What are this java while loops doing in merge sort?

Question

Step one and step two (step three) seem like repeatedly running to me. Why is it programmed like this?

    int i = 0, j = 0; 
    int k = l; 
    while (i < n1 && j < n2) {     ----step one
        if (L[i] <= R[j]){ 
            arr[k] = L[i]; 
            i++; 
        } 
        else{ 
            arr[k] = R[j]; 
            j++; 
        } 
        k++; 
    } 
    while (i < n1){             ---step two
        arr[k] = L[i]; 
        i++; 
        k++; 
    } 
    while (j < n2){         ----step three
        arr[k] = R[j]; 
        j++; 
        k++; 
    } 
}

This code is just doing the merge step, merging arrays L and R into arr. Step 2 and 3 are adding the remaining numbers, only one of the 2 will ever be executed, this is after the end of L or R was reached, to fill in the remaining values of the other array. — maraca, Nov 02 '19 at 22:19

score 2 · Accepted Answer · answered Nov 02 '19 at 22:23

"Step one" does the work of merging from two source arrays into the destination. When L or R is exhausted there may still be unmerged elements remaining in the other source array. "Step two" exists to copy any remaining elements from L to the destination. "Step three" serves the same purpose for R.

Xgh05t · Answer 2 · 2020-01-25T18:18:37.290

You can choose to skip those steps and just use a for loop, if that is easier for you in the following way:

        for(int i = 0; i < arr.size(); i++) {
            if(r >= right.size() || (l < left.size() && left[l] < right[r])) {
                arr[i] = left[l++];
            } else {
                arr[i] = right[r++];
            }
        }
arr.size() = n1 + n2 in your implementation

or even this:

while(len--) {
            if(r >= right.size() || (l < left.size() && left[l] < right[r])) {
                arr[i] = left[l++];
            } else {
                arr[i] = right[r++];
            }
}
where len = n1 + n2

I personally find this way more readable and easier, but to each their own! (this is unstable and can be made stable, but I leave that part for the reader to figure out!)

edit: I noticed it's java, maybe len-- won't work you can do len >= 0 and len-- inside the loop.

What are this java while loops doing in merge sort?

2 Answers2