Having problems while inserting values into a max binary heap

Question

I created a binary heap using C. Here are the functions to initialize the heap and insert the values

int *heapArray; // pointer to array of elements in heap
int capacity = 0; // maximum possible size of min heap, initially set to 0
int heap_size; // Current number of elements in min heap
int d = 2;  //max number of children per node

int getParent(int i) //get the parent index of i
{
    return (i/d);
}

bool isFull() //check if heap is full
{
    if(heap_size == capacity)
        return true;
    if(heap_size < capacity)
        return false;
}

void initializeHeap(int cap) //initialize the heap
{
    heap_size = 0;
    capacity = cap;
    heapArray = (int*)malloc(cap*sizeof(int));
}

void insertValue(int x) //insert a value into the heap
{
    if(isFull()) //If heap is already full, double the heap size
    {
        capacity = 2*capacity;
        heapArray = realloc(heapArray,capacity*sizeof(int));
    }

    heap_size = heap_size + 1;
    heapArray[heap_size] = x; //insert new value into heap (not zero-indexed)
    maxHeapSwim(heap_size);
}

void maxHeapSwim(int k) //move value up the heap to maintain max-heap order
{
    while(k > 1 && heapArray[getParent(k)] < heapArray[k])
    {
        swap(&heapArray[k],&heapArray[parent(k)]);
        k = parent(k);
    }
}

Then in the main() method, I tried to insert values into the heap and print out the values:

int main()
{

  initializeHeap(1); //Set the heap capacity to 1

  insertValue(2);
  insertValue(3);
  insertValue(1);
  insertValue(6);
  insertValue(4);

  printf("\n");
  printf("%i", heapArray[1]);
  printf("\n");
  printf("%i", heapArray[2]);
  printf("\n");
  printf("%i", heapArray[3]);
  printf("\n");
  printf("%i", heapArray[4]);
  printf("\n");
  printf("%i", heapArray[5]);
  printf("\n");

  return 0;

}

Since this is a max-heap, I was hoping the output would look like this:

But instead, it looked like this:

I don't understand. Why is the 2 turning into a 0? I have a feeling it has something to do with how I'm doubling the heap size in the insertValue() function....maybe the way I'm using realloc. Is there something I'm doing incorrectly?

Another thing to note is that my binary heap isn't zero-indexed. The first value is inserted into heapArray[1], the second value into heapArray[2], and so on.

@stark I'm aware of that. But I've seen many examples where a binary heap is built by keeping the 0th index of the array empty and the first value is placed at index 1. That's what I'm trying to do here. — Bob K, Dec 31 '18 at 13:25

EsmaeelE · Answer 1 · 2018-12-31T13:55:58.123

Some changes on first section of your code works for me.

Implement swap() and some minor changes.

Code

#define false 0
#define true 1

int *heapArray; // pointer to array of elements in heap
int capacity = 0; // maximum possible size of min heap, initially set to 0
int heap_size; // Current number of elements in min heap
int d = 2;  //max number of children per node

int getParent(int i) //get the parent index of i
{
    return (i/d);
}

void maxHeapSwim(int k) //move value up the heap to maintain max-heap order
{
    while(k > 1 && heapArray[getParent(k)] < heapArray[k])
    {
        //swap(&heapArray[k],&heapArray[parent(k)]);
        int x=heapArray[k];
        heapArray[k]=heapArray[getParent(k)];
        heapArray[getParent(k)]=x;
        k = getParent(k);
    }
}

int  isFull() //check if heap is full
{
    if(heap_size == capacity)
        return true;
    if(heap_size < capacity)
        return false;
}

void initializeHeap(int cap) //initialize the heap
{
    heap_size = 0;
    capacity = cap;
    heapArray = (int*)malloc(cap*sizeof(int));
}

void insertValue(int x) //insert a value into the heap
{
    if(isFull()) //If heap is already full, double the heap size
    {
        capacity = 2*capacity;
        heapArray = realloc(heapArray,capacity*sizeof(int));
    }

    heap_size = heap_size + 1;
    heapArray[heap_size] = x; //insert new value into heap (not zero-indexed)
    maxHeapSwim(heap_size);
}




int main()
{  

  initializeHeap(1); //Set the heap capacity to 1

  insertValue(2);
  insertValue(3);
  insertValue(1);
  insertValue(6);
  insertValue(4);

  printf("\n");
  printf("%i", heapArray[1]);
  printf("\n");
  printf("%i", heapArray[2]);
  printf("\n");
  printf("%i", heapArray[3]);
  printf("\n");
  printf("%i", heapArray[4]);
  printf("\n");
  printf("%i", heapArray[5]);
  printf("\n");

  return 0;
}

output

./code 


6
4
1
2
3

It didn't work. I'm still getting the output 6,4,1,0,3 instead of 6,4,1,2,3 — Bob K, Dec 31 '18 at 13:44
Still not working. The only things you changed in my code is the maxHeapSwim function and `define false 0` and `define true 1`. Are there any other changes that I'm missing? — Bob K, Dec 31 '18 at 14:20
Yes thats true i only change`maxHeapSwim()` but as i show in output its works on my terminal. — EsmaeelE, Dec 31 '18 at 15:04
this is output of code on online compiler: https://www.jdoodle.com/a/SPC — EsmaeelE, Dec 31 '18 at 15:19
Yeah but it still didn't work. I managed to solve the problem though. I'll post it as an answer later. — Bob K, Jan 06 '19 at 11:39

score 0 · Answer 2 · answered Jan 16 '19 at 13:18

This worked for me:

void insertValue(int x)
{
    if(capacity < 10)
    {
        capacity = 10;
        heapArray = (int*)realloc(heapArray,capacity*sizeof(int));
    }
    if(heap_size >= capacity/2)
    {
        capacity += 10;
        heapArray = (int*)realloc(heapArray,capacity*sizeof(int));
    }

    heap_size = heap_size + 1;
    heapArray[heap_size] = x;
    maxHeapSwim(heap_size);
}

The key is to make sure the binary heap is never full.

Having problems while inserting values into a max binary heap

2 Answers2