Syntax and semantics{week 03}

The College of Saint RoseCIS 433 – Programming LanguagesDavid Goldschmidt, Ph.D.

from Concepts of Programming Languages, 9th edition by Robert W. Sebesta, Addison-Wesley, 2010, ISBN 0-13-607347-6

Syntax

Syntax is the expected form or structure of the expressions, statements, and program units of a programming language

Syntax of a Java while statement: while ( <boolean_expr> ) <statement>

Partial syntax of an if statement: if ( <boolean_expr> ) <statement>

Semantics

Semantics is the meaning of the expressions, statements, and program units of a given programming language

Semantics of a Java while statement while ( <boolean_expr> ) <statement> Execute <statement> zero or more times

as long as <boolean_expr> evaluates to true

Syntax and semantics

Together, syntax and semantics definea programming language

Syntax errors are detectedand reported by a compiler

Errors related to semantics are defects in program logic that cause incorrect resultsor program crashes

Defining syntax

Terminology to describe syntax: A sentence is a string of characters over an

alphabet of symbols A language is a set of sentences A lexeme is the lowest-level syntactic unit

of a language (e.g. +, *, sum, while)▪ One step above individual characters

A token is a set of lexemes▪ e.g. identifier, equal_sign, integer_literal, etc.

Language recognizers

A language recognizer reads an input string and determines whether it belongs to the given language

This is the syntax analysis partof a compiler or interpreter

input strings(source code)

languagerecognizer

accept or rejecteach input string

Language generators

A language generator produces syntactically acceptable strings of a given language

Not practical to generate all valid strings Instead, inspect generator rules (the

grammar) to determine if a sentence is acceptable for a given language

languagegenerator valid strings

of the language

Noam Chomsky

In the mid-1950s, linguist Noam Chomsky (born 1928) developed four classesof generative grammars Context-free grammars (CFGs) are

useful for describing programminglanguage syntax

Regular grammars are useful for describingvalid tokens of a programming language

Backus-Naur Form (BNF)

In 1960, John Backus and Peter Naur developed a formal notationfor specifying programminglanguage syntax Backus-Naur Form (BNF) is nearly

identical to Chomsky’s context-free grammars

Syntax of an assignment statement in BNF:▪ <assign> <var> = <expression> ;

BNF structure

Syntax of an assignment statement in BNF: BNF rule or production defining <assign>:

<assign> <var> = <expression> ; abstraction

being defineddefinition

of <assign> The definition consists of other abstractions, as well as lexemes and tokens

Example language

<program> begin <stmts> end<stmts> <stmt> | <stmt> ; <stmts><stmt> <var> = <expr><var> a | b | c | d | e<expr> <term> + <term> | <term> - <term><term> <var> | literal-integer-value

a vertical barindicates an OR

a token, which is simplya grouping of lexemes

Write a sentence that conforms to this grammar

Derivations

A derivation is a repeated application of rules Start with a start symbol and end with a

sentence <program> => begin <stmts> end => begin <stmt> end => begin <var> = <expr> end => begin b = <expr> end => begin b = <term> + <term> end => begin b = <var> + <term> end => begin b = c + <term> end => begin b = c + 123 end

Many possible (often infinite) derivations

<program> begin <stmts> end<stmts> <stmt> | <stmt> ; <stmts><stmt> <var> = <expr><var> a | b | c | d | e<expr> <term> + <term> | <term> - <term><term> <var> | literal-integer-value

Leftmost and rightmost derivations

A leftmost derivation is one in which the leftmost abstraction is always the next one expanded

Write both a leftmost and rightmost derivation to obtain this sentence: begin d = 10 - a end

Why is the leftmost derivation important?

<program> begin <stmts> end<stmts> <stmt> | <stmt> ; <stmts><stmt> <var> = <expr><var> a | b | c | d | e<expr> <term> + <term> | <term> - <term><term> <var> | literal-integer-value

Working with grammars

Given this simple grammar:

Which of the following sentences aregenerated by this grammar?▪ baaabbccc▪ abc▪ bbaabbaabbaabbaac▪ aabbbbccccccccccccccccccccc

<S> <A> <B> <C><A> a <A> | a<B> b <B> | b<C> c <C> | c

What next? (i)

Write BNF for the following constructs from your favorite programming language: Assignment statement▪ Include operators +, -, *, /, %, ++, --

Complete while and if statements Class header for Java/C++/C# etc.

What next? (ii)

Given this grammar:

Show both leftmost and rightmost derivations for the following sentences:▪ A = A * ( B + ( C * A ) ) ▪ B = B * ( (D) + C ) ▪ C = A + B + C * D + A

<assign> <var> = <expr> <var> A | B | C | D <expr> <var> + <expr> | <var> * <expr> | ( <expr> ) | <var>

What next? (iii)

Use BNF to write a grammarfor reverse Polish notation Use <expr> as your start symbol

Valid sentences include:▪ 5 8 19 + * ▪ 2 3 + 5 7 * / ▪ 2 3 + 5 7 * / 3 4 + * 1 – ▪ 9 9 * 8 7 * * 5 5 * * 4 -

What next? (iv)

Read and study Chapter 3

Do Exercises at the end of Chapter 3