Slide 1

Chapter 2-b

• Syntax, Semantics

Slide 2

Syntax, Semantics - Definition

• The syntax of a programming language is the form of its expressions, statements and programming units.

• The semantics is the meaning of these expressions, statements and programming units.

• A grammar is a formal set of rules that describes a valid syntax of a language.

Slide 3

Syntax, Semantics - Examples

• Syntax of Date: DD/DD/DDDD

where D represents a digit.

The semantics describes which parts stand for the date, month and year.

• Syntax of “if” statement:

if ( <logic_expr> ) <stmt>

Semantics: <stmt> will be executed only if <logic_expr> evaluates to “true”

Slide 4

Lexemes

• Lexemes are the lowest level syntactic units.

Example:

val = (int)(xdot + y*0.3) ;

In the above statement, the lexemes are

val, = , ( , int, ), (, xdot, +, y, * , 0.3, ), ;

Slide 5

Tokens

The category of lexemes are tokens.

• Identifiers: Names chosen by the programmer. Eg. val, xdot, y

• Keywords: Names chosen by the language designer to help syntax and structure. Eg. int, return, void. (Keywords that cannot be used as identifiers are known as reserved words )

Slide 6

Tokens (Contd.)

• Operators: Identify actions. Eg. +, &&, !

• Literals: Denote values directly. Eg. 3.14, -10, ‘a’, true, null

• Punctuation Symbols: Supports syntactic structure. Eg. (, ), ;, {, }

Slide 7

Backus Naur Form (BNF)

Useful for describing the syntax of progr. languages. Eg. Pascal “if”:

Terminals

<if_stmt> if <logic_expr> then <stmt> Production

LHS Non-terminals

Non-terminals are abstractions for syntactic structures.Terminals are lexemes or tokens.

Slide 8

Logical OR in BNF

Logical OR in BNF is denoted by |

Eg.<digit> 0|1|2|3|4|5|6|7|8|9

<if_stmt> if <logic_expr> then <stmt>

| if <logic_expr> then <stmt> else <stmt>

<sign> + |

Slide 9

Recursive rules in BNF

A BNF rule is recursive if LHS appears on RHS.

Eg:<ident_list> <identifier>

| <identifier> , <ident_list>

<integer> <digit>

| <digit> <integer>

Slide 10

Extended BNF

• [ ] Optional element:

<if_stmt> if <logic_expr> then <stmt> [ else <stmt>]

<real_num> [<int_num>] . <int_num>

• { } Unspecified number of repititions: Repeated infefinitely or left out altogether.

<ident_list> <identifier> { , <identifier> }

Slide 11

EBNF (Contd.)

• ( …| …) Multiple choice options. A single element must be chosen from a group.

“for” loop in Pascal:

<for_stmt>for <var> := <expr> (to|downto) <expr> do <stmt>

EBNF enhances the readability and writability of

BNF.

Slide 12

Syntax Graphs

BNF Syntax Graph

• LHS <if_stmt> if_stmt

• Non-terminal <stmt>

• Terminal if

stmt

if

Slide 13

Syntax Graph - Example

BNF:

<if_stmt> if <logic_expr> then <stmt>

Syntax Graph:

if_stmt if logic_expr then stmt

Slide 14

Syntax Graph Constructs

• Alternatives:

<stmt>|<funct>

• Optional

[<expr>]

stmt

funct

expr

Slide 15

Syntax Graph Constructs (Contd.)

• Unspecified repetitions:

{<digit>}

• Repetition with minimum one occurrence

<digit>{<digit>}

digit

digit

Slide 16

A simple grammar

<assign><ident>=<expr>

<ident> A|B|C

<expr> <ident>+<expr>

| <ident>*<expr>

| ( <expr> )

| <ident>

Slide 17

Sentences

A sentence is got by replacing the non terminals by strings of symbols according to the rules in the grammar.

Egs. (Based on the grammar on previous slide)

A = B*(A+C)

C = A+B*A

B = A

Slide 18

Parse Trees

Parse trees describe the hierarchical structure of sentences. It has the following properties:

• The root is labeled by LHS.

• Every non-leaf node (internal node) is a non-terminal.

• Each leaf is labeled with a terminal.

Slide 19

Parse tree for A=B*C

<assign>

<ident> <expr>=

<ident> <expr> * A

B

C

<ident>

Slide 20

Ambiguous Grammar

A grammar that generates a sentence which has two or more distinct parse trees is said to be an ambiguous grammar.

Eg. If we rewrite the grammar on slide 15 as below,<assign><ident>=<expr>

<ident> A|B|C

<expr> <expr>+<expr>

| <expr>*<expr>

| ( <expr> )

| <ident>

then the sentence A = B*C+A would have two distinct parse trees, and therefore the above grammar is ambiguous.

Slide 21

Derivation

Derivation is a mechanism by which the rules of a grammar can be repeatedly applied to generate a sentence.

At each stage, a non-terminal is replaced by the right-hand side of a rule, till finally the whole sentence is generated.

Slide 22

Derivation - Example.

<assign> <ident> = <expr>

A = <expr>

A = <ident> * <expr>

A = B * <expr>

A = B * <ident>

A = B * C