Programming Concepts: Queues

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UNIT 3 - ⇑ Programming Concepts ⇑

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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[edit]

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).

Representation of a Linear FIFO Queue

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.

A Linear queue is constantly working its way through memory, it does not re-use memory. As a result if a large number of items are being added and removed from the list this means the lists will grow very large without containing much data. It is not a very efficient solution and will use up large amounts of memory. 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).

State 1 State 2 State 3 State 4 State 5 State 6
start state H joins queue Start is served J joins queue Start is served P joins queue
1 2 3 4 5
D G A
 FrontPtr = 1
 NextFree = 4
1 2 3 4 5
D G A H
 FrontPtr = 1
 NextFree = 5
1 2 3 4 5
D G A H
 FrontPtr = 2
 NextFree = 5

Note that no data is
'deleted', but the
pointers changed

1 2 3 4 5
D G A H J
 FrontPtr = 2
 NextFree = 6
1 2 3 4 5
D G A H J
 FrontPtr = 3
 NextFree = 6
1 2 3 4 5 6
D G A H J P
 FrontPtr = 3
 NextFree = 7
plus pointFast to implement, with simple code

However:

minus point Uses up large amounts of memory, might overwrite important memory locations


Circular[edit]

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
Circular buffer - XX123XX with pointers.svg 3 5
Data is specified as being between the StartPtr and the EndPtr
Circular buffer - XX1234X with pointers.svg 3 6
Adding another element to the queue moves the EndPtr up one
Circular buffer - XXX234X with pointers.svg 4 6
Deleting an element from the front of the queue is achieved by increasing the StartPtr by one
Circular buffer - XXX2345 with pointers.svg 4 7
Adding another element to the queue moves the EndPtr up one, it is now at the end of the buffer
Circular buffer - 6XX2345 with pointers.svg 4 1
Adding another element to the queue makes the EndPtr loop around to the free space at the beginning of the buffer
Circular buffer - 67X2345 with pointers.svg 4 2
Adding another element to the queue moves the EndPtr up one, it is now closing in the on the StartPtr
Circular buffer - 6782345 with pointers.svg 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

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

and drawbacks:

minus point Involve storing pointers as well as data, taking up more space linear queues
minus point Limited by the amount of data available in the buffer (array)


Priority[edit]

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&E is an example of a priority queue

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.

State 1 State 2 State 3
1. broken arm
2. grazed knee
1. heart attack
2. broken arm
3. grazed knee
1. heart attack
2. severe concussion
3. broken arm
4. grazed knee
initial queue heart attack top priority jumps the queue severe concussion less important than heart attack, so moves to second

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:

Process Priority
Web Browser Normal
Media Player Below Normal
OS Security High
Scheduled Virus Scanner Low
Printer Queue Low
Word Processor Normal
Extension Scheduling Algorithms

There are several methods we can use to handle processor scheduling, each have their own benefits and drawbacks:

Scheduling algorithm CPU Utilization Throughput Turnaround time Response time
First In First Out Low Low High Low
Shortest Job First Medium High Medium Medium
Priority based scheduling Medium Low High High
Round-robin scheduling High Medium Medium High
Multilevel Queue scheduling High High Medium Medium
Exercise: Queues

Why would you use a circular queue over a linear queue?

Answer :

A circular queue uses a fixed amount of memory and reuses memory as it moves through the queue. A linear queue does not reuse memory and can waste memory.

Name three pointers used in implementing a circular queue

Answer :

  • one to the actual buffer in memory
  • one to point to the start of valid data (StartPtr)
  • one to point to the end of valid data (EndPtr)

What happens in a priority queue when a task of higher priority wants the processor's attention?

Answer :

The lower priority task is queued and the higher priority task is given the processors attention

For what would be the value of the pointers when Monkey and Snake are added, the first item removed and then Bear added to the following queue:
1 2 3 4 5
Badger Ferret Weasel
If it were a circular queue?

Answer :

1 2 3 4 5
Bear Ferret Weasel Monkey Snake
StartPtr = 2
EndPtr = 1

If it were a linear queue?

Answer :

This would break, as there is no more space left in the queue!

Describe the functioning of a queue:

Answer :

A first in first out data type

Name two uses for a queue in a computer:

Answer :

  • keyboard buffer
  • printer queue