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{{Short description|Lexical structure of source code grouped together}}
{{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.
Blocks have two functions: to group statements so that they can be treated as one statement, and to define [[scope (computer science)|scopes]] for [[name binding|names]] to distinguish them from the same name used elsewhere. In a block-structured programming language, the objects named in outer blocks are visible inside inner blocks, unless they are [[Name masking|masked]] by an [[Object (computer science)|object]] declared with the same name.
==History==
Ideas of block structure were developed in the 1950s during the development of the first [[autocode]]s, and were formalized in the [[Algol 58]] and [[Algol 60]] reports. Algol 58 introduced the notion of the "compound statement", which was related solely to [[control flow]].<ref name="algol58_report">{{cite journal
|last1= Perlis |first1 = A. J. |author-link1=Alan Perlis
|last2= Samelson |first2= K.
| date = 1958
| title = Preliminary report: international algebraic language
| journal = Communications of the ACM
| doi = 10.1145/377924.594925
| publisher =ACM
| ___location = New York, NY, USA
| volume = 1
| 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. Several broad families are:
* 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>
==Limitations==
Some languages which support blocks with declarations do not fully support all declarations; for instance many C-derived languages do not permit a function definition within a block ([[nested function]]s). And unlike its ancestor Algol, Pascal does not support the use of blocks with their own declarations inside the begin and end of an existing block, only compound statements enabling sequences of statements to be grouped together in '''if''', '''while''', '''repeat''' and other control statements.
==Basic semantics==
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="fortranfixed">
C LANGUAGE: ANSI STANDARD FORTRAN 66
C INITIALIZE VALUES TO BE CALCULATED
PAYSTX = .FALSE.
PAYSST = .FALSE.
TAX = 0.0
SUPTAX = 0.0
C SKIP TAX DEDUCTION IF EMPLOYEE EARNS LESS THAN TAX THRESHOLD
IF (WAGES .LE. TAXTHR) GOTO 100
PAYSTX = .TRUE.
TAX = (WAGES - TAXTHR) * BASCRT
C SKIP SUPERTAX DEDUCTION IF EMPLOYEE EARNS LESS THAN SUPERTAX THRESHOLD
IF (WAGES .LE. SUPTHR) GOTO 100
PAYSST = .TRUE.
SUPTAX = (WAGES - SUPTHR) * SUPRAT
100 TAXED = WAGES - TAX - SUPTAX
</syntaxhighlight>
The logical structure of the program is not reflected in the language, and analyzing when a given statement is executed can be difficult.
Blocks allow the programmer to treat a group of statements as a unit, and the default values which had to appear in initialization in this style of programming can, with a block structure, be placed closer to the decision:
<syntaxhighlight lang="pascal">
{ Language: Jensen and Wirth Pascal }
if wages > tax_threshold then
begin
paystax := true;
tax := (wages - tax_threshold) * tax_rate
{ The block structure makes it easier to see how the code could
be refactored for clarity, and also makes it easier to do,
because the structure of the inner conditional can easily be moved
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
end
else begin
pays_supertax := false;
supertax := 0
end
end
else begin
paystax := false; pays_supertax := false;
tax := 0; supertax := 0
end;
taxed := wages - tax - supertax;
</syntaxhighlight>
Use of blocks in the above fragment of [[Pascal (programming language)|Pascal]] clarifies the programmer's intent, and enables combining the resulting blocks into a nested hierarchy of [[Conditional (computer programming)|conditional]] statements. The structure of the code reflects the programmer's thinking more closely, making it easier to understand and modify.
The above source code can be made even clearer by taking the inner if statement out of the outer one altogether, placing the two blocks one after the other to be executed consecutively. Semantically there is little difference in this case, and the use of block structure, supported by indenting for readability, makes it easy for the programmer to refactor the code.
In primitive languages, variables had broad scope. For instance, an integer variable called IEMPNO might be used in one part of a Fortran subroutine to denote an employee social security number (ssn), but during maintenance work on the same subroutine, a programmer might accidentally use the same variable, IEMPNO, for a different purpose, and this could result in a bug that was difficult to trace. Block structure makes it easier for programmers to control scope to a minute level.
<syntaxhighlight lang="scheme">
;; Language: R5RS Standard Scheme
(let ((empno (ssn-of employee-name)))
(while (is-manager empno)
(let ((employees (length (underlings-of empno))))
(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,
;; referring to the manager's ssn, is shadowed.
(printf "Name: ~a, role: ~a~%"
(name-of empno)
(role-of empno)))
(underlings-of empno)))))
</syntaxhighlight>
In the above [[Scheme (programming language)|Scheme]] fragment, empno is used to identify both the manager and their underlings each by their respective ssn, but because the underling ssn is declared within an inner block it does not interact with the variable of the same name that contains the manager's ssn. In practice, considerations of clarity would probably lead the programmer to choose distinct variable names, but they have the choice and it is more difficult to introduce a bug inadvertently.
==Hoisting==
In some languages, a variable can be declared at function scope even within enclosed blocks. For example, in JavaScript, variables declared with <code>var</code> have function scope.
==See also==
{{Portal|Computer programming}}
* [[Basic block]]
* [[Block scope]]
* [[Closure (computer programming)]]
* [[Control flow]]
==References==
{{Reflist}}
{{Programming paradigms navbox}}
{{DEFAULTSORT:Statement Block}}
[[Category:Programming constructs]]
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