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Block (programming)

January 20, 2023

Not to be confused with Block programming.
"Code block" redirects here. For the IDE, see Code::Blocks. For block-based programming, see Visual programming language.

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 declarations and 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 structures are formed from blocks.

Blocks have two functions: to group statements so that they can be treated as one statement, and to define scopes for 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 masked by an object declared with the same name.

History

Ideas of block structure were developed in the 1950s during the development of the first autocodes, 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.[1] 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."[2]

Syntax

Blocks use different syntax in different languages. Two broad families are:

  • the ALGOL family in which blocks are delimited by the keywords "begin" and "end" or equivalent. In C, blocks are delimited by curly braces - "{" and "}". ALGOL 68 uses parentheses.
  • Parentheses - "(" and ")", are used in the MS-DOS batch language
  • indentation, as in Python
  • s-expressions with a syntactic keyword such as prog or let (as in the Lisp family)
  • In 1968 (with ALGOL 68), then in Edsger W. Dijkstra's 1974 Guarded Command Language the conditional and iterative code block are alternatively terminated with the block reserved word reversed: e.g. if ~ then ~ elif ~ else ~ fi, case ~ in ~ out ~ esac and for ~ while ~ do ~ od

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 functions). 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. Conditionals were implemented using conditional goto statements: 

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

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: 

 { 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;

Use of blocks in the above fragment of Pascal clarifies the programmer's intent, and enables combining the resulting blocks into a nested hierarchy of 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. 

;; 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)))))

In the above 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 var have function scope.

See also

References

  1. ^ Perlis, A. J.; Samelson, K. (1958). "Preliminary report: international algebraic language". Communications of the ACM. New York, NY, USA: ACM. 1 (12): 8–22. doi:10.1145/377924.594925. S2CID 28755282.
  2. ^ Backus, J. W.; Bauer, F. L.; Green, J.; Katz, C.; McCarthy, J.; Perlis, A. J.; Rutishauser, H.; Samelson, K.; Vauquois, B.; Wegstein, J. H.; van Wijngaarden, A.; Woodger, M. (May 1960). Naur, Peter (ed.). "Report on the Algorithmic Language ALGOL 60". Communications of the ACM. New York, NY, USA: ACM. 3 (5): 299–314. doi:10.1145/367236.367262. ISSN 0001-0782. S2CID 278290. Retrieved 2009-10-27.

January 20, 2023
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Not to be confused with Block programming Code block redirects here For the IDE see Code Blocks For block based programming see Visual programming language This article needs additional citations for verification Please help improve this article by adding citations to reliable sources Unsourced material may be challenged and removed Find sources Block programming news newspapers books scholar JSTOR May 2010 Learn how and when to remove this template message 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 declarations and 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 structures are formed from blocks Blocks have two functions to group statements so that they can be treated as one statement and to define scopes for 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 masked by an object declared with the same name Contents 1 History 2 Syntax 3 Limitations 4 Basic semantics 5 Hoisting 6 See also 7 ReferencesHistory EditIdeas of block structure were developed in the 1950s during the development of the first autocodes 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 1 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 2 Syntax EditBlocks use different syntax in different languages Two broad families are the ALGOL family in which blocks are delimited by the keywords begin and end or equivalent In C blocks are delimited by curly braces and ALGOL 68 uses parentheses Parentheses and are used in the MS DOS batch language indentation as in Python s expressions with a syntactic keyword such as prog or let as in the Lisp family In 1968 with ALGOL 68 then in Edsger W Dijkstra s 1974 Guarded Command Language the conditional and iterative code block are alternatively terminated with the block reserved word reversed e g u b if b u u then u u elif u u else u u b fi b u u b case b u u in u u out u u b esac b u and u for u u while u u b do b u u b od b u Limitations EditSome 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 functions 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 EditThe 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 Conditionals were implemented using conditional goto statements 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 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 Language Jensen and Wirth Pascal if wages gt 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 gt 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 Use of blocks in the above fragment of Pascal clarifies the programmer s intent and enables combining the resulting blocks into a nested hierarchy of 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 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 In the above 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 EditIn some languages a variable can be declared at function scope even within enclosed blocks For example in JavaScript variables declared with var have function scope See also Edit Computer programming portalBasic block Block scope Closure computer programming Control flowReferences Edit Perlis A J Samelson K 1958 Preliminary report international algebraic language Communications of the ACM New York NY USA ACM 1 12 8 22 doi 10 1145 377924 594925 S2CID 28755282 Backus J W Bauer F L Green J Katz C McCarthy J Perlis A J Rutishauser H Samelson K Vauquois B Wegstein J H van Wijngaarden A Woodger M May 1960 Naur Peter ed Report on the Algorithmic Language ALGOL 60 Communications of the ACM New York NY USA ACM 3 5 299 314 doi 10 1145 367236 367262 ISSN 0001 0782 S2CID 278290 Retrieved 2009 10 27 Retrieved from https en wikipedia org w index php title Block programming amp oldid 1116872562, wikipedia, wiki, book, books, library,

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