How do i fix my interpretation of this run-length encoding algorithm?

Question

For school, i have to build myself a method in java that compresses an array using RLE(run-length encoding). I can't find a solution online because my teacher wants me to solve the problem myself. I, unfortunately, cannot do this for i am a busy man with some busy plans.

RLE turns this: {1,1,1,1,2,2,3,3,6,6,6,7,8,8,8} into this: {4,1,2,2,2,3,3,6,1,7,3,8} it basically makes a new array that follows this formula {# of this value, this value, # of this value, this value, cont...} there are 4 1's so {4,1} you get my drift.

Here is what i tried to do(forgive me for my crappy code, i am merely a high school student):

public class tester{
public static void main(String[] args){
    int[] potato = {1,1,1,2,2,4,4,4,6,6,6,6};
    printArray(compress(potato));
}

public static void printArray(int[] arr){
    for(int i = 0; i < arr.length; i++){
        System.out.println(arr[i]);
    }
}

public static int[] compress(int[] a) {
    //figure out how many different numbers there are.
    int diffNums = 1;
    for(int i = 0; i < a.length; i++){
        if(i != a.length-1 && a[i] != a[i+1]){
            diffNums++;
        }
    }

    //make compressed array at the correct length.
    int[] compressed = new int[diffNums * 2];

    //figure out what each number is. 
    int[] nums = new int[diffNums];
    nums[0] = a[0];
    int spot = 0;
    for(int i = 0; i < a.length; i++){
        if(i != a.length-1 && a[i] != a[i+1]){
            nums[spot] = a[i+1];
        }
    }

    //figure out quantity of each number.
    int[] quantities = new int[diffNums];
    int spot2 = 0;
    int spotcur = 0;
    for(int i = 0; i < diffNums; i++){
        int quant = 0;
        while(a[spotcur] == a[spot2]){
            quant++;
            spotcur++;
        }
        spot2 = spotcur;
        quantities[i] = quant;
    }

    //add them together and return compressed array
    int spotter = 0;
    for(int i = 0; i < diffNums; i++){
        compressed[spotter] = quantities[i];
        spotter++;
        compressed[spotter] = nums[i];
        spotter++;
    }
    return compressed;
  }
}

Does anyone know how i can fix this crappy code? i am stuck on it

Answer 1

I think this problem could be solved with a lot less code. You could use an outer/inner loop construct something like the following:

public static int[] compress(int[] a) {
    List<Integer> encoded = new ArrayList<>();
    for (int i=0; i<a.length; i++) {
        int num = a[i];
        int count = 1;
        for (int j=i+1; j<a.length; j++) {
            int nextNum = a[j];
            if (nextNum != num) 
                break;
            count++;
            i++;
        }
        encoded.add(count);
        encoded.add(num);
    }
    return encoded.stream().mapToInt(i->i).toArray();
  }

Also, the Arrays class contains a useful toString method already defined.

System.out.println(Arrays.toString(compress(potato)));