Question

I followed this recursive algorithm for merge sort detailed on Wikipedia.

Here is the code that I came up with:

int* merge(int left[], int leftSize, int right[], int rightSize){
int result[leftSize + rightSize];   //The merged array
int resultPointer = 0;  //Index position of where to insert element
int leftPointer = 0;
int rightPointer = 0;

//While length of either of the lists is > 0
while((leftSize > 0) || (rightSize > 0)){

    cout << "Got here" << endl;
    //If length of both left and right lists is > 0
    if((leftSize > 0) && (rightSize > 0)){

        //Compare first elements from both lists and put smallest one in the result list
        if(left[0] < right[0]){
            result[resultPointer] = left[0];
            leftPointer++;
            leftSize--;
        }else if(right[0] < left[0]){
            result[resultPointer] = right[0];
            rightPointer++;
            rightSize--;
        }else{
            //if both elements are the same, put them both in the result list
            result[resultPointer] = left[0];
            leftPointer++;
            leftSize--;
            result[resultPointer++] = right[0];
            rightPointer++;
            rightSize--;
        }
        resultPointer++;    //Increment pointer to point to next empty element

    }else if(leftSize > 0){
        result[resultPointer] = left[0];
        leftPointer++;
        leftSize--;
    }else if(rightSize > 0){
        result[resultPointer] = right[0];
        rightPointer++;
        rightSize--;
    }


}

//int* resultList = result;

return result;
}

int* merge_sort(int list[], int size){

//If list has 1 element then it is sorted so just return that
if(size<=1){
    return list;
}

int middle = size/2;    //Get mid point of given list

//Create left and right arrays
int left[middle];
int right[size-middle];

for(int i = 0; i<size-middle; i++){

    if(i<middle){
        left[i] = list[i];
    }
    right[i] = list[i+middle];
}

//Recursively call merge sort to sort out the sublists
int* leftList = merge_sort(left, middle);
int* rightList = merge_sort(right, size-middle);

//Merge the sorted lists and return a fully sorted list

int* merged = merge(leftList, middle, rightList, size-middle);

return merged;
}

The resulting output is just a bunch of random numbers(which I think happens when something hasn’t been initialized). This implementation might be stupid but I would still like to know what is going wrong here.

Cheers

Answer 1

You are returning a pointer from your merge function which points to a local variable. The local variable will go out of scope the moment you return from your merge function. So you return a pointer that is not pointing to any valid memory.