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Concept
Block (programming)
Summary
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.
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. 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."
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.
Official source
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