Syntax analysis

Structure of Programming Languages

SYNTAX ANALYSISVSRivera

Syntax

“The arrangement of words as elements in a sentence to show their relationship” describes the sequence of symbols that make up valid programs.

Form of expressions, statements and program units.

The General Problem of Describing Syntax: Terminology

A sentence is a string of characters over some alphabet

A language is a set of sentences A lexeme is the lowest level syntactic

unit of a language (e.g., *, sum, begin)

A token is a category of lexemes (e.g., identifier)

Syntactic elements of the Language

Character set – ASCII, Unicode Identifiers –restrictions on length

reduces readability Operator symbols - + and –

represents two basic arithmetic operations.


Keywords and reserved words – is an identifier used as a fixed part of the syntax of a statement. It is a reserved word if it may not be used as a programmer-chosen identifier.

Noise words – optional words that are inserted in a statements to improved readability.


Comments – important part of the documentation. REM, /* */, or //

Blank (spaces) Delimiters – a syntactic element used

simply to mark the beginning or end of some syntactic unit such as a statement or expression. “begin”…”end”, or { }.


Expressions – functions that access data objects in a program and return some value.

Statements

Syntactic Analysis (parsing)

2nd stage in translation Determines if the program being

compiled is a valid sentence in the syntactic model programming language.

Role of the Parser Where lexical analysis splits the input

into tokens, the purpose of syntax analysis (also known as parsing) is to recombine these tokens to reflect the data structure of the text.

The parse must also reject invalid texts by reporting syntax errors, and recover from commonly occurring errors so that it can continue processing the remainder of its input.

Role of the Parser

Lexical Analyzer

Sourceprogram

Get next token

token

ParserRest of

front end

Parser

Parsetree

Intermediaterepresentation

Formal Methods of Describing Syntax

Grammars Parse Trees Syntax Diagrams

Grammars

Formal definition of the syntax of a programming language.

Collection of rules that define, mathematically, which strings of symbols are valid sentences.

Parts of Grammar

Set of tokens/terminal symbols symbols that are atomic / non-divisible can be combined to form valid constructs

in the language

Set of non-terminal symbols symbols used to represent intermediate

definitions within the language defined by productions syntactic classes or categories

Parts of Grammar

Set of rules called productions a definition of a non-terminal symbol has the form

x ::= ywhere x is a non-terminal symbol and y is a sequence of symbols (non-terminal or terminal)

Parts of Grammar LHS: abstraction being defined RHS: tokens, lexemes, references to

other abstractions

Goal symbol one of the set of non-terminal symbols also referred to as the start symbol

Rules to form Grammar

Every non-terminal symbol must appear to the left of the ::= at least one production

The goal symbol must not appear to the right of the ::= of any production

A rule is recursive if its LHS appears in its RHS

Context Free Grammar (CFG)

Backus-Naur Form (BNF) Grammar originally presented by John Backus (to

describe ALGOL 58)and later modified by Peter Naur

Composed of finite set of grammar rules which define a programming language.

Examples

<conditional stmt> ::= if <boolean expr> then

<stmt> else

<stmt>| if <boolean expr> then

<stmt>

Examples

<unsigned int> ::=<digit> | <unsigned int>

<digit> A rule is recursive if its LHS appears in

its RHS

Examples <program> ::= begin

<stmt_list>end

<stmt_list> ::=<stmt> | <stmt> <stmt_list>

<stmt> ::= <var> := <expression><var> ::= A | B | C<expression> ::= <var> + <var>

Grammar Derivation

BNF is a generative device for defining language.

The sentences of the language are generated through a sequence of applications of the rules, beginning with a special non-terminal (start symbol) of the grammar.

Example

<program> ::= begin <stmt_list> end

begin <stmt> endbegin <var> := <expression> endbegin <var> := <var> + <var> endbegin A := B + C end

Example

A := B * ( A + C) <assign> ::= <id> := <expr>

:= A := <expr>:= A := <id> * <expr>:= A := B * <expr>:= A := B * (<expr>):= A := B * ( A + <expr>):= A := B * ( A + <id>):= A := B * ( A + C)

When does derivation stop?

By exhaustingly choosing all combinations of choices, the entire language can generate.

Exercise

BNF of signed integer? begin A := B + C; B := C; end

Extended BNF (EBNF)

Enhance the descriptive power of BNF Increases the readability and

writability of BNF

Extended BNF (EBNF)

Notational Extensions An optional element may be indicated by

enclosing the element in square brackets,[ … ].

A choice of alternative may use the symbol | within the single rule, optionally enclosed by parenthesis ( [ , ] ) if needed.

An arbitrary sequence of instances of element may be indicated by enclosing the element in braces followed by an asterisk, { … }+.

Example

BNF <expr> ::= <expr> + <term>

| <expr> - <term>| <term>

<term> ::= <term> * <factor>| <term> / <factor>| <factor>

Example

EBNF <expr> ::= <term> { (+|-) <term> }

<term> ::= <factor> { (*|/) <factor>}

Example

BNF <program> ::= begin

<stmt_list>

end

Example

EBNF <program> ::= begin

<stmt> {<stmt>}

end <program> ::= begin

{<stmt>}+

end

Example

BNF <signed int> ::= + <int> | - <int> <int> ::= <digit> | <int> <digit>

EBNF <signed int> ::= [+|-] <digit>

{<digit>}+

Exercise

EBNF of identifier?

Solution

EBNF of identifier <identifier> ::= <letter> {<letter> |

<digit> }+

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Syntax analysis

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