Java program to check if a number is Kaprekar number or not

Java program to check if a number is Kaprekar number or not:

In this post, we will learn how to check if a number is a Kaprekar number or not in Java. We will write one Java program that will take a number as input from the user and print if it is a Kaprekar number or not.

Kaprekar number:

We can use the following steps to check if a number is a Kaprekar number or not:

Find the square value of the number.
Divide the square value into two parts. For example, if the square of the number is 565504, we can divide it in 565 and 504 or 55 and 5504 etc.
Find the sum of these two parts. If it is equal to the number itself, it is a Kaprekar number.

For example, 2728 is a Kaprekar number.

The square value of 2728 is 7441984.
Divide the square value in two parts: 744 and 1984.
The sum of these parts is 744 + 1984, which is 2728, i.e. the number itself.

Algorithm to follow:

We will use the below algorithm in this program:

Take the number as an input from the user.
Find the square value of the number and save it in a different variable.
Find the total number of digits in the square value.
By using a loop, check the sum of all possible parts of the number.
If the sum is equal to the number itself for any two part, it is a Kaprekar number.

Let’s write down the program.

Java program to check Kaprekar number:

import java.util.Scanner;

public class Main {

    public static int digitsCount(int n) {
        int d = 0;

        while (n > 0) {
            d++;
            n /= 10;
        }
        return d;
    }

    public static boolean isKaprekar(int num) {
        int square = num * num;
        int totalDigits = digitsCount(square);
        int sum;

        if (num == 1) {
            return true;
        }

        for (int i = 1; i < totalDigits; i++) {
            int power = (int) Math.pow(10, i);

            if (square % power == 0) {
                continue;
            }

            sum = (square / power) + (square % power);

            if (sum == num) {
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args) {
        int no;
        Scanner sc = new Scanner(System.in);

        System.out.println("Enter the number: ");
        no = sc.nextInt();

        if (isKaprekar(no)) {
            System.out.println(no + " is a Kaprekar number");
        } else {
            System.out.println(no + " is not a Kaprekar number");
        }

    }
}

Here,

digitsCount method is used to find the count of digits in the number. It takes an integer as the input parameter and returns the count of digits in that integer.
isKaprekar method is used to check if a number is Kaprekar number or not. It takes an integer value as the parameter and returns a boolean value.
The program takes a number as an input from the user and prints a message if it is a Kaprekar number or not.

It will print output as like below:

Enter the number: 
703
703 is a Kaprekar number

Enter the number: 
333
333 is not a Kaprekar number

Find all Kaprekar numbers between 1 to 10000:

You can use the same method with a loop to find all Kaprekar numbers in a range.

package com.company;

public class Main {

    public static int digitsCount(int n) {
        int d = 0;

        while (n > 0) {
            d++;
            n /= 10;
        }
        return d;
    }

    public static boolean isKaprekar(int num) {
        int square = num * num;
        int totalDigits = digitsCount(square);
        int sum;

        if (num == 1) {
            return true;
        }

        for (int i = 1; i < totalDigits; i++) {
            int power = (int) Math.pow(10, i);

            if (square % power == 0) {
                continue;
            }

            sum = (square / power) + (square % power);

            if (sum == num) {
                return true;
            }
        }
        return false;
    }

    public static void main(String[] args) {
        for (int i = 1; i < 10000; i++) {
            if (isKaprekar(i)) {
                System.out.println(i + " is a Kaprekar number");
            }
        }
    }
}

It will print all Kaprekar numbers between 1 to 10000:

1 is a Kaprekar number
9 is a Kaprekar number
45 is a Kaprekar number
55 is a Kaprekar number
99 is a Kaprekar number
297 is a Kaprekar number
703 is a Kaprekar number
999 is a Kaprekar number
2223 is a Kaprekar number
2728 is a Kaprekar number
4879 is a Kaprekar number
4950 is a Kaprekar number
5050 is a Kaprekar number
5292 is a Kaprekar number
7272 is a Kaprekar number
7777 is a Kaprekar number
9999 is a Kaprekar number

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Java program to check if a number is Kaprekar number or not