Question

It compiles fine, but when it runs, it adds random high numbers to the list, as well as duplicates of existing numbers. I’ve had a couple of people look over this, and none of them can figure it out.

void mergeSort(int list[], int length) {
    recMergeSort(list, 0, length - 1);
}

void recMergeSort(int list[], int first, int last) {

    if (first < last) {
        int mid = (first + last) / 2;
        recMergeSort(list, first, mid);
        recMergeSort(list, mid + 1, last);
        merge(list, first, last, mid);
    }
}

void merge(int list[], int first, int last, int mid) {

    int arraySize = last - first + 1;
    int* tempList = new int[arraySize];
    int beginPart1 = first;
    int endPart1 = mid;
    int beginPart2 = mid + 1;
    int endPart2 = last;


    int index = beginPart1;


    while (beginPart1 <= endPart1 && beginPart2 <= endPart2) {
        if (list[beginPart1] < list[beginPart2]) {
            tempList[index] = list[beginPart1];
            beginPart1++;
        }
        else {
            tempList[index] = list[beginPart2];
            beginPart2++;
        }
        index++;
    }

    while (beginPart1 <= endPart1) {
        tempList[index] = list[beginPart1];
        index++;
        beginPart1++;
    }

    while (beginPart2 <= endPart2) {
        tempList[index] = list[beginPart2];
        index++;
        beginPart2++;
    }


    for (int i = first; i <= last; i++) {
        list[i] = tempList[i - first];
    }

    delete[] tempList;
}

Answer 1

In function merge(), you’re incorrectly calculating the index variable:

Assuming begin = 10, mid = 14, end = 19 (for a total array size of 0 .. 19, and you’re recMergeSort()ing the higher half), your index = 10, but the tempList array is indexed 0..9 (because arraySize = last - first + 1 == 10).

So you’re overflowing your tempList array, and when you “merge”, you get data corruption.

Fix your index variable to be 0-based (rather than beginPart1 based).