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Loop-invariant code motion is a compiler optimization, not part of the language semantics; define hoisting more clearly |
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{{Short description|Lexical structure of source code grouped together}}
{{redirect|Code block|the IDE|Code::Blocks}}▼
{{distinguish|Block programming}}
▲{{redirect|Code block|the IDE|Code::Blocks|block-based programming|Visual programming language}}
{{refimprove|date=May 2010}}
In [[computer programming]], a '''block''' or '''code block''' or '''block of code''' is a lexical structure of [[source code]] which is grouped together. Blocks consist of one or more [[Declaration (computer programming)|declarations]] and [[Statement (computer science)|statements]]. A [[programming language]] that permits the creation of blocks, including blocks [[Nesting (computing)|nested]] within other blocks, is called a '''block-structured programming language'''. Blocks are fundamental to [[structured programming]], where [[control structure]]s are formed from blocks.▼
{{Programming paradigms}}▼
▲In [[computer programming]], a '''block''' or '''code block''' or '''block of code''' is a lexical structure of [[source code]] which is grouped together. Blocks consist of one or more [[Declaration (computer programming)|declarations]] and [[Statement (computer science)|statements]]. A programming language that permits the creation of blocks, including blocks nested within other blocks, is called a '''block-structured programming language'''. Blocks are fundamental to [[structured programming]], where [[control structure]]s are formed from blocks.
Blocks have two functions: to group statements so that they can be treated as one statement
==History==
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| issue = 12
| pages = 8–22
|s2cid = 28755282 | doi-access = free
}}</ref> The subsequent ''Revised Report'' which described the syntax and semantics of Algol 60 introduced the notion of a block and [[block scope]], with a block consisting of " A sequence of declarations followed by a sequence of statements and enclosed between begin and end..." in which "[e]very declaration appears in a block in this way and is valid only for that block."<ref name="algol60_report">{{cite journal |last1=Backus |first1=J. W. |author-link1=John Backus |last2=Bauer |first2=F. L. |author-link2=Friedrich L. Bauer |last3=Green |first3=J. |last4=Katz |first4=C. |last5=McCarthy |first5=J. |last6=Perlis |first6=A. J. |last7=Rutishauser |first7=H. |author-link7=Heinz Rutishauser |last8=Samelson |first8=K. |last9=Vauquois |first9=B. |author-link9=Bernard Vauquois |last10=Wegstein |first10=J. H. |last11=van Wijngaarden |first11=A. |last12=Woodger |first12=M. |date=May 1960 |editor1-last=Naur |editor1-first=Peter |title=Report on the Algorithmic Language ALGOL 60 |url=http://www.masswerk.at/algol60/report.htm |journal=Communications of the ACM |___location=New York, NY, USA |publisher=ACM |volume=3 |issue=5 |pages=299–314 |doi=10.1145/367236.367262 |issn=0001-0782 |s2cid=278290 |access-date=2009-10-27 |doi-access=free |editor1-link=Peter Naur}}</ref>
==Syntax==
Blocks use different syntax in different languages.
* the [[ALGOL]] family in which blocks are delimited by the keywords "<code>begin</code>" and "<code>end</code>" or equivalent. In [[C (programming language)|C]], blocks are delimited by curly braces - "<code>{</code>" and "<code>}</code>". [[ALGOL 68]] uses parentheses.
* Parentheses - "<code>(</code>" and "<code>)</code>", are used in the MS-DOS [[batch language]]
* [[off-side rule|indentation]], as in [[Python (programming language)|Python]] and [[Haskell]]
* [[s-expression]]s with a syntactic keyword such as <code>prog</code> or <code>let</code> (as in the [[Lisp (programming language)|Lisp]] family)
* In 1968 (with [[ALGOL 68]]), then in [[Edsger W. Dijkstra]]'s 1974 [[Guarded Command Language#Selection: if|Guarded Command Language]] the conditional and iterative code block are alternatively terminated with the block reserved word ''reversed'': e.g. <code><u>'''if'''</u> ~ <u>then</u> ~ <u>elif</u> ~ <u>else</u> ~ <u>'''fi'''</u></code>, <code><u>'''case'''</u> ~ <u>in</u> ~ <u>out</u> ~ <u>'''esac'''</u></code> and <code><u>for</u> ~ <u>while</u> ~ <u>'''do'''</u> ~ <u>'''od'''</u></code>
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The semantic meaning of a block is twofold. Firstly, it provides the programmer with a way for creating arbitrarily large and complex structures that can be treated as units. Secondly, it enables the programmer to limit the scope of variables and sometimes other objects that have been declared.
In early languages such as [[Fortran IV]] and [[BASIC]], there were no statement blocks or control structures other than simple forms of loops. Conditionals were implemented using conditional [[goto]] statements:
<syntaxhighlight lang="
C LANGUAGE: ANSI STANDARD FORTRAN 66
C INITIALIZE VALUES TO BE CALCULATED
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out of the outer conditional altogether and the effects of doing
so are easily predicted. }
if wages > supertax_threshold then begin
pays_supertax := true;
supertax := (wages - supertax_threshold) * supertax_rate
else begin
pays_supertax := false;
supertax := 0
end▼
end
else begin
paystax := false; pays_supertax := false;
tax := 0; supertax := 0
taxed := wages - tax - supertax;
</syntaxhighlight>
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(printf "~a has ~a employees working under him:~%" employee-name employees)
(for-each
(lambda (empno)
;; Within this lambda expression the variable empno refers to the ssn
;; of an underling. The variable empno in the outer expression,
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</syntaxhighlight>
In the above [[Scheme (programming language)|Scheme]] fragment, empno is used to identify both the manager and
==Hoisting==
In some languages, a variable can be declared at function scope even within enclosed blocks. For example, in
==See also==
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==References==
{{Reflist}}
▲{{Programming paradigms navbox}}
{{DEFAULTSORT:Statement Block}}
[[Category:Programming constructs]]
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