1 / 24

Describing Syntax and Semantics Chapter 3

Describing Syntax and Semantics Chapter 3. Describing Syntax and Semantics. SYNTAX - the form of the expressions, statements and program units in a programming language. SEMANTICS - the meaning of those expressions, statements and program units. Describing Syntax.

casta
Download Presentation

Describing Syntax and Semantics Chapter 3

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Describing Syntaxand SemanticsChapter 3

  2. Describing Syntaxand Semantics SYNTAX - the form of the expressions, statements and program units in a programming language SEMANTICS - the meaning of those expressions, statements and program units

  3. Describing Syntax The strings of a language are called sentences or statements The lowest level syntactic units of a programming language are called lexemes. (identifiers, constants, operators, etc.) A token is a category of lexemes.

  4. Describing Syntax index = 2 * count + 17; LEXEME:TOKEN: index identifier = equal_sign 2 int_constant * mult_op count identifier + plus_op 17 int_constant ; semicolon

  5. Describing Syntax Languages may be formally defined by one of two methods: recognition or generation.

  6. Describing Syntax Given a language, L, that uses the alphabet , a recognizer will indicate if the given string of characters from is in the language L. A compiler is an example of a recognizer.

  7. Describing Syntax A generator is a device which can be used to generate sentences or statements of a language. Useful for humans as a guide for the generation of valid statements in the language. A grammar is an example of a generator.

  8. Describing Syntax Noam Chomsky, linguist, described four language generation devices, called grammars, that can be used to generate four different classes of languages. Regular grammars - used to describe tokens Context-free grammars - used to describe whole programming languages

  9. Describing Syntax John Backus, introduced a new formal notation for describing a programming language syntax. (Algol 58) The notation was modified by Peter Naur. Backus-Naur form (BNF)

  10. Describing Syntax Metalanguage - a language that can be used to describe another language BNF is a metalanguage for the description of programming languages.

  11. Describing Syntax BNF uses abstractions to denote various syntactic structures. <assign> is an abstraction which denotes a valid assignment statement

  12. LHS RHS Describing Syntax BNF uses rules or productions to describe valid programs, statements and expressions within the language. <assign> <var> := <expression> is a rule which describes a valid assignment statement in Pascal

  13. Describing Syntax GRAMMAR - a set of rules or productions which describe a language RULE or PRODUCTION- describes valid programs, statements and expressions within the language. NON-TERMINAL - an abstraction which can be expanded by the application of some rule TERMINAL - corresponds to lexemes or tokens of the language

  14. LHS RHS Describing Syntax BNF rule or production: <assign> <var> := <expression> The non-terminal on the LHS may be replaced with the string of terminals and non-terminals on the RHS

  15. Multiple definitions can be written in a single rule by separating different definitions with the OR symbol Describing Syntax BNF rule or production: <id> A | B | C

  16. Describing Syntax BNF rules or productions for lists: <id_list> <id> | <id>, <id_list> Recursion is used to describe rules for variable length lists. A rule is recursive if its LHS also appears in its RHS

  17. Describing Syntax Derivations: Sentences in a language may be generated through a sequence of applications of the rules, beginning with a start symbol. Parse Trees: The hierarchical syntactic structure of a sentence in the language can be described with a parse tree.

  18. Describing Syntax Example 3.1 <program> begin <stmt_list> end <stmt_list> <stmt> | <stmt>; <stmt_list> <stmt> <var> := <expression> <var> A | B | C <expression> <var> + <var> | <var> - <var> | <var>

  19. Grammar for Simple Assignment Statements Example 3.2 <assign> <id> := <expr> <id> A | B | C <expr> <id> + <expr> | <id> * <expr> | ( <expr> ) | <id>

  20. An Ambiguous Grammar for Simple Assignment Statements Example 3.3 <assign> <id> := <expr> <id> A | B | C <expr> <expr> + <expr> | <expr> * <expr> | ( <expr> ) | <id>

  21. More on RECURSION IN GRAMMAR RULES <expr> <id> + <expr> | <id> * <expr> | ( <expr> ) | <id> When a BNF rule has the non-terminal symbol of its left hand side appearing as the rightmost symbol on its right hand side, the rule is said to be right recursive.

  22. RECURSION IN GRAMMAR RULES A rule which is right recursive can be used to specify right associativity (meaning that operators of equal precedence are evaluated from right to left) Many programming languages specify that addition, subtraction, multiplication and division follow rules of left associativity. Those that contain that exponentiation operators, specify that right associativity be used for exponentiation.

  23. An Unambiguous Grammar with Operator Precedence Example 3.4 <assign> <id> := <expr> <id> A | B | C <expr> <expr> + <term> | <term> <term> <term> * <factor> | <factor> <factor> ( <expr> ) | <id>

  24. expr term + - expr Syntax Graphs

More Related