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Chapter 3 Syntax Part 1. CMSC 331 Shon Vick. Syntax and Semantics. Syntax - the form or structure of the expressions – whether an expression is well formed Semantics – the meaning of an expression. Syntactic Structure.

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## Chapter 3 Syntax Part 1

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**Chapter 3Syntax Part 1**CMSC 331 Shon Vick**Syntax and Semantics**• Syntax - the form or structure of the expressions – whether an expression is well formed • Semantics – the meaning of an expression**Syntactic Structure**• Syntax almost always expressed using some variant of a notation called a context-free grammar (CFG) or simply grammar • BNF • EBNF • Syntax Graph**A CFG has 4 parts**• A set of tokens (lexemes), known as terminal symbols • A set of non-terminals • A set of rules (productions) where each production consists of a left-hand side (LHS) and a right-hand side (RHS) The LHS is a non-terminal and the RHS is a sequence of terminals and/or non-terminal symbols. • A special non-terminal symbol designated as the start symbol**An example of BNF syntax for real numbers**<r> ::= <ds> . <ds> <ds> ::= <d> | <d> <ds> <d> ::= 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7| 8 | 9 < > encloses non-terminal symbols ::= 'is' or 'is made up of ' or 'derives' (sometimes denoted with an arrow ->) | or**Example**• On the example from the previous slide: • What are the tokens? • What are the lexemes? • What are the non terminals? • What are the productions?**BNF Points**• A non terminal can have more than RHS or an OR can be used • Lists or sequences are expressed via recursion • A derivation is just a repeated set of production (rule) applications • Examples**Example Grammar**<program> -> <stmts> <stmts> -> <stmt> | <stmt> ; <stmts> <stmt> -> <var> = <expr> <var> -> a | b | c | d <expr> -> <term> + <term> | <term> - <term> <term> -> <var> | const**Example Derivation**<program> => <stmts> => <stmt> => <var> = <expr> => a = <expr> => a = <term> + <term> => a = <var> + <term> => a = b + <term> => a = b + const**Parse Trees**• Alternative representation for a derivation • Example parse tree for the previous example stmts stmt expr var = term term + a var const b**Parse Trees**PS -> P | P PS P -> e | '(' PS ')' | '<' PS '>' | '[' PS ']' What’s the parse tree for this statement ? < [ ] [ < > ] >**Ambiguity**• Two parse trees for the same expression • Consider the following grammar string -> string + string | string - string digit -> 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 What are the two trees for 9 - 5 + 2**EBNF - Extended BNF**• Like BNF except that • Non-terminals start w/ uppercase • Parens are used for grouping terminals • Braces {} represent zero or more occurrences (iteration ) • Brackets [] represent an optional construct , that is a construct that appears either once or not at all.**EBNF example**Exp -> Term { ('+' | '-') Term } Term -> Factor { ('*' | '/') Factor } Factor -> '(' Exp ')' | variable | constant**EBNF/BNF**• EBNF and BNF are equivalent • How can {} be expressed in BNF? • How can ( ) be expressed? • How can [ ] be expressed?**Syntax Graphs**Terminal in circles nonterminals in rectangles; Syntax Graphs - put the terminals in circles or ellipses and put the nonterminals in rectangles; connect with lines with arrowheads e.g., Pascal type declarations type_identifier ( identifier ) , constant .. constant**Recursive Descent**• An easy way to build a parser • Example • Does work in the face of left recursion • Purge left recursion

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