Question

I am attempting to implement Heap sort in my program to learn more about sorting algorithms. However I am running in to an issue.

I call Heap sort in main like this:

Main:

heap_sort(h_vector);

Where h_vector is a randomly sized vector with random ordered elements. My heap sort algorithm looks like.

Heap Sort:

void max_heapify(std::vector<int>& v, int i)
{
    int left = i + 1, right = i + 2;
    int largest;

    if( left <= v.size() && v[left] > v[i])
    {
        largest = left;
    }

    else
    {
        largest = i;
    }

    if( right <= v.size() && v[right] > v[largest])
    {
        largest = right;
    }

    if( largest != i)
    {
        std::swap(v[i], v[largest]);
        max_heapify(v,largest);
    }



}

void build_max_heap(std::vector<int>& v)
{
    for( int i = v.size() - 2; i >= 0; --i)
    {
        max_heapify(v, i);
    }

}

void heap_sort(std::vector<int>& v)
{
    build_max_heap(v);
    int x = 0;
    int i = v.size() - 1;
    while( i > x)
    {
        std::swap(v[i],v[x]);
        ++x;
        --i;
    }

}

Whenever I add this sort to my program I get the following error.

Error:

*** glibc detected *** ./a.out: free(): invalid next size (normal): 0x096c82d0 ***

I am not sure what could be causing this. I thought at first that my alogrithm might be going out of bounds of the vector but I have checked a few times and I do not see where. Any ideas? Thanks for your help in advance.

Answer 1

At the very first invokation of max_heapify(), you invoke it with i = v.size() - 2

Thus, when you set right = i + 2; you actually set: right = v.size()

Now, look at this:

 if( right <= v.size() && v[right] > v[largest])

Note that right <= v.size(), and you are now trying to access v[right], which is out of bound.

Note that the last index of v is v[v.size() -1] – so all your if statements should be right < v.size() [instead <=]

I assume solving these issues will solve your bug eventually.