Java Program to check for a Kaprekar number

A Kaprekar number is a number whose square can be broken/partitioned in such a manner that the sum of the partitions adds up to bring the number itself.

Example: 99² = 9801 : 98+01 = 99  (hence a kaprekar number)

Why Kaprekar number. Kaprekar number is named after a great Indian Mathematician Kaprekar

DISCLAIMER:  This program is however limited to check until 99999 because int in java is of 4 bytes= (32 bits) i.e. range of -2³¹ to +2³¹ -1 (including 0) which makes: -2147483648 to +2147483647, Now since in this program 99999 ² exceeds the range for int so program is limited to produce kaprekar numbers until 99999.

import java.util.*;

class kaprekar

{

 

int digitcount(int x)

{

int digit=0;

while(x!=0)

{

digit++;

x/=10;

}

return digit;

}

 

int return_A(int x)

{

while(x%10==0)

x/=10;

return x;

}

 

public static void main(String args[])

{

int n,A,B,sq,temp;

int digits;

Scanner sc=new Scanner(System.in);

kaprekar ob=new kaprekar();

System.out.print("Enter number : "); //  to check whether it is a Kaprekar number or not

n=sc.nextInt();

sq=n*n;

//count the number of digits in the square of that number

digits=ob.digitcount(n*n);

 

//now partition the square number using X=Ab^n + B ; where b=10 (decimal number system)

// partitioning theory is written above

A= sq/ (int)Math.pow(10,digits/2); //But this will get Ab^n  so remove b^n:-

A=ob.return_A(A);

B= sq% (int)Math.pow(10,digits/2);

 

temp=A+B;

if(temp==n)

System.out.print("\nIt is a Kaprekar number \n");

else

System.out.print("\n Not a kaprekar number");

}

}