2 - n1;
    }

    cout << "Prime numbers between " << n1 << " and " << n2 << " are: ";

    for(int i = n1+1; i < n2; ++i) {
        // If i is a prime number, flag will be equal to 1
        flag = checkPrimeNumber(i);

        if(flag)
            cout << i << " ";
    }

    return 0;
}

// user-defined function to check prime number
int checkPrimeNumber(int n){
    bool isPrime = true;

    // 0 and 1 are not prime numbers
    if (n == 0 || n == 1) {
        isPrime = false;
    }
    else {
        for(int j = 2; j <= n/2; ++j) {
            if (n%j == 0) {
                isPrime = false;
                break;
            }
        }
    }

    return isPrime;
}

Output

Enter two positive integers: 12
55
Prime numbers between 12 and 55 are: 13 17 19 23 29 31 37 41 43 47 53 

To print all prime numbers between two integers, checkPrimeNumber() function is created. This function checks whether a number is prime or not.

All integers between n1 and n2 are passed to this function.

If a number passed to checkPrimeNumber() is a prime number, this function returns true, if not the function returns false.

If the user enters the larger number first, this program will swap the numbers. Without swapping, this program won’t work.