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

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  • 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

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