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Java Programming Exercies

Java Exercises: Generate and show all Kaprekar numbers less than 1000

Java Numbers: Exercise-4 with Solution

Write a Java program to generate and show all Kaprekar numbers less than 1000.

In number theory, a Kaprekar number for a given base is a non-negative integer, the representation of whose square in that base can be split into two parts that add up to the original number again. For instance, 45 is a Kaprekar number, because 452 = 2025 and 20 + 25 = 45.
The first few Kaprekar numbers in base 10 are:
1, 9, 45, 55, 99, 297, 703, 999, 2223, 2728, 4879, 4950, 5050, 5292, …

Pictorial Presentation:

Java: Generate and show all Kaprekar numbers less than 1000

Sample Solution:

Java Code:

 public class Example4 {
      public static void main(String[] args){
        int ctr = 0;
        int base = (args.length > 0) ? Integer.parseInt(args[0]) : 10;
        for(long n = 1; n <= 1000; n++){
            String sqr_Str = Long.toString(n * n, base);
            for(int j = 0; j < sqr_Str.length() / 2 + 1; j++){
                String[] parts = split_num(sqr_Str, j);
                long first_Num = Long.parseLong(parts[0], base);
                long sec_Num = Long.parseLong(parts[1], base);
                if(sec_Num == 0) break;
                if(first_Num + sec_Num == n){
                    System.out.println(Long.toString(n, base) +
                            "\t" + sqr_Str + "\t  " + parts[0] + " + " + parts[1]);
                    ctr++;
                    break;
                }
            }
        }
        System.out.println(ctr + " Kaprekar numbers.");
    }
     private static String[] split_num(String str, int idx){
        String[] ans1 = new String[2];
        ans1[0] = str.substring(0, idx);
        if(ans1[0].equals("")) ans1[0] = "0"; 
        ans1[1] = str.substring(idx);
        return ans1;
    }	
}

Sample Output:

1       1         0 + 1                                                                                       
9       81        8 + 1                                                                                       
45      2025      20 + 25                                                                                     
55      3025      30 + 25                                                                                     
99      9801      98 + 01                                                                                     
297     88209     88 + 209                                                                                    
703     494209    494 + 209                                                                                   
999     998001    998 + 001                                                                                   
8 Kaprekar numbers. 

Flowchart:

Flowchart: Generate and show all Kaprekar numbers less than 1000

Java Code Editor:

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Previous: Write a Java program to generate random integers in a specific range.
Next: Write a Java program to find the number of seed Lychrel number candidates and related numbers for n in the range 1..10000 inclusive. (With that iteration limit of 500).

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