Chapter 2 b Syntax Semantics 1 Syntax Semantics

Chapter 2 -b • Syntax, Semantics 1

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. 2

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” 3

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, ), ; *, 4

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 ) 5

Tokens (Contd. ) • Operators: Identify actions. Eg. +, &&, ! • Literals: Denote values directly. Eg. 3. 14, -10, ‘a’, true, null • Punctuation Symbols: Supports syntactic structure. Eg. (, ), ; , {, } 6

Backus Naur Form (BNF) Useful for describing the syntax of progr. languages. Eg. Pascal “if”: Terminals <if_stmt> if <logic_expr> then <stmt> LHS Production Non-terminals are abstractions for syntactic structures. Terminals are lexemes or tokens. 7

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> + | 8

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> 9

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> } 10

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. 11

Syntax Graphs • LHS BNF <if_stmt> • Non-terminal <stmt> • Terminal if Syntax Graph if_stmt if 12

Syntax Graph - Example BNF: <if_stmt> if <logic_expr> then <stmt> Syntax Graph: if_stmt if logic_expr then stmt 13

Syntax Graph Constructs • Alternatives: <stmt>|<funct> stmt funct • Optional [<expr>] expr 14

Syntax Graph Constructs (Contd. ) • Unspecified repetitions: {<digit>} digit • Repetition with minimum one occurrence digit <digit>{<digit>} 15

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 17

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 nonterminal. • Each leaf is labeled with a terminal. 18

Parse tree for A=B*C <assign> <ident> A = <expr> <ident> * B <expr> <ident> C 19

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> ) | <ident> then the sentence A = B*C+A would have two distinct parse trees, and therefore the above grammar is ambiguous. 20

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. 21

Derivation - Example. <assign> <ident> = <expr> A = <ident> * <expr> A = B * <ident> A = B * C 22