Question

Good afternoon ladies and gents. So, it is not my day for errors. Implementing Mergesort (not in-place) in C++, and I’m having real trouble with the code, with no idea why. The second-to-last line of the mergeSort() function assigns the result of the merge() to a vector of ints, result. This line (the actual allocation, not the function) throws a bad_alloc error, and I have no idea why.

The internet suggests that bad_alloc is mostly thrown due to out-of-memory errors, but this cannot be the case as the first time its called is on a vector of 500 ints, which should be nowhere near too much memory (what is that, like 2 Kb on a 32-bit int?). I assume I’m doing something silly and incorrect for C++, but I don’t know what. I tried calling what() on the exception, but that just returns its name. The code:

vector<int> Sorting::mergeSort(vector<int> A) {

    // If the length of A is 0 or 1, it is sorted.
    if(A.size() < 2) return A;

    // Find the mid-point of the list.
    int midpoint = A.size() / 2;

    // Declare the left/right vectors.
    vector<int> left, right;

    // Declare the return vector.
    vector<int> result (A.size());

    for(int i = 0; i < midpoint; ++i) {
        left.push_back(A.at(i));
    }

    for(int i = midpoint; i < A.size(); ++i) {
        right.push_back(A.at(i));
    }

    left = mergeSort(left);
    right = mergeSort(right);
    result = merge(left, right);

    return result;

}


vector<int> merge(vector<int> left, vector<int> right) {

    vector<int> result;

    while(left.size() > 0 && right.size() > 0) {

        if(left.front() <= right.front()) {
            result.push_back(left.front());
            left.erase(left.begin());
        } else {
            result.push_back(right.front());
            right.erase(right.begin());
        }
    }

    if(left.size() > 0) {
        for(int i = 0; i < left.size(); ++i) {
            result.push_back(left.at(i));
        }
    } else {
        for(int i = 0; i < right.size(); ++i) {
            result.push_back(right.at(i));
        }
    }

}

If I re-write the merge function to just take a reference to result and edit it during the function, it works fine, but I wanted to keep the code as close as possible to the ‘standard’ psuedo-code given for merge-sort.

I appreciate any help, thanks.

Answer 1

In the Merge function, vector<int> result is not being returned.