Building a priority queue with a heap

Question

If the heap is based on an array and the root is extracted, the code I'm using is suppose to reshuffle the nodes so that the heap property is maintained.

From my understanding the array and heap aren't exactly the same thing, but it seems that in the code, building the heap is just rearranging the array so that it holds the heap property. Is this the standard way to create the heap? Is it necessary to fill another array instead of just altering the array it is based on?

I'm having trouble understanding this concept because when I remove the root using a function extract-max , unless I pop the element from the array the array remains the same size. And the heap is supposed to be smaller but all that happens is that the node that moved up to replace the root is indicated by a zero in the array and it is not removed.

If I do keep the structure where the heap is basically the array obeying the heap property, how do I remove the node that should no longer be part of the heap? In my code this node is denoted by a 0 when printing the array.

#include <iostream>
#include <deque>
#include "print.h"
#include "random.h"

int parent(int i)
{
    return (i - 1) / 2;
}

int left(int i)
{
    if(i == 0)
        return 1;
    else
        return 2*i;
}

int right(int i)
{
    if(i == 0)
        return 2;
    else
        return 2*i + 1;
}

void max_heapify(std::deque<int> &A, int i, int heapsize)
{
    int largest;
    int l = left(i);
    int r = right(i);
    if(l <= heapsize && A[l] > A[i])
        largest = l;
    else
        largest = i;
    if(r <= heapsize && A[r] > A[largest])
        largest = r;
    if(largest != i) {
        exchange(A, i, largest);
        max_heapify(A, largest, heapsize);
    }
}

void build_max_heap(std::deque<int> &A)
{
    int heapsize = A.size() - 1;
    for(int i = (A.size() - 1) / 2; i >= 0; i--)
        max_heapify(A, i, heapsize);
}

int heap_extract_max(std::deque<int> &A, int heapsize)
{
    if(heapsize < 0)
        throw std::out_of_range("heap underflow");
    int max = A.front();
    //A.pop_front();
    A[0] = A[heapsize--];
    //A.pop_back();
    max_heapify(A, 0, heapsize);
    return max;
}

int main(int argc, const char * argv[])
{
    std::deque<int> B = deq(7);
    print(B);
    build_max_heap(B);
    print(B);
    std::cout << "Extract max ->\t" << heap_extract_max(B, B.size()) << std::endl;
    print(B);
    std::cout << "Extract max ->\t" << heap_extract_max(B, B.size()) << std::endl;
    print(B);
    return 0;
}

Output:

79 92 86 29 27 42 6 
92 86 79 29 27 42 6 
Extract max ->  92
86 79 42 29 27 0 6 
Extract max ->  86
79 42 27 29 0 0 6 

Answer 1

So. I changed max-heapify to return an index to the node that should be eliminated.

int max_heapify(std::deque<int> &A, int i, int heapsize)
{
    int largest;
    int l = left(i);
    int r = right(i);
    if(l <= heapsize && A[l] > A[i])
        largest = l;
    else
        largest = i;
    if(r <= heapsize && A[r] > A[largest])
        largest = r;
    if(largest != i) {
        exchange(A, i, largest);
        int j = max_heapify(A, largest, heapsize);
        return j;
    }
    return i;
}

Then I erased the element in heap_extract_max

int heap_extract_max(std::deque<int> &A, int heapsize)
{
    if(heapsize < 0)
        throw std::out_of_range("heap underflow");
    int max = A.front();
    A[0] = A[heapsize--];
    int i = max_heapify(A, 0, heapsize);
    A.erase(A.begin() + i);
    return max;
}

I get this output for 11 elements:

79 10 61 99 28 12 51 96 13 37 83 
99 96 83 79 37 61 51 10 13 28 12 
Extract max ->  99
96 83 61 79 37 12 51 10 13 28 
Extract max ->  96
83 79 61 51 37 12 10 13 28 

I really thought initially that I'd just be able to pop the last element after its value was recorded into the root and then reorganize the heap, but it wasn't cooperating. So the alternative was to delete the node position after the heap was reorganized.

Going back to the original code:

I realized where the problem was. I need to decrement the heapsize before I index the array using the variable, not after. Then I can just pop the last element the way I intended.

int heap_extract_max(std::deque<int> &A, int heapsize)
{
    if(heapsize < 0)
        throw std::out_of_range("heap underflow");
    int max = A.front();
    A[0] = A[--heapsize];
    A.pop_back();
    max_heapify(A, 0, heapsize);
    //int i = max_heapify(A, 0, heapsize);
    //A.erase(A.begin() + i);
    return max;
}

Answer 2

Your heap_extract_max function is discarding the last element in the std::deque .

int heap_extract_max(std::deque<int> &A, int heapsize)
{
    if(heapsize < 0)
        throw std::out_of_range("heap underflow");
    int max = A.front();  // Get a copy of the first element
    A[0] = A[heapsize--]; // Copy the last element to the first element
                          // and reduce the heap size, effectively 
                          // discarding the last element.
    max_heapify(A, 0, heapsize);
    return max;
}

after this, you can safely use pop_back() on your std::deque to remove that element from the container if you want.