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Posted 28 Mar 2004
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# Priority queue

, 28 Mar 2004
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.

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.

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## About the Author

 Software Developer (Senior) Germany
No Biography provided

## Comments and Discussions

 First PrevNext
 Danka !!! :) Samedi Ishmael Medivh25-Oct-14 11:53 Samedi Ishmael Medivh 25-Oct-14 11:53
 Thanks a lot majdaldin122-Apr-11 3:59 majdaldin1 22-Apr-11 3:59
 My vote of 3 LassarRRRRRR17-Feb-11 6:01 LassarRRRRRR 17-Feb-11 6:01
 My vote of 4 s.hamed_rezvani12-Jan-11 4:48 s.hamed_rezvani 12-Jan-11 4:48
 Generic version Olmo del Corral12-Mar-09 12:01 Olmo del Corral 12-Mar-09 12:01
 Updated for non duplicate value in list. bestwasin2-Feb-09 6:26 bestwasin 2-Feb-09 6:26
 implemntinig priority Quesues Member 331791728-Aug-08 8:42 Member 3317917 28-Aug-08 8:42
 implemntinig priority Quesues Member 331791728-Aug-08 8:35 Member 3317917 28-Aug-08 8:35
 Fast Priority Queue Implementation of Dijkstra Shortest Path Algorithm Tolga Birdal31-Mar-08 6:38 Tolga Birdal 31-Mar-08 6:38
 remove method belal_haija22-Mar-08 16:41 belal_haija 22-Mar-08 16:41
 hashtable warnockm20-Nov-07 16:03 warnockm 20-Nov-07 16:03
 Correction David Krantz15-Dec-06 0:49 David Krantz 15-Dec-06 0:49
 A-Star Implementation CastorTiu22-Sep-06 22:02 CastorTiu 22-Sep-06 22:02
 Take care while using multiple threads novak.pavel31-May-06 3:08 novak.pavel 31-May-06 3:08
 Dijsktra's algorithm vit.ruzicka26-May-06 6:01 vit.ruzicka 26-May-06 6:01
 Re: Dijsktra's algorithm warnockm20-Nov-07 16:05 warnockm 20-Nov-07 16:05
 Remove functionality bffgdggd6-Jan-06 4:59 bffgdggd 6-Jan-06 4:59
 License for Priority Queue tosboy21-Dec-05 0:40 tosboy 21-Dec-05 0:40
 confused re: updating vmy1-Jun-04 17:23 vmy 1-Jun-04 17:23
 Re: confused re: updating BenDi3-Jun-04 2:34 BenDi 3-Jun-04 2:34
 Re: confused re: updating James Kolpack12-Jan-06 7:35 James Kolpack 12-Jan-06 7:35
 Thanks!! abc8769-Apr-04 20:21 abc876 9-Apr-04 20:21
 What is wrong with Queue? sandeman2-Apr-04 10:16 sandeman 2-Apr-04 10:16
 Re: What is wrong with Queue? BenDi2-Apr-04 10:36 BenDi 2-Apr-04 10:36
 Re: What is wrong with Queue? BenDi2-Apr-04 10:38 BenDi 2-Apr-04 10:38
 Last Visit: 12-Nov-18 20:32     Last Update: 12-Nov-18 20:32 Refresh 12 Next »

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