 # C++ 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 understand this example, you should have the knowledge of the following C++ programming topics:

## Example: Prime Numbers Between two Intervals

``````#include <iostream>
using namespace std;

int check_prime(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:\n";

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

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

return 0;
}

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

// 0 and 1 are not prime numbers
if (n == 0 || n == 1) {
is_prime = false;
}

for(int j = 2; j <= n/2; ++j) {
if (n%j == 0) {
is_prime = false;
break;
}
}

return is_prime;
}``````

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, the `check_prime()` 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 `check_prime()` 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.