error while trying to create a minHeap from array

Question

I'm trying to get a minHeap from an array, but I can't get it right The input I've tried is: 4 3 2 1 The output I got is: 2 3 4 1

First I tried with using only an array of int for storing the heap and it worked, then I changed and used an array of struct node, but the final heap isn't a minHeap

Here is the main code:

int main(){

    makeMinHeap(v,vSize-1); // v is the pointer to the array of struct node, and vSize is the 
                            // size of the array
}
    void makeMinHeap(struct node *h, int size) {
    for (int i = floor(size/2); i >= 0 ; i--) {
        heapify(h, i,size);
    }
}

void heapify(struct node *h, int i,int size) {
    int l = left(i);
    int r = right(i);

    int m = i;

    if (l < size && h[l].value < h[i].value) {
        m = l;
    }
    else if (r < size && h[r].value < h[i].value) {
        m = r;
    }

    if (m != i) {
        swap(&h[m].value, &h[i].value);
        heapify(h, m, size);
    }

}

int left(int i) { return 2 * i; }

int right(int i) { return (2 * i + 1); }

void swap(int *x, int *y) { 
    int tmp = *x;
    *x = *y;
    *y = tmp;
}

A heap is a structure, not just a list of numbers. It has links relating parent nodes to children nodes. “2 3 4 1” is not a heap and does not, by itself, show any problem creating a heap. Why did you expect any other output? How did you get output at all; the posted code does not have any calls to output routines? Edit the question to provide a [mre]. — Eric Postpischil, Apr 28 '22 at 11:26
A binary heap is often represented as an array -- which implicitly defines the relationship between parent and child, as is also visible from the code, albeit that also that part in the code is not correct. — trincot, Apr 28 '22 at 12:15

trincot · Accepted Answer · 2022-04-28T12:26:22.223

Here are some of the issues:

The first (left) child of a node is not at i*2, but at i*2+1. The right child is at i*2+2.
The else if condition in heapify should really be a separate if, and you don't want to compare with h[i].value but with h[m].value, since you want to compare with the least value so far (which might be at the left child)
As vSize is the size of the array, you should not make the initial call with makeMinHeap(v, vSize-1), as you will then never look at the last value in the array. The -1 makes sense only for the heapify loop, which indeed can start at i = floor((size-1)/2), and so that subtraction should only be applied there.

Here are the relevant functions that needed correction:

int left(int i)  { return 2 * i + 1; } // corrected
int right(int i) { return 2 * i + 2; } // corrected

void heapify(struct node *h, int i, int size) {
    int l = left(i);
    int r = right(i);
    int m = i;

    if (l < size && h[l].value < h[i].value) {
        m = l;
    } // Not else-if here
    if (r < size && h[r].value < h[m].value) { // h[m]!
        m = r;
    }
    if (m != i) {
        swap(&h[m].value, &h[i].value);
        heapify(h, m, size);
    }
}

void makeMinHeap(struct node *h, int size) {
    for (int i = floor((size-1)/2); i >= 0 ; i--) { // -1 here
        heapify(h, i, size);
    }
}

int main(){
    int vSize = 4;
    struct node v[4] = {4, 3, 2, 1};
    makeMinHeap(v, vSize); // No -1 here! 
    for (int i = 0; i < vSize; i++) printf("%i ", v[i].value);
    printf("\n");    
}

error while trying to create a minHeap from array

1 Answers1