Check whether given integer number is prime or not in java (example)

• Given an integer, check whether input number is prime or not.
• A prime number is a natural number greater than 1 that has no positive divisors other than 1 & itself.
  • e.g. if input number 7
    • 7 have two factor i.e. 1 & 7, then 7 is prime number.
  • e.g. if input number is 4
    • 4 have factors of 1, 2 & 4, so 4 is not a prime number.
  • List of prime numbers from 0 – 50 are 2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47.
• We will discuss following methods, to whether check given number is prime, say n (where n > 1).
  1. Iterate from 2 to n -1 to check given number is prime.
  2. Iterate from 2 to n /2 to check given number is prime.
  3. Iterate from 2 to SquareRoot(n) to check given number is prime.

1. Check given integer number is prime or not in java (Loop from 2 to n – 1)

private static boolean checkPrime(int number) {

	for (int range = 2; range < number ; range++ ) {
		
		if(number % range == 0)
			return false;
	}
	return true;
}

2. Check given integer number is prime or not in java (Loop from 2 to n/2)

private static boolean checkPrimeLoopTillHalf(int number) {

	for (int range = 2; range <= number / 2; range++ ) {
		
		if(number % range == 0)
			return false;
	}
	return true;
}

3. Check given integer number is prime or not in java (Loop from 2 to sqrt(n))

private static boolean checkPrimeLoopTillSqrt(int number) {

	int sqrt = (int) Math.sqrt(number);
	for (int range = 2; range <= sqrt; range++ ) {
		
		if(number % range == 0)
			return false;
	}
	return true;
}

Program: check given integer number is prime or not in java (example)

package org.learn;

import java.util.Scanner;

public class CheckPrimeNumber {

	private static boolean checkPrime(int number) {

		for (int range = 2; range < number ; range++ ) {
			
			if(number % range == 0)
				return false;
		}
		return true;
	}
	
	private static boolean checkPrimeLoopTillHalf(int number) {

		for (int range = 2; range <= number / 2; range++ ) {
			
			if(number % range == 0)
				return false;
		}
		return true;
	}
	
	private static boolean checkPrimeLoopTillSqrt(int number) {

		int sqrt = (int) Math.sqrt(number);
		for (int range = 2; range <= sqrt; range++ ) {
			
			if(number % range == 0)
				return false;
		}
		return true;
	}

	public static void main(String[] args) {
		try (Scanner scanner = new Scanner(System.in)) {
			System.out.printf("1. Enter input number : ");
			int inputNumber = scanner.nextInt();
			
			if(inputNumber < 2) {
				System.out.println("Enter number greater than 1");
				return;
			}
			
			boolean isPrime = checkPrime(inputNumber);
			System.out.printf("2. Is prime number : %b", isPrime);
			
			isPrime = checkPrimeLoopTillHalf(inputNumber);
			System.out.printf("\n3. Is prime number - Loop till number/2: %b", isPrime);
			
			isPrime = checkPrimeLoopTillSqrt(inputNumber);
			System.out.printf("\n4. Is prime number - Loop till sqrt(number): %b", isPrime);
		}
	}
}

Output: check given integer number is prime or not in java

1. Enter input number : 4
2. Is prime number : false
3. Is prime number - Loop till number/2: false
4. Is prime number - Loop till sqrt(number): false

1. Enter input number : 7
2. Is prime number : true
3. Is prime number - Loop till number/2: true
4. Is prime number - Loop till sqrt(number): true

1. Enter input number : 12
2. Is prime number : false
3. Is prime number - Loop till number/2: false
4. Is prime number - Loop till sqrt(number): false