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Posted 28 Mar 2004

Priority queue

, 28 Mar 2004
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Another addition to the System.Collections namespace - a priority queue, also known as a heap.


Many algorithms - for instance Dijkstra's well-known search for the shortest path - run most efficient with a 'semi-sorted' list, which always has the smallest element up front. The most efficient structure for this purpose is a heap, also called priority queue.


The PriorityQueue class is built in the guidelines of the System.Collections namespace. Comparison is done through the IComparer interface, or, if none is specified, through the IComparable implementation of the provided objects. The objects in the PQ need not have all the same type but they have to be comparable with each other.

Next to the standard System.Collections methods (Count, Contains, ... you name it), the PQ supports 3 main operations:

  • Push(): This will add an object to the PQ, reforming the structure so that this element is at the front if it is the currently smallest. The complexity of the operation is O(ld n).
  • Pop(): This will remove and return the current head of the PQ, which always is the smallest element. Complexity O(ld n).
  • Update(): It might be necessary to change elements that are already in the queue. Because this is not very common (you need to find the element first), it can only be done by the explicit IList implementation (the indexer should not be used in any other case). Once you set the indexer, the PQ will automatically reorder. Complexity O(ld n) (surprise;))


/// We create a PQ, add some random numbers and retreive them in sorted order:

IPriorityQueue PQ = new BinaryPriorityQueue();
Random R = new Random();
int i;


  • 30.3.04 - Inserted.
  • 31.3.04 - renamed the main class to BinaryPriorityQueue.

    Created IPriorityQueue interface.


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About the Author

Software Developer (Senior)
Germany Germany
I did my diploma in Dresden and Sydney where I dealt with algorithms, agents and other cool AI stuff. Now I moved to Frankfurt to work on my PhD dealing with software structures for artificial intelligence systems. If I can, I do things in C# and ASP.NET, but if I have to, my C++, Java and SQL are not that bad.
Long Live .NET.

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Comments and Discussions

GeneralDanka !!! :) Pin
Samedi Ishmael Medivh25-Oct-14 10:53
memberSamedi Ishmael Medivh25-Oct-14 10:53 
GeneralThanks a lot Pin
majdaldin122-Apr-11 2:59
membermajdaldin122-Apr-11 2:59 
GeneralMy vote of 3 Pin
LassarRRRRRR17-Feb-11 5:01
memberLassarRRRRRR17-Feb-11 5:01 
GeneralMy vote of 4 Pin
s.hamed_rezvani12-Jan-11 3:48
members.hamed_rezvani12-Jan-11 3:48 
GeneralGeneric version Pin
Olmo del Corral12-Mar-09 11:01
memberOlmo del Corral12-Mar-09 11:01 
GeneralUpdated for non duplicate value in list. Pin
bestwasin2-Feb-09 5:26
memberbestwasin2-Feb-09 5:26 
First of all thanks to BenDi for your great PQ.

Only one feature that I want more is 'allow no duplicated value in list', so I decide to extend his code.

What I have done is just to perform the binary search before Push() any object in the list, just to check that it is already there or not. If it's there then we do nothing.
I choose binary search because this will limit the big-o to O(log N), so it will be like almost the same complexity with PQ.

(I'm not sure whether Contains() method in ArrayList() is perform in linear or binary so I do it for myself here.)

This is modified Push() method
/// If the object is already existed in the list then return -99.
public int Push(object O)
	int p = InnerList.Count, p2;
	if (CheckValueInList(O) == -1)
		InnerList.Add(O); // E[p] = O
		return -99;  //return, we dont do anything
		if (p == 0)
		p2 = (p - 1) / 2;
		if (OnCompare(p, p2) < 0)
			SwitchElements(p, p2);
			p = p2;
	}while (true);
	return p;

And the binary search method is here
            // Check the existance of the specified object in the list,
            // if it's exist then return the index of the existed object, otherwise return -1
            int CheckValueInList(Object O)
                //if no element
                if (InnerList.Count == 0)
                    return -1;
                //binary search
                int low = 0;
                int high = InnerList.Count - 1;
                int mid;
                while (low <= high)
                    mid = low + (high - low) / 2;
                    if (Comparer.Compare(InnerList[mid], O) < 0)
                        high = mid - 1;
                    else if (Comparer.Compare(InnerList[mid], O) > 0)
                        low = mid + 1;
                        return mid; // found
                return -1;

Any suggest is welcomed. Smile | :)
Questionimplemntinig priority Quesues Pin
Member 331791728-Aug-08 7:42
memberMember 331791728-Aug-08 7:42 
Questionimplemntinig priority Quesues Pin
Member 331791728-Aug-08 7:35
memberMember 331791728-Aug-08 7:35 
NewsFast Priority Queue Implementation of Dijkstra Shortest Path Algorithm Pin
Tolga Birdal31-Mar-08 5:38
memberTolga Birdal31-Mar-08 5:38 
Questionremove method Pin
belal_haija22-Mar-08 15:41
memberbelal_haija22-Mar-08 15:41 

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