C++ Exercises: Count the prime numbers less than a given positive number

Last update on December 20 2024 12:30:27 (UTC/GMT +8 hours)

Write a C++ program to count prime numbers less than a given positive number.

Sample Input: n = 8
Sample Output: Number of prime numbers less than 8 is 2
Sample Input: n = 30
Sample Output: Number of prime numbers less than 30 is 10

Sample Solution:

C++ Code :

#include <iostream>
#include <cmath>

using namespace std;

class Solution {
public:
    // Function to count prime numbers less than 'n'
    int count_Primes(int n) {
        int ctr = 0; // Initialize counter for prime numbers
        for (int i = 2; i < n; i++) {
            if (is_Prime(i)) { // Check if 'i' is a prime number
                ctr++; // Increment counter if 'i' is prime
            }
        }
        return ctr; // Return the count of prime numbers
    }

    // Function to check if a number is prime
    bool is_Prime(int n) {
        int n_ = int(sqrt(n)); // Compute square root of 'n'
        for (int i = 2; i <= n_; i++) {
            if (0 == n % i) { // Check if 'n' is divisible by 'i'
                return false; // If divisible, 'n' is not a prime number
            }
        }
        return true; // If 'n' is not divisible by any 'i', 'n' is a prime number
    }
};

int main() {
    Solution *solution = new Solution(); // Create an instance of Solution class
    int n = 8;
    // Display the number of prime numbers less than 'n'
    cout << "Number of prime numbers less than " << n << " is " << solution->count_Primes(5) << endl;
    n = 30;
    cout << "Number of prime numbers less than " << n << " is " << solution->count_Primes(30) << endl;
    n = 100;
    cout << "Number of prime numbers less than " << n << " is " << solution->count_Primes(100) << endl;
    return 0;
}

Sample Output:

Number of prime numbers less than 8 is 2
Number of prime numbers less than 30 is 10
Number of prime numbers less than 100 is 25

Flowchart:

C++ Code Editor:

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