Editing Statement (computer science) (section)

===Statements and keywords===

Some programming language grammars [[reserved word|reserve keywords]] or [[Stropping (syntax)|mark them specially]], and do not allow them to be used as [[Identifier (computer languages)|identifiers]]. This often leads to [[Formal grammar|grammars]] which are easier to [[parsing|parse]], requiring less [[Parsing#Lookahead|lookahead]].

====No distinguished keywords====

Fortran and PL/1 do not have reserved keywords, allowing statements like:

* in PL/1:
**<code>IF IF = THEN THEN ...</code>    (the second <code>IF</code> and the first <code>THEN</code> are variables).
* in Fortran:
**<code>IF (A) X = 10...           </code>   conditional statement (with other variants)
**<code>IF (A) = 2           </code>    assignment to a subscripted variable named <code>IF</code>
::As spaces were optional up to Fortran 95, a typo could completely change the meaning of a statement:
:*<code>DO 10 I = 1,5        </code>    start of a loop with I running from 1 to 5
:*<code>DO 10 I = 1.5        </code>    assignment of the value 1.5 to the variable <code>DO10I</code>

====Flagged words====
{{main|Stropping (syntax)}}

In Algol 60 and Algol 68, special tokens were distinguished explicitly: for publication, in boldface e.g. <code>'''begin'''</code>; for programming, with some special marking, e.g., a flag (<code>'begin</code>), quotation marks (<code>'begin'</code>), or underlined (<code><u>begin</u></code> on the [[Elliott 503]]). This is called "stropping".

Tokens that are part of the language syntax thus do not conflict with programmer-defined names.

====Reserved keywords====
{{main|Reserved word}}
Certain names are reserved as part of the programming language and can not be used as programmer-defined names.
The majority of the most popular programming languages use reserved keywords. Early examples include [[FLOW-MATIC]] (1953) and [[COBOL]] (1959). Since 1970 other examples include Ada, C, C++, Java, and Pascal.  The number of reserved words depends on the language: C has about 30 while COBOL has about 400.