C++ Programming Code Examples C++ > Mathematics Code Examples Program to Display Prime Numbers Between Two Intervals Using Functions Program to Display Prime Numbers Between Two Intervals Using Functions Example to print all prime numbers between two numbers (entered by the user) by making a user-defined function. 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 larger number first, this program will not work as intended. To solve this issue, you need to swap numbers first. #include <iostream> using namespace std; int checkPrimeNumber(int); int main() { int n1, n2; bool flag; cout << "Enter two positive integers: "; cin >> n1 >> n2; 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 flag = true; for(int j = 2; j <= n/2; ++j) { if (n%j == 0) { flag = false; break; } } return flag; }