Collection (computing)
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In computer science, a collection is a grouping of some variable number of data items (possibly zero) that have some shared significance to the problem being solved and need to be operated upon together in some controlled fashion. Generally, the data items will be of the same type or, in languages supporting inheritance, derived from some common ancestor type. A table (or array) is usually not considered a collection because it holds a fixed number of items, although tables/arrays commonly play a role in the implementation of collections.
Some different kinds of collections are lists, sets, bags (or multisets), trees and graphs.
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[edit] Lists
In a list, the order of data items is significant. Duplicate data items are permitted. Examples of operations on lists are searching for an item in the list and determining its location (if it is present) removing an item from the list, adding an item at a specific location, etc. If the principal operations on the list are to be the addition of items at one end and the removal of items at the other, it will generally be called a queue or FIFO. If the principal operations are the addition and removal of items at just one end, it will be called a stack or LIFO. In both cases, items are maintained within the collection in the same order (unless they are removed and re-inserted somewhere else) and so these are special cases of the list collection. Other specialized operations on lists include sorting, where, again, the order of items is of great importance.
[edit] Sets
In a set, the order of data items is of no consequence, but duplicate items are not permitted. Examples of operations on sets are the addition and removal of items and searching for an item in the set. Items held in a set are frequently compound, i.e. they contain structured data rather than having a single value. One value may be a 'key', used to find the item in the set. Having found the item from the key, other data associated with the item may be retrieved. A set used for this purpose will generally be called a lookup table and may be implemented by a hash table.
[edit] Bags
In a bag, the order of data items is of no consequence plus duplicate items are permitted. Examples of operations on bags are the addition and removal of items and determining how many of a particular item are present in the bag. Bags can be transformed into lists by the action of sorting.
[edit] Trees
In a tree, a 'root' data item has associated with it some number of data items which in turn have associated with them some number of other items in what is frequently viewed as parent-child relationships. Every item (other than the root) has a single parent (the root has no parent) and some number of children, possibly zero. Examples of operations on trees are the addition of data items so as to maintain a specific property of the tree to perform sorting, etc. and traversals to visit data items in a specific sequence. A tree used for sorting operations is usually called a heap. Tree collections are also used to store data that is presented in a tree-like manner, such as menu systems and files in directories on a data storage system.
[edit] Graphs
In a graph, data items have associations with one or more other data items in the collection and are somewhat like trees without the concept of a root or the parent-child relationship so that all data items are peers. Examples of operations on trees are traversals and searches which explore the associations of data items looking for some specific property. Graphs are frequently used to model real-world situations and to solve related problems. An example is the Spanning tree protocol, which creates a graph (or mesh) representation of a data network and figures out which associations between switching nodes need to be broken to turn it into a tree and thus prevent data going around in loops.
[edit] Abstract Concept vs. Implementation
As described here, a collection and the various kinds of collections are abstract concepts. There exists in the literature considerable confusion between the abstract concepts of computer science and their specific implementations in various languages or kinds of languages. Assertions that collections, lists, sets, trees, etc. are data structures, abstract data types or classes must be read with this in mind. Collections are first and foremost abstractions that are useful in formulating solutions to computing problems. Viewed in this light, they retain important links to underlying mathematical concepts which can be lost when the focus is on the implementation.
