Editing Reserved word (section)

{{short description|Word in a programming language that cannot be used as an identifier}}

In a [[programming language]], a '''reserved word''' (sometimes known as a '''reserved identifier''') is a word that cannot be used by a [[programmer]] as an [[Identifier (computer languages)|identifier]], such as the name of a variable, function, or [[Label (programming language)|label]] – it is "reserved from use". In brief, an ''identifier'' starts with a letter, which is followed by any sequence of letters and digits (in some languages, the [[underscore]] '_' is treated as a letter).

In an [[Imperative programming|imperative programming language]] and in many [[Object-oriented programming|object-oriented programming languages]], apart from assignments and subroutine calls, keywords are often used to identify a particular [[Statement (computer science)|statement]], e.g. '''if''', '''while''', '''do''', '''for''', etc.  Many languages treat keywords as reserved words, including [[Ada (programming language)|Ada]], [[C (programming language)|C]], [[C++]], [[COBOL]], [[Java (programming language)|Java]], and [[Pascal (programming language)|Pascal]].  The number of reserved words varies widely from one language to another: C has about 30 while COBOL has about 400.

A few languages do not have any reserved words; [[Fortran]] and [[PL/I]] identify keywords by context, while [[Algol 60]] and [[Algol 68]] generally use [[Stropping (syntax)|stropping]] to distinguish keywords from programmer-defined identifiers, e.g. <code>.if</code> or <code>'if</code> or <code>'if'</code> or <code><u>if</u></code>is a keyword distinct from identifier <code>if</code>.

Most programming languages have a standard library (or libraries), e.g. mathematical functions sin, cos, etc. The names provided by a library are not reserved, and can be redefined by a programmer if the library functionality is not required.