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C++ Program to Check Whether a Number is Prime or Not

Write a C++ program to accept a number input from the user and determine whether it is a prime number or not.
A prime number is a natural number greater than 1 that has no positive divisors other than 1 and itself. 

Input and Output Examples

Example 1:

  • Input: 7
  • Output: 7 is a prime number.

Example 2:

  • Input: 12
  • Output: 12 is not a prime number.

Algorithm to Check Prime Number

  1. Start the program.
  2. Prompt the user to enter a number.
  3. Read the number input.
  4. Iterate from 2 to the square root of the number.
  5. Check if the number is divisible by any integer in the range.
  6. If it is divisible by any integer, it is not prime; otherwise, it is prime.
  7. End the program.

Below is the C++ code to check whether a number is prime or not:

cpp
#include <iostream>
#include <cmath> // Include the cmath library for mathematical functions
using namespace std;

int main() {
    // Declare variables to store the number and a flag to indicate whether it is prime
    int num;
    bool isPrime = true;

    // Prompt the user to input a number
    cout << "Enter a number: ";
    cin >> num;

    // Check if the number is less than 2
    if (num < 2) {
        isPrime = false; // Numbers less than 2 are not prime
    } else {
        // Iterate from 2 to the square root of the number
        for (int i = 2; i <= sqrt(num); ++i) {
            // If the number is divisible by any integer in this range, it is not prime
            if (num % i == 0) {
                isPrime = false;
                break;
            }
        }
    }

    // Display the result based on the flag
    if (isPrime) {
        cout << num << " is a prime number." << endl;
    } else {
        cout << num << " is not a prime number." << endl;
    }

    // Ending the program
    return 0;
}

Testing the Program with Different Input Values

  • Input: 7
    Output: 7 is a prime number.
  • Input: 12
    Output: 12 is not a prime number.

Practice Problem

Write a C++ program to find and print all prime numbers less than a given number N. (Hint: You can use the prime-checking algorithm within a loop to iterate through numbers.)

Programming

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