C++ Program to check whether a number is a prime or composite number.

Pass-by-Value

In the pass-by-value approach, the function receives a copy of the argument, and any changes made to the parameter inside the function do not affect the original argument.

#include <iostream>
using namespace std;

// Function to check if a number is prime or composite (pass-by-value)
bool isPrime(int num) {
    if (num <= 1) return false; // Numbers less than or equal to 1 are not prime
    for (int i = 2; i <= num / 2; ++i) {
        if (num % i == 0) return false; // Number is divisible by i, so it's not prime
    }
    return true; // Number is prime
}

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number;
    
    if (isPrime(number)) {
        cout << number << " is a prime number." << endl;
    } else {
        cout << number << " is a composite number." << endl;
    }
    
    return 0;
}

Pass-by-Reference

In the pass-by-reference approach, the function receives a reference to the argument, allowing modifications to the argument inside the function to affect the original variable.

#include <iostream>
using namespace std;

// Function to check if a number is prime or composite (pass-by-reference)
bool isPrime(int& num) {
    if (num <= 1) return false; // Numbers less than or equal to 1 are not prime
    for (int i = 2; i <= num / 2; ++i) {
        if (num % i == 0) return false; // Number is divisible by i, so it's not prime
    }
    return true; // Number is prime
}

int main() {
    int number;
    cout << "Enter a number: ";
    cin >> number;
    
    if (isPrime(number)) {
        cout << number << " is a prime number." << endl;
    } else {
        cout << number << " is a composite number." << endl;
    }
    
    return 0;
}

Explanation

• Pass-by-Value: The isPrime function receives a copy of number. It performs the prime-checking logic on this copy. The original variable in main remains unaffected by changes within isPrime.
• Pass-by-Reference: The isPrime function receives a reference to number. Although the logic is the same as in the pass-by-value version, here it operates directly on the original variable. Any changes to num within isPrime would reflect in main, but in this case, there are no modifications to num.

Sample Output

Assuming you enter 7 as the number:

For Pass-by-Value:

Enter a number: 7
7 is a prime number.

For Pass-by-Reference:

Enter a number: 7
7 is a prime number.