Cocktail Sort code segfaults - not sure why.

Question

Hi there. I wrote a Cocktail Sort algorithm, and am testing it by generating random vectors of size 500 up to 10,000 - running it 10 times per vector. After about the 2000-3000 length vector mark, the code segfaults. I expect it isn't the test code as the same test is used for multiple sorting algorithms and it runs fine for every other algorithm. I am assuming somewhere I miss an end-condition and it tries to access an element of the input array that doesn't exist... but I'm not entirely sure if that would cause a segfault to be run.

This is the code, I hope someone can spot my error. (I also would enjoy any comments on how it could be better - but please note I do value readability over speed for this code.)

void Sorting::cocktailSort(vector<int>& A) {

    int temp;

    // The first/last indexes to check. Anything before/after these indexes
    // is already sorted.
    int firstIndex = -1;
    int lastIndex = A.size()-1;
    bool swapped;

    do {
        firstIndex += 1;
        swapped = false;
        for(int i = firstIndex-1; i < lastIndex; i++) {
            if(A[i] > A[i+1]) {
                temp = A[i];
                A[i] = A[i+1];
                A[i+1] = temp;
                swapped = true;
            }
        }
        if(!swapped) break;
        swapped = false;
        lastIndex -= 1;
        for(int i = lastIndex; i >= firstIndex; i--) {
            if(A[i] < A[i-1]) {
                temp = A[i];
                A[i] = A[i-1];
                A[i-1] = temp;
                swapped = true;
            }
        }
    }while (swapped);

}

This is not homework.

Answer 1

If you use A.at(i) instead of A[i], bounds checking will be done, and out-of-range exceptions thrown. That may be helpful for debugging.

It appears to me that the access here...

    for(int i = firstIndex-1; i < lastIndex; i++) {
        if(A[i] > A[i+1]) {

will be out-of-bounds when firstIndex is zero (the first iteration of the main loop).