Heap implementation of priority queue?

Question

I'm trying to implement a queue of patients using a heap (with root smaller than children) but when I print the queue, the queue of patients doesn't look prioritized.

Insertion method works fine but it's the enqueue that doesn't prioritize items?

// Heap class
.....some code

//insertion: inserts patients into minimum heap using an array.
//expand array as needed and reorder heap to maintain its properties

public void insert(ER_Patient patient) {
    if(heapArray.length==count)
        expand();
    heapArray[count] = patient;
    count++;
    if(count>1)
        reorder();
}

// Priority Queue class
.......some code

public void enqueue(ER_Patient patient) {
    try {
        heap.insert(patient);
    } catch (NoSuchCategoryException exception) {
        System.out.println("Can't enqueue");
    }

}

// copy content of original's array to a new larger array
private void expand(){
    ER_Patient[] tempArray = new ER_Patient[heapArray.length * 8];
    for(int i=0;i<=heapArray.length-1;i++)
        tempArray[i]=heapArray[i];
    heapArray = tempArray;
}

// maintain heap property by keeping roots smaller than children
private void reorder(){
    ER_Patient temp;
    int next = count -1;
    temp = heapArray[next];
    while((next!=0) && temp.compareTo(heapArray[(next-1)/2])<0){
        heapArray[next] = heapArray[(next-1)/2];
        next = (next-1)/2;
    }
    heapArray[next] = temp;
}

Maybe you should post your reorder and expand methods. You current peace of code is not related to your problem. — Marcinek, Nov 25 '15 at 07:40
Print it how? Unless you successively remove the first item and print it, you won't get an ordered listing. The heap array itself isn't sequential. — user207421, Nov 25 '15 at 07:56
this is how i print: public void display(Heap h){ for(int i=0;i — Square-root, Nov 25 '15 at 08:13
a@EJP are you saying i have to remove 1st item before printing the array? — Square-root, Nov 25 '15 at 08:23
Why are you substracting 1 to variable `next`in the `reorder()` method in the sentence `int next = count -1;`? and also when you swap items it seems that you swap with the element in position `(N-1)/2` I guess you have to do it with the element at `N/2`. Just a guess anyway — sir psycho sexy, Nov 25 '15 at 08:24
@Square-root what EJP means, is the structure have just a partial order, that's the idea of a binary heap. If you want to see al the elements you added to the structure returned in order you have to remove them all. — sir psycho sexy, Nov 25 '15 at 08:27
Normally, a PriorityQueue (as it is in the JDK) has the lowest element first. However it make no guarantee about the order of any other element. — Peter Lawrey, Nov 25 '15 at 08:31
What part of 'successively remove the first item and print' don't you understand? — user207421, Nov 25 '15 at 08:40

score 2 · Accepted Answer · answered Nov 25 '15 at 08:44

2

This is how I print:

public void display(Heap h)
{
    for(int i=0;i<h.count;i++)
        System.out.println(heapArray[i]);
}

Wrong.

Unless you successively remove the first item and print it, you won't get an ordered listing. The heap array itself isn't sequential.

answered Nov 25 '15 at 08:44

user207421

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thing is heap uses patient id to order so what u suggested will print id in order. but i want to print priority in order: e.g 1 2 3 4 – Square-root Nov 25 '15 at 08:52
I understand that you want to print it in order, and that's what my answer tells you how to do. You don't seem to understand how priority heaps actually work. They aren't sorted arrays; they are essentially partially-sorted binary trees. The first thing you need to do is implement a `dequeue()` method as @virtualagent07 suggested. – user207421 Nov 25 '15 at 08:54
i have dequeue. it just removes the root. i'll try again – Square-root Nov 25 '15 at 08:56
1

It only has to remove the root and re-establish the heap invariant. Do you understand what 'successively remove and print' means? – user207421 Nov 25 '15 at 09:00
i guess so but my dequeue and removerootitem both work fine. – Square-root Nov 25 '15 at 09:04
So therefore your revised print method that successively removes and prints should also work fine. As opposed to `for(int i=0;i – user207421 Nov 25 '15 at 09:05

score 0 · Answer 2 · edited Nov 25 '15 at 08:55

0

Shouldn't the last line of reorder method be part of the while loop?

heapArray[next] = temp;

Also, there should be proper dequeue() method out there

edited Nov 25 '15 at 08:55

user207421

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answered Nov 25 '15 at 08:06

virtual agent 07

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3

Heap implementation of priority queue?

2 Answers2