Implement Heap Sort using sift down approach in c - c

Question

I'm trying to implement heap sort using sift down approach in c, The output of the program is missing 0th index of the array. I'm unable to find the issue where the problem is, Is it due to array overflow?
Here is my code
#include<stdio.h>
void heapSort(int[], int);
void siftdown(int[], int, int, int);
int main(void)
{
int array[] = {5, 2, 1, 4, 3};
int nodeCount = sizeof(array)/sizeof(array[0]);
heapSort(array, nodeCount);
printf("Sorted array:\n");
for(int i=0; i < nodeCount; i++)
{
printf("%d\n", array[i]);
}
return 0;
}
void heapSort(int array[], int nodeCount){
//creating a heap
for(int i = nodeCount / 2; i >= 1; i--){
siftdown(array, array[i], i, nodeCount);
}
//perform heapsort
for(int lastNode = nodeCount; lastNode > 1; lastNode--){
int lastNodeValue = array[lastNode];
array[lastNode] = array[1];
array[1] = lastNodeValue;
siftdown(array, lastNodeValue, 1, lastNode -1);
}
}
void siftdown(int array[], int nodeValue, int root, int last){
int leftChild = 2 * root + 1 ;
while(leftChild <= last){ //at least has one child
if(leftChild < last){ //has right child
if(array[leftChild + 1] > array[leftChild]){
leftChild++;
}
}
if(nodeValue >= array[leftChild]){
break;
}
else{
array[root] = array[leftChild];
root = leftChild;
leftChild = 2 * root + 1;
}
array[root] = nodeValue;
}
}
The output of the program:
Sorted array: 5 1 2 3 4

Answer 1

The loop in your heapSort function is:
for(int i = nodeCount / 2; i >= 1; i--){
siftdown(array, array[i], i, nodeCount);
}
Because i is never 0, you don't ever sift that node down. Change the comparison to i >= 0.
In your heapSort function, you have this loop:
//perform heapsort
for(int lastNode = nodeCount; lastNode > 1; lastNode--){
int lastNodeValue = array[lastNode];
array[lastNode] = array[1];
array[1] = lastNodeValue;
siftdown(array, lastNodeValue, 1, lastNode -1);
}
Arrays in C start at 0. So if you have 5 items in the array, the indexes are 0, 1, 2, 3, 4. Your loop is using array indexes 1, 2, 3, 4, 5. You need to change that loop as well. It will look something like:
//perform heapsort
for(int lastNode = nodeCount-1; lastNode > 0; lastNode--){
int lastNodeValue = array[lastNode];
array[lastNode] = array[0];
array[0] = lastNodeValue;
siftdown(array, lastNodeValue, 0, lastNode-1);
}