A queue is a first-in first-out (FIFO) abstract data type that is heavily used in computing. Uses for queues involve anything where you want things to happen in the order that they were called, but where the computer can't keep up to speed. For example:

• Keyboard Buffer - you want the letters to appear on the screen in the order you press them. You might notice that when your computer is busy the keys you press don't appear on the screen until a little while after you press them. When they do appear they appear in the order you press them.
• Printer Queue - you want print jobs to complete in the order you sent them, i.e. page 1, page 2, page 3, page 4 etc. When you are sharing a printer several people may send a print job to the printer and the printer can't print things instantly, so you have to wait a little while, but the items output will be in the order you sent them.

There are several different types of queues such as the ones described below:

Linear

In this queue form, elements are able to join the queue at one end and can exit from the queue at the other end. First In First Out (FIFO).

A real life example of this would be queuing to pay at the supermarket. The first person into the queue is the first person to be served. And if you join the queue last you are going to be served last (no pushing!). However, there is one difference, in a supermarket queue once the first person has been served the rest of the queue shuffles forward, making the previously second person now first, this is possible in a computer but very costly in terms of processing. The linear queue you are about to see 'crawls' through memory, devouring everything it comes across.

The most basic implementation of a linear queue would work its way through its allotted memory, without reusing memory. The front and end pointers can only move forwards and sooner or later would reach the end of the allotted memory - at this point, no more items could be added to the queue. Take a look at this example and the values of the two pointers storing the front of the queue (FrontPtr) and the next free memory location (NextFree).

 FrontPtr = 1
 NextFree = 4

 FrontPtr = 1
 NextFree = 5

 FrontPtr = 2
 NextFree = 5
 Output = D

 FrontPtr = 2
 NextFree = 6

 FrontPtr = 3
 NextFree = 6
 Output = G

 FrontPtr = 3
 NextFree = 7

A better implementation would reuse memory by shifting each item one to the left, each time an item is removed from the front. However, if there are many items in the queue this could become very slow. See below (note that we no longer need a front pointer as item 1 is always the front):

 NextFree = 4

 NextFree = 5

 NextFree = 4
 Output = D

 NextFree = 5

 NextFree = 4
 Output = G

 NextFree = 5

Fast to implement, with simple code

However:

Removing items from the front of the queue can be slow

Circular

Unlike linear queues Circular queues allow for memory to be re-used:

Generally, a circular buffer requires three pointers:

• one to the actual buffer in memory
• one to point to the start of valid data
• one to point to the end of valid data

The following example shows a partially-full fixed length buffer and how it handles additions and deletions from a queue. Take particular care to note the pointers.

Diagram	StartPtr	EndPtr
	3	5
Data is specified as being between the StartPtr and the EndPtr
	3	6
Adding another element to the queue moves the EndPtr up one
	4	6
Deleting an element from the front of the queue is achieved by increasing the StartPtr by one
	4	7
Adding another element to the queue moves the EndPtr up one, it is now at the end of the buffer
	4	1
Adding another element to the queue makes the EndPtr loop around to the free space at the beginning of the buffer
	4	2
Adding another element to the queue moves the EndPtr up one, it is now closing in the on the StartPtr
	4	3
Adding another element to the queue moves the EndPtr up one, it is now closing in the on the StartPtr. If we tried to add another element to the circular queue we couldn't without first removing something from the front of the queue

Circular queues are very common in computer science and they are used to store things, they have the following benefits

They reuse memory space, meaning they won't overwrite data or run out of memory (within limits)

and drawbacks:

Involve storing pointers as well as data, taking up more space than linear queues

Limited by the amount of data available in the buffer (array)

Priority

This is based on the order of priority of the elements in the queue. In other words, each element of this type of queue has different level of priority attached to it; e.g., high priority or low priority.

A real world example of this would be an Accident and Emergency Department waiting room, someone who had fallen over, would probably be low priority compared with someone who had cut their head open, and as such even if the head injury patient came in later they would probably jump the queue.

In a computer we use priority queues to handle the execution of processes. Each process will have a priority attached to it, with the more important processes jumping ahead of the less important for the processor's attention when they require it. For example:

StartPtr = 2
EndPtr = 1