CODEWITHSUNDEEP
javaarraysmethods
JimBelushi2
JimBelushi2

Reputation: 285

Heapify with ArrayList

I wrote this method to test my priority queue, which has as parameter an ArrayList and a Comparator:

@Test
  public void testPriorityQueue_ExtractMax() {
    PriorityQueue queue = new PriorityQueue(new IntegerComparator());
    Integer[] arrayExp={14, 9};
    queue.insert(16);
    queue.insert(9);
    queue.insert(14);
    queue.extractMax();
    assertArrayEquals(arrayExp, queue.getArray().toArray());
  }

Now, if I execute it, it says that the first elements in my result is 9, but I has to be 14 (the new root of my queue). These are the methods extract max and heapify. How can I solve it?

public void heapify(int index) {
    int largest = index;
    int leftIndex = 2 * index + 1;
    int rightIndex = 2 * index + 2;

    if (leftIndex < queue.size() && c.compare(queue.get(index), (queue.get(leftIndex))) < 0)
      largest = leftIndex;

    if (rightIndex < queue.size() && c.compare(queue.get(largest), queue.get(rightIndex)) < 0)
      largest = rightIndex;

    if (largest != index) {
      swap(index, largest);
      heapify(largest);
    }
  }

  public T extractMax() {
    if (queue.size() == 0) return null;

    T min = queue.get(0);
    queue.remove(0);
    queue.set(0, queue.get(queue.size() - 1));
    heapify(0);
    return min;
  }

This is the IntegerComparator:

public class IntegerComparator implements Comparator<Integer>{
  @Override
  public int compare(Integer l1, Integer l2) {
    int result = l1.compareTo(l2);
    if(result != 0)
      return result;
    return -1;
   }
}

EDIT_2: This is my insert method, can it be the problem:

public void insert(T elem) {
    int i = queue.size();
    int parentIndex = (i - 1) / 2;
    while (i > 0 && c.compare(elem, queue.get(parentIndex)) == 1) {
      queue.set(i, queue.get(parentIndex));
      i = parentIndex;
      parentIndex = (i - 1) / 2;
    }
    queue.add(i, elem);
  }

Upvotes: 2

Views: 2520

Answers (2)

KunLun
KunLun

Reputation: 3225

When you do this queue.remove(i); automatically shift all elements, after index i, to left with one position.

https://www.tutorialspoint.com/java/util/arraylist_remove.htm

And with queue.set(0, queue.get(queue.size() - 1)); You just set value of last index from queue on index 0 and lose value from index 0, but it still remain on last position.

After reading insert method

At queue.set(i, queue.get(parentIndex)); if i = queue.size() in first step, that mean index i don't exist in queue.

Upvotes: 1

Joshua Nicholl
Joshua Nicholl

Reputation: 73

As a suggestion, consider adding an unused element at index 0 so that accessing the parents/children of each node is more intuitive.

Example:

Consider the heap heap = [-1, 6, 4, 5, 2, 1, 4, 3] where the root is defined as heap[1], and the data for the heap is located from index = 1 to the heap's size.

To access the children of a node at index, it is intuitive to say that the left child is defined as heap[2 * index] and the right child is defined as heap[2 * index + 1]. Similarly, to access the parent node of a node at index, one can use int truncation to access parents: 

int parentInd = (int)(index/2);
T parent = heap[parentInd];

Solution:

As raul1ro has pointed out, you are losing the data from index 0 that you were not intending to remove.

In extractMax():

T min = queue.get(0);
queue.remove(0);
queue.set(0, queue.get(queue.size() - 1));

should be:

T min = queue.get(0); //Get min
T temp = queue.get(queue.size() - 1); //Get the last element in heap as temp
queue.remove(queue.size - 1); //Remove the last element
queue.set(0, temp); //Set the root to the temp value so that you can heapify

This will make it so that you only lose 1 element when you extractMax()

Hope that helps!

Upvotes: 1

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