Print content of priority queue

Question

How do I make the print_queue work properly in Java? This is my own implementation of a queue.

Using Iterator() works fine, except it prints numbers in random order.

package data_structures_java ;
import java.util.Iterator;
import java.util.PriorityQueue ;
import java.util.* ;
public class Queue_implementation {
 
    PriorityQueue<Integer> actual_queue ;
    
    public Queue_implementation(){
        actual_queue = new PriorityQueue<Integer>() ;
        
    }
    
    public  void add(int num){
        actual_queue.add(num) ;
    }
    
    public int remove(){
          return actual_queue.remove() ;          
    }
    
    public int peek(){
        if( actual_queue.isEmpty()) return -1 ;
        else return actual_queue.peek() ;
    }
    
    public int element(){
        return actual_queue.element() ;
    }
    
    public void print_queue(){      
        PriorityQueue<Integer>copy = new PriorityQueue<Integer>();
        copy.addAll(actual_queue) ;        
        Iterator<Integer> through = actual_queue.iterator() ;
        while(through.hasNext() ) {
                System.out.print(through.next() + " ") ;
        }
        System.out.println() ;
                
        actual_queue.addAll(copy) ;
        
    }
    public static void main(String[] args) {            
        Queue_implementation x = new Queue_implementation() ;
        x.add(10) ;
        x.add(9) ;
        x.add(8) ;
        x.add(7) ;
        x.add(6) ;
        x.print_queue() ;
    }

}

I tried to use toArray() but it returns Object[], which I don't know how to traverse:

Object[] queue_object_array = x.toArray() ;
Arrays.sort(queue_object_array) ;

Is your Queue supposed to be a Queue or is is supposed to be a PriorityQueue? They do two different things. — Will Newton, Apr 08 '14 at 05:33
@WillNewton, just queue actually , but i m using std.priority queue from java api — ERJAN, Apr 08 '14 at 05:45

score 10 · Answer 1 · answered Apr 08 '14 at 06:34

10

Using Iterator() works fine, except it prints numbers in random order.

That's exactly what it says it will do in the Javadoc. The only way to get the ordering in the PriorityQueue is to use the poll() or remove() methods.

answered Apr 08 '14 at 06:34

user207421

305,947
44
307
483

2

yes i dont' see the reason why he did it - me, topic starter gives you +1 – ERJAN Apr 08 '14 at 19:36
ejp, but when i use remove() - i delete elements from the queue itself, do i need to store it somehow and copy back after remove()? – ERJAN Apr 08 '14 at 19:38
@ERJAN Yes, if you need the contents afterwards. – user207421 Apr 08 '14 at 22:07

hqt · Answer 2 · 2020-04-17T07:47:47.840

8

One line solution: it is useful when you need to fast debug.

System.out.println(Arrays.toString(priorityQueue.toArray()));

Edit: Another concise code:

System.out.println(priorityQueue);

edited Apr 17 '20 at 07:47

answered May 21 '19 at 20:10

hqt

29,632
51
171
250

1

what is the difference between the code and `System.out.println(priorityQueue)` – ZhaoGang Apr 15 '20 at 14:48
@ZhaoGang thanks for the note. Actually it is the same after I checked. It just because my thinking process causes me to do so :D You can add a new answer, or I will edit my answer. – hqt Apr 15 '20 at 16:18
1

They are much the same, so no need for a new answer I think, please edit your answer if you would like to, thx. – ZhaoGang Apr 16 '20 at 01:32
1

@user207421 I don't say this method will print in order. It's useful for debugging. I am lazy to check but I remember the original post just wants to print out the content. – hqt Feb 07 '21 at 03:21
This does not print the PQ in priority order. Please read the question. Printing in order is what the question is about. *Ergo* you haven't answered it. Being too lazy to check is no excuse. I did. – user207421 Aug 20 '23 at 22:56

score 1 · Answer 3 · answered May 26 '21 at 06:06

1

If you need to print content of priorityQueue where each element is a 2D array you can try using:

System.out.println(Arrays.deepToString(priorityQueue.toArray());

If it's a 1D array:

System.out.println(Arrays.toString(priorityQueue.toArray());

answered May 26 '21 at 06:06

Tushar

528
4
20

This does not print the PQ in priority order. Please read the question. – user207421 Aug 20 '23 at 22:57

Print content of priority queue

3 Answers3

Linked

Related