Queue (data structure)

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Linear data structures
Array Deque Linked list Queue Stack

A queue (pronounced /kjuː/) is a particular kind of collection in which the entities in the collection are kept in order and the principal (or only) operations on the collection are the addition of entities to the rear terminal position and removal of entities from the front terminal position. This makes the queue a First-In-First-Out (FIFO) data structure. In a FIFO data structure, the first element added to the queue will be the first one to be removed. This is equivalent to the requirement that whenever an element is added, all elements that were added before have to be removed before the new element can be invoked. A queue is an example of a linear data structure.

Queues provide services in computer science, transport and operations research where various entities such as data, objects, persons, or events are stored and held to be processed later. In these contexts, the queue performs the function of a buffer.

Queues are common in computer programs, where they are implemented as data structures coupled with access routines, as an abstract data structure or in object-oriented languages as classes. Common implementations are Circular buffers and Linked lists.

1 Operations
2 Example C++ Program
3 Representing a Queue
4 Practical Usage
5 Additional Information
6 See also
7 References
8 External links

[edit] Operations

Common operations from the C++ Standard Template Library include the following:

bool empty(): Returns True if the queue is empty, and False otherwise.
T& front(): Returns a reference to the value at the front of a non-empty queue. There is also a constant version of this function, const T& front().
void pop(): Removes the item at the front of a non-empty queue.
void push(const T& foo): Inserts the argument foo at the back of the queue.
int size(): Returns the total number of elements in the queue.

[edit] Example C++ Program

Based on an example in Ford and Topp Data Structures with C++ page 387.

queue< char > theQueue; // creates a queue of chars named "theQueue"
theQueue.push('a');     // theQueue now contains "a"
theQueue.push('b');     // theQueue now contains "a b"
theQueue.push('c');     // theQueue now contains "a b c"
cout << "theQueue contains: a b c" << endl << endl;
while( !theQueue.empty() )      // while the queue is not empty...
{
    cout << "Size = " << theQueue.size() << endl;   // ...output queue size
    cout << "Value at front = " << theQueue.front() << endl << endl;     // ...and output the first element value
    theQueue.pop();         // ...and remove it
}

[edit] Representing a Queue

The defining attribute of a queue data structure is the fact that allows access to only the front and back of the structure. Furthermore, elements can only be removed from the front and can only be added to the back. In this way, an appropriate metaphor often used to represent queues is the idea of a checkout line (Ford/Topp p. 385). Other examples of queues are people traveling up an escalator, machine parts on an assembly line, or cars in line at a gas station. The recurring theme is clear: queues are essentially waiting lines.

In each of the cases, the customer or object at the front of the line was the first one to enter, while at the end of the line is the last to have entered. Every time a customer finishes paying for their items (or a person steps off the escalator, or the machine part is removed from the assembly line, etc.) that object leaves the queue from the front. This represents the queue “dequeue” function. Every time another object or customer enters the line to wait, they join the end of the line and represent the “enqueue” function. The queue “size” function would return the length of the line, and the “empty” function would return true only if there was nothing in the line.

[edit] Practical Usage

Queues can be very useful in many different situations. As seen in the grocery store metaphor, they are a data structure ordered as first-in-first-out (FIFO) or first-come-first-served (FCFS). This specific type of structure is used for applications that require items to be retrieved in their order of addition to the list (their order of occurrence). Examples of such applications could be an event scheduler (perhaps for a hair salon, or massage parlor), a simple to-do list, or specific operations of the radix sort algorithm (see below under the “See also” header for further information on Radix Sort).

[edit] Additional Information

Theoretically, one characteristic of a queue is that it does not have a specific capacity. Regardless of how many elements are already contained, a new element can always be added. It can also be empty, at which point removing an element will be impossible until a new element has been added again.

A practical implementation of a queue e.g. with pointers of course does have some capacity limit, that depends on the concrete situation it is used in. For a data structure the executing computer will eventually run out of memory, thus limiting the queue size. Queue overflow results from trying to add an element onto a full queue and queue underflow happens when trying to remove an element from an empty queue.

A bounded queue is a queue limited to a fixed number of items.

[edit] See also

Array
Circular buffer
Congestion
Deque
Differential execution
Linked list
Pipeline
Priority queue
Queueing theory
Radix sort
Spooling
Stack (data structure) - the "opposite" of a queue: LIFO (Last In First Out)

[edit] References

Donald Knuth. The Art of Computer Programming, Volume 1: Fundamental Algorithms, Third Edition. Addison-Wesley, 1997. ISBN 0-201-89683-4. Section 2.2.1: Stacks, Queues, and Deques, pp. 238–243.
Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Clifford Stein. Introduction to Algorithms, Second Edition. MIT Press and McGraw-Hill, 2001. ISBN 0-262-03293-7. Section 10.1: Stacks and queues, pp.200–204.
William Ford, William Topp. Data Structures with C++ and STL, Second Edition. Prentice Hall, 2002. ISBN 0-13-085850-1. Chapter 8: Queues and Priority Queues, pp.386–390.
Adam Drozdek. Data Structures and Algorithms in C++, Third Edition. Thomson Course Technology, 2005. ISBN 0-534-49182-0. Chapter 4: Stacks and Queues, pp.137–169.