Editing Statement (computer science) (section)

==Syntax==

{{main|Syntax (programming languages)}}

Apart from assignments and subroutine calls, most languages start each statement with a special word (e.g. goto, if, while, etc.) as shown in the above examples.  Various methods have been used to describe the form of statements in different languages; the more formal methods tend to be more precise:

* Algol 60 used [[Backus–Naur form]] (BNF) which set a new level for language grammar specification.<ref name=ALGOL60RPT>{{cite web|url=http://www.masswerk.at/algol60/report.htm |at=Section "1.1" |title=Revised Report on the Algorithmic Language Algol 60 |first1=J.W. |last1=Backus |first2=F.L. |last2=Bauer |first3=J. |last3=Green |first4=C. |last4=Katz |first5=J. |last5=McCarthy |first6=P. |last6=Naur |first7=A.J. |last7=Perlis |first8=H. |last8=Rutishauser |first9=K. |last9=Samuelson |first10=B. |last10=Vauquois |first11=J.H. |last11=Wegstein |first12=A. |last12=van Wijngaarden |first13=M. |last13=Woodger |editor-first1= Peter |editor-last1=Naur |website=mass:werk  |access-date=January 23, 2021}}</ref>
* Up until Fortran 77, the language was described in English prose with examples,<ref name=FORTRAN66>{{cite web|url=https://wg5-fortran.org/ARCHIVE/Fortran66.pdf|title=FORTRAN |date=1966 |publisher=United States of America Standards Institute |via=WG5 Fortran Standards |access-date=February 19, 2021}}</ref>  From Fortran 90 onwards, the language was described using a variant of BNF.<ref name=FORTRAN95>{{cite web|url=https://j3-fortran.org/doc/year/04/04-007.pdf|title=Working draft J3/04-007 |date=May 10, 2004 |publisher=J3 Fortran |access-date=February 19, 2021}}</ref>
* Cobol used a two-dimensional metalanguage.<ref name=COBOL1959>{{cite web|url=https://public.support.unisys.com/2200/docs/CP18.0/PDF/78307709-002.pdf|title=ASCII COBOL Programming Reference Manual |date= June 2010 |publisher=unisys |access-date=January 23, 2021}}</ref>
* Pascal used both [[syntax diagram]]s and equivalent BNF.<ref name=PASCAL>{{cite web|url=http://prog.vub.ac.be/~tjdhondt/ESL/Pascal_files/PASCAL%20user%20manual%20and%20report.pdf|title=PASCAL User Manual and Report |first1=Kathleen |last1=Jensen |first2=Niklaus |last2=Wirth |editor-first1=G. |editor-last1=Goos |editor-first2=J. |editor-last2=Hartmanis |date=1974 |work=Lecture Notes in Computer Science |at=Appendix D |access-date=February 19, 2021}}</ref>
BNF uses recursion to express repetition, so various [[Extended Backus–Naur form|extensions]] have been proposed to allow direct indication of repetition.

===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.