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Priority queue

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

Introduction

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.

Usage

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;))

Example

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

IPriorityQueue PQ = new BinaryPriorityQueue();
Random R = new Random();
int i;
for(i=0;i<50;i++)
{
    PQ.Push(R.Next(1000));
}
while(PQ.Count>0)
    Console.WriteLine(PQ.Pop());

History

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

    Created IPriorityQueue interface.

License

This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here

About the Author

BenDi
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.

Comments and Discussions

 
GeneralThanks a lot Pinmembermajdaldin122-Apr-11 2:59 
GeneralMy vote of 3 PinmemberLassarRRRRRR17-Feb-11 5:01 
GeneralMy vote of 4 Pinmembers.hamed_rezvani12-Jan-11 3:48 
GeneralGeneric version PinmemberOlmo del Corral12-Mar-09 11:01 
GeneralUpdated for non duplicate value in list. Pinmemberbestwasin2-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
	else
		return -99;  //return, we dont do anything
	do
	{
		if (p == 0)
			break;
		p2 = (p - 1) / 2;
		if (OnCompare(p, p2) < 0)
		{
			SwitchElements(p, p2);
			p = p2;
		}
		else
			break;
	}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;
                    else
                        return mid; // found
                }
                return -1;
            }
 
Any suggest is welcomed. Smile | :)
Questionimplemntinig priority Quesues PinmemberMember 331791728-Aug-08 7:42 
Questionimplemntinig priority Quesues PinmemberMember 331791728-Aug-08 7:35 
NewsFast Priority Queue Implementation of Dijkstra Shortest Path Algorithm PinmemberTolga Birdal31-Mar-08 5:38 
Questionremove method Pinmemberbelal_haija22-Mar-08 15:41 
Questionhashtable Pinmemberwarnockm20-Nov-07 15:03 
GeneralCorrection PinmemberDavid Krantz14-Dec-06 23:49 
GeneralA-Star Implementation PinmemberCastorTiu22-Sep-06 21:02 
GeneralTake care while using multiple threads Pinmembernovak.pavel31-May-06 2:08 
QuestionDijsktra's algorithm Pinmembervit.ruzicka26-May-06 5:01 
GeneralRe: Dijsktra's algorithm Pinmemberwarnockm20-Nov-07 15:05 
GeneralRemove functionality Pinmemberbffgdggd6-Jan-06 3:59 
GeneralLicense for Priority Queue Pinmembertosboy20-Dec-05 23:40 
Generalconfused re: updating Pinmembervmy1-Jun-04 16:23 
GeneralRe: confused re: updating PinmemberBenDi3-Jun-04 1:34 
GeneralRe: confused re: updating Pinmemberjames kolpack12-Jan-06 6:35 
GeneralThanks!! PinmemberM.Shoaib Khan9-Apr-04 19:21 
QuestionWhat is wrong with Queue? Pinmembersandeman2-Apr-04 9:16 
AnswerRe: What is wrong with Queue? PinmemberBenDi2-Apr-04 9:36 
GeneralRe: What is wrong with Queue? PinmemberBenDi2-Apr-04 9:38 
GeneralRe: What is wrong with Queue? Pinmembersandeman2-Apr-04 14:00 

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