kth smallest element using min-heap - c++

Question

I was solving a problem of kth smallest element using min-heap but got stuck as it's always giving me the first smallest element, so I guess my extractmin function is not working correctly.My approach was to make array into a min heap data structure in which root is the minimum element and then remove k-1 smallest elements and then simply return the value of root.
#include <iostream>
using namespace std;
void swap(int &x, int &y)
{
int u = x;
x = y;
y = u;
}
int l(int p)
{
return 2 * p + 1;
}
int r(int p)
{
return 2 * p + 2;
}
void heapify(int arr[], int i, int n);
void makeheap(int arr[], int n)
{
int last = n - 1;
for (int i = (last - 1) / 2; i >= 0; i--)
{
heapify(arr, i, n);
}
}
void heapify(int arr[], int i, int n)
{
int smallest = arr[i],y=0;
if (l(i) <= n - 1 && arr[l(i)] < arr[i])
{
smallest = arr[l(i)];
}
if (r(i) <= n - 1 && arr[r(i)] < smallest)
{
smallest = arr[r(i)];
y=1;
}
if (smallest != arr[i])
{
if(y==0){
swap(arr[l(i)], arr[i]);}
else if(y==1)
{
swap(arr[r(i)],arr[i]);
}
heapify(arr, i, n);
}
}
void extractmin(int arr[], int &n)
{
swap(arr[0], arr[n - 1]);
n--;
heapify(arr, 0, n);
}
int getmin(int arr[])
{
return arr[0];
}
int main()
{
int T;
cin >> T;
while (T--)
{
int n;
cin >> n;
int arr[n];
for (int i = 0; i < n; i++)
{
cin >> arr[i];
}
int k;
cin >> k;
makeheap(arr, n);
for (int i = 0; i < k - 1; i++) {
extractmin(arr, n);
}
cout << getmin(arr) << "\n";
}
}
Input:
2
6
7 10 4 3 20 15
3
5
7 10 4 20 15
4
Expected Output:
7
15
My Output:
3
4

Answer 1

In heapify() you use
swap(smallest,arr[i]);
That exchanges the values in smallest and arr[i]. But arr[l(i)] or arr[r(i)] is not changed. What this actually does is copy the smaller value of arr[l(i)] and arr[r(i)] into arr[i].
Also in main you do
for (int i = 0; i < k - 1; i--) {
which counts i down till it hits INT_MIN instead of counting up to k. This quickly causes the heap to be empty and the code to segfault.
PS: Why not use the std::make_heap and friends?