MergeSort and Merge C++

Question

Having an issue with my MergeSort() and Merge() in my homework for school. Im new to C++ (I've been out of school for two years, they changed my major with new classes. I was learning java....) Long story short, my Merge() and MergeSort() functions aren't working in VS Code when I debug them. I'm having issues with the function working in general. I don't know if I'm passing the parameters correctly.

I've tried to change up the Merge() function by moving it up above MergeSort() .

void Merge(int B[], int &lStart, int &lEnd, int &rStart, int &rEnd) {
    int length;
    int temp[length];
    int theSaved = lStart;
    int i = lStart;

    while (lStart < lEnd && rStart <= rEnd) {
        if (B[lStart] < B[rStart]) {
            temp[i++] = B[lStart++];
        }

        else {
            temp[i++] = B[rStart++];
        }
    }

    while (lStart <= lEnd) {
        temp[i++] = B[lStart++];
    }

    while (rStart <= rEnd) {
        temp[i++] = B[rStart++];
    }

    for (int j = theSaved; j < rEnd; j++) {
        B[j] = temp[j];
    }
}

void MergeSort(int B[], int start, int end) {
    int middle;
    int theMiddle = middle + 1;
    if (start < end) {
        middle = (start + end) / 2;
        MergeSort(B, start, middle);
        MergeSort(B, middle + 1, end);
        Merge(B, start, middle, theMiddle, end);
    }
}

int main() {
    int length = 7;
    int begin = 0;
    int end = (length - 1);
    int B[length] = {10, 50, 90, 60, 5, 20, 40};

    for (int i = 0; i < length; i++) {
        MergeSort(B, begin, end);
        cout << "\n" << B[i];
    }
    return 0;
}

I just wanted some insight into what's going on. I tried to do it logically but it ended up being too confusing. The output should be 5,10,20,40,50,60,90

Answer 1

merge() should not be taking parameters by reference, use int lStart instead of int &lStart. rStart will always be lEnd + 1, so only 3 parameters are needed for merge. The if in merge should be lStart <= lEnd.

Although not a problem, the code would be a bit simpler if the initial call from main is MergeSort(B, 0, length). Then Merge could be called with (B, start, middle, end). The loop in merge() would have... lStart < middle... rStart < end... for the loops and if statements.