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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.

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Software Developer (Senior)
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.

 First Prev Next
 Thanks a lot majdaldin1 22-Apr-11 2:59
 My vote of 3 LassarRRRRRR 17-Feb-11 5:01
 My vote of 4 s.hamed_rezvani 12-Jan-11 3:48
 Generic version Olmo del Corral 12-Mar-09 11:01
 Updated for non duplicate value in list. bestwasin 2-Feb-09 5:26
 implemntinig priority Quesues Member 3317917 28-Aug-08 7:42
 implemntinig priority Quesues Member 3317917 28-Aug-08 7:35
 remove method belal_haija 22-Mar-08 15:41
 hashtable warnockm 20-Nov-07 15:03
 Correction David Krantz 14-Dec-06 23:49
 A-Star Implementation CastorTiu 22-Sep-06 21:02
 Take care while using multiple threads novak.pavel 31-May-06 2:08
 Dijsktra's algorithm vit.ruzicka 26-May-06 5:01
 Re: Dijsktra's algorithm warnockm 20-Nov-07 15:05
 Remove functionality bffgdggd 6-Jan-06 3:59
 License for Priority Queue tosboy 20-Dec-05 23:40
 confused re: updating vmy 1-Jun-04 16:23
 I don't understanbd what you're supposed to do if you want to change an object's key while it's in the priority queue and then re-enforce the queue. Can you post a small example of what you mean in the instructions for Update()?
 Re: confused re: updating BenDi 3-Jun-04 1:34
 Re: confused re: updating james kolpack 12-Jan-06 6:35
 Thanks!! M.Shoaib Khan 9-Apr-04 19:21
 What is wrong with Queue? sandeman 2-Apr-04 9:16
 Re: What is wrong with Queue? BenDi 2-Apr-04 9:36
 Re: What is wrong with Queue? BenDi 2-Apr-04 9:38
 Re: What is wrong with Queue? sandeman 2-Apr-04 14:00
 Re: What is wrong with Queue? BenDi 2-Apr-04 21:57
 Your PriorityQueue code converted to Visual Basic Enigmativity 30-Mar-04 22:51
 Re: Your PriorityQueue code converted to Visual Basic BenDi 31-Mar-04 0:14
 Re: Care to join a "all collection" project ? BenDi 30-Mar-04 3:16
 Re: Care to join a "all collection" project ? BenDi 2-Apr-04 22:45
 Which heap ? Jonathan de Halleux 29-Mar-04 11:15
 Re: Which heap ? BenDi 30-Mar-04 2:55
 Re: Which heap ? Jonathan de Halleux 30-Mar-04 3:00
 Re: Which heap ? BenDi 30-Mar-04 3:24
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