Block (programming)

In computer programming, a block or code block 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.

The function of blocks in programming is to enable groups of statements to be treated as if they were one statement, and to narrow the lexical scope of variables, procedures and functions declared in a block so that they do not conflict with variables having the same name used elsewhere in a program for different purposes. In a block-structured programming language, the names of variables and other objects such as procedures which are declared in outer blocks are visible inside other inner blocks, unless they are shadowed by an object of the same name.

Ideas of block structure were developed in the 1950s during the development of the first , 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:

Some other techniques used are as follows :

Some languages which support blocks with variable 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.

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