C++ Exercises: List non-prime numbers from 1 to an upperbound

Last update on August 14 2025 12:55:08 (UTC/GMT +8 hours)

16. List Non-Prime Numbers from 1 to Upper Bound

Write a C++ program to list non-prime numbers from 1 to an upperbound.

Visual Presentation:

C++ Exercises: List non-prime numbers from 1 to an upperbound

Sample Solution :-

C++ Code :

#include <iostream> // Include input-output stream header
#include <cmath>    // Include math functions header

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

int main() // Start of the main function
{
    int ult; // Declaration of variable 'ult' to store the upper limit
    cout << "\n\n List non-prime numbers from 1 to an upperbound:\n"; // Display message on the console
    cout << "----------------------------------------------------\n";
    cout << " Input the upperlimit: "; // Prompt the user to input the upper limit
    cin >> ult; // Read the input value as the upper limit

    cout << " The non-prime numbers are: " << endl; // Display message for non-prime numbers

    // Loop through numbers from 2 to the upper limit to check for non-prime numbers
    for (int num = 2; num <= ult; ++num) 
    {
        int mfactor = (int)sqrt(num); // Find the square root of the number and cast it to an integer as the maximum factor to check

        // Loop to check for factors of the current number from 2 to its square root
        for (int fact = 2; fact <= mfactor; ++fact) 
        {
            // If the current number is divisible by 'fact', it's not a prime number
            if (num % fact == 0) 
            {
                cout << num << " "; // Display the non-prime number
                break; // Break the loop to move to the next number
            }
        }
    }
    cout << endl; // Output a newline
    return 0; // Indicates successful termination of the program
}

Sample Output:

 List non-prime numbers from 1 to an upperbound:                       
----------------------------------------------------                   
 Input the upperlimit: 25                                              
 The non-prime numbers are:                                            
4 6 8 9 10 12 14 15 16 18 20 21 22 24 25

Flowchart:

For more Practice: Solve these Related Problems:

Write a C++ program to display all composite numbers between 1 and a user-specified upper limit.
Write a C++ program that reads an upper bound and prints non-prime numbers within that range using a loop.
Write a C++ program to list numbers between 1 and n that are not prime by checking for divisibility.
Write a C++ program that outputs all non-prime numbers between 1 and a given limit and counts their total.

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