Adding numbers digits to number

Question

For my program, I'd want to code an array 1-100. In that array, I want to store the number + their digits. For example, if the number is 6 the stored value would be 6 because of 6 + 6 = 12. If the number is 17 the stored value should be 25 because 17 + 1 + 7 = 25. I want to do it for every number. My code has a method and 2 for loops but currently outputs everything as 0; Here's my code.

public class GeneratedNums {

public static void main(String[] args) {

    int [] numbers = new int [101]; 

    for ( int x=0; x < 101; x++){
        numbers[x] = sumDigits (x);


    }

    for ( int x=0; x < numbers.length; x++){
        System.out.println(x + ": " + numbers[x]);
    }


}

public static int sumDigits ( int num) {

   int sum = num;

   while ( num != 0){
       num += num%10;
       num /= 10;
   }
      return num;
}
}

Answer 1

You should be adding the result of modulo operation to sum . And you should return sum .

while ( num != 0){
    sum += num % 10;
    num /= 10;
 }
return sum;

Answer 2

You don't need these many loops, you can improve it by caching the previous outputs and reusing it (Dynamic programming tabulation) . Considering you don't have values greater than 100 following code can work for you

public static int sumDigits(int num) {
    int[] cache = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 1 };
    int sum = num + cache[num % 10] + cache[num / 10];
    return sum;
}

basically, I've cached outputs for first 10 inputs.

FYI, you can make scale the program for larger inputs by storing the previous outputs in HashMap