String (computer science): Difference between revisions

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[[File:String example.png|alt=Diagram of String data in computing. Shows the word "example" with each letter in a separate box. The word "String" is above, referring to the entire sentence. The label "Character" is below and points to an individual box.|thumb|Strings are typically made up of [[character (computing)|characters]], and are often used to store human-readable data, such as words or sentences.]]

In [[computer programming]], a '''string''' is traditionally a [[sequence]] of [[character (computing)|characters]], either as a [[literal (computer programming)|literal constant]] or as some kind of [[Variable (computer science)|variable]]. The latter may allow its elements to be [[Immutable object|mutated]] and the length changed, or it may be fixed (after creation). A string is generally considered as a [[data type]] and is often implemented as an [[array data structure]] of [[byte]]s (or [[word (computer architecture)|word]]s) that stores a sequence of elements, typically characters, using some [[character encoding]]. More general, ''Stringstring'' may also denote more general [[Array data type|arrays]] or othera sequence (or [[List (abstract data type)|list]]) of data typesother andthan structuresjust characters.

Depending on the programming language and precise data type used, a [[variable (programming)|variable]] declared to be a string may either cause storage in memory to be statically allocated for a predetermined maximum length or employ [[dynamic allocation]] to allow it to hold a variable number of elements.

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=== Concatenation and substrings ===

''[[Concatenation]]'' is an important [[binary operation]] on Σ^*. For any two strings ''s'' and ''t'' in Σ^*, their concatenation is defined as the sequence of symbols in ''s'' followed by the sequence of characters in ''t'', and is denoted ''st''. For example, if Σ = {a, b, ..., z}, ''s'' = {{code|footbear}}, and ''t'' = {{code|ballhug}}, then ''st'' = {{code|footballbearhug}} and ''ts'' = {{code|ballfoothugbear}}.

String concatenation is an [[associative]], but non-[[commutative]] operation. The empty string ε serves as the [[identity element]]; for any string ''s'', ε''s'' = ''s''ε = ''s''. Therefore, the set Σ^* and the concatenation operation form a [[monoid]], the [[free monoid]] generated by Σ. In addition, the length function defines a [[monoid homomorphism]] from Σ^* to the non-negative integers (that is, a function <math>L: \Sigma^{*} \mapsto \mathbb{N} \cup \{0\}</math>, such that <math>L(st)=L(s)+L(t)\quad \forall s,t\in\Sigma^*</math>).