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Chapter 2-b

Chapter 2-b. Syntax, Semantics. 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.

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Chapter 2-b

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  1. Chapter 2-b • Syntax, Semantics

  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.

  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”

  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, ), ;

  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 )

  6. Tokens (Contd.) • Operators: Identify actions. Eg. +, &&, ! • Literals: Denote values directly. Eg. 3.14, -10, ‘a’, true, null • Punctuation Symbols: Supports syntactic structure. Eg. (, ), ;, {, }

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

  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>  + | 

  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>

  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> }

  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.

  12. Syntax Graphs BNFSyntax Graph • LHS <if_stmt> if_stmt  • Non-terminal <stmt> • Terminal if stmt if

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

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

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

  16. A simple grammar <assign><ident>=<expr> <ident> A|B|C <expr>  <ident>+<expr> | <ident>*<expr> | ( <expr> ) | <ident>

  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

  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.

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

  20. Ambiguous Grammar A grammar that generates a sentence which has two or more distinct parse trees is said to be anambiguous 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.

  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.

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

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