C++ Exercises: Check whether a number is prime or not

Last update on April 07 2025 13:01:35 (UTC/GMT +8 hours)

5. Check Whether a Number is Prime or Not

Write a program in C++ to check whether a number is prime or not.

Visual Presentation:

Sample Solution :-

C++ Code :

#include <iostream> // Preprocessor directive to include the input/output stream header file

using namespace std; // Using the standard namespace to avoid writing std::

int main() // Start of the main function
{
    int num1, ctr = 0; // Declaration of integer variables 'num1' and 'ctr', 'ctr' initialized to 0

    cout << "\n\n Check whether a number is prime or not:\n"; // Display a message indicating the purpose
	cout << "--------------------------------------------\n"; // Display a separator line
	cout << " Input a number to check prime or not: "; // Prompting the user to input a number
	cin >> num1; // Reading the input number entered by the user

    for (int a = 1; a <= num1; a++) // Start of a 'for' loop from 1 to the input number 'num1'
    {
        if (num1 % a == 0) // Check if 'num1' is divisible by 'a' without any remainder
        {
            ctr++; // Increment 'ctr' when 'num1' is divisible by 'a'
        }
    }

    if (ctr == 2) // Check if 'ctr' is equal to 2 (prime numbers have only two factors: 1 and the number itself)
    {
        cout << " The entered number is a prime number. \n"; // Display a message indicating that the number is prime
    }
    else {
        cout << " The number you entered is not a prime number. \n"; // Display a message indicating that the number is not prime
    }
}

Sample Output:

 Check whether a number is prime or not:                               
--------------------------------------------                           
 Input a number to check prime or not: 13                              
 The entered number is a prime number. 

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C++ program to determine if a given number is prime using the trial division method.
• Write a C++ program to check primality of an input number using a for loop with optimized checks up to √n.
• Write a C++ program that determines whether a number is prime and prints an appropriate message using conditional operators.
• Write a C++ program to verify if a number is prime by iterating through possible divisors and using a flag variable.

C++ Code Editor:

Click to Open Editor

Contribute your code and comments through Disqus.

Previous: Write a program in C++ to find the perfect numbers between 1 and 500.
Next: Write a program in C++ to find prime number within a range.