Java: Print the number of prime numbers which are less than or equal to a given integer

Last update on May 13 2025 13:00:10 (UTC/GMT +8 hours)

Count Prime Numbers ≤ N

Write a Java program to print the number of prime numbers less than or equal to a given integer.

Input:

n (1 ≤ n ≤ 999,999).

Visual Presentation:

Sample Solution-1:

Java Code:

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        // Prompting the user to input the number (n)
        System.out.println("Input the number(n):");

        // Creating a Scanner object for user input
        Scanner s = new Scanner(System.in);

        // Reading the input number (n) from the user
        int c = s.nextInt();

        // Calling the check method to find the number of prime numbers
        int ans = check(c);

        // Displaying the number of prime numbers which are less than or equal to n
        System.out.println("Number of prime numbers which are less than or equal to n:");
        System.out.println(ans);
    }

    // Method to check the number of prime numbers
    static int check(int c) {
        // Creating a boolean array to mark numbers as prime or not
        boolean[] prime = new boolean[c + 1];

        // Initializing a counter for prime numbers
        int count = 0;

        // Loop to mark non-prime numbers in the array
        for (int i = 2; i <= Math.sqrt(c); i++) {
            for (int j = i + i; j <= c; j += i) {
                prime[j] = true;
            }
        }

        // Counting the number of prime numbers
        for (int i = 2; i <= c; i++) {
            if (!prime[i]) {
                count++;
            }
        }

        // Returning the total number of prime numbers
        return count;
    }
} 

Sample Output:

Input the number(n):
 1235
Number of prime numbers which are less than or equal to n.:
202

Flowchart:

Sample Solution-2:

Java Code:

// Importing the Scanner class to read user input
import java.util.Scanner;

// Main class named "test"
public class test {
    // Main method
    public static void main(String[] args) {
        // Prompting the user to input the number (n)
        System.out.println("Input the number(n):");

        // Creating a Scanner object for user input
        Scanner s = new Scanner(System.in);

        // Reading the input number (n) from the user
        int c = s.nextInt();

        // Initializing a counter for prime numbers
        int prime_ctr = 0;

        // Loop to iterate through numbers and check for primes
        for (int i = 2; i <= c; i++) {
            // Checking if the current number is prime using the Check_Prime method
            if (Check_Prime(i)) {
                prime_ctr++;
            }
        }

        // Displaying the number of prime numbers which are less than or equal to n
        System.out.println("Number of prime numbers which are less than or equal to " + c + ": " + prime_ctr);
    }

    // Method to check if a given number is prime
    public static boolean Check_Prime(int n) {
        // Loop to check for factors of the number
        for (int divisor = 2; divisor <= n / 2; divisor++) {
            // If the number has a factor other than 1 and itself, it is not prime
            if (n % divisor == 0) {
                return false;
            }
        }

        // If no factors were found, the number is prime
        return true;
    }
} 

Sample Output:

Input the number(n):
 1235
Number of prime numbers which are less than or equal to 1235: 202

Flowchart:

For more Practice: Solve these Related Problems:

• Write a Java program to count the number of twin prime pairs less than or equal to n.
• Write a Java program to list and count all prime numbers within a given range using the Sieve of Eratosthenes.
• Write a Java program to count primes less than n while excluding primes that end with a specific digit.
• Write a Java program to count the number of primes ≤ n and then compute the sum of these prime numbers.

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