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Lecture # 17

Lecture # 17. Syntax Directed Definition. Translation Schemes. A translation scheme is a CF grammar embedded with semantic actions. rest  + term { print(“+”) } rest. Embedded semantic action. rest. +. term. { print(“+”) }. rest. Example Translation Scheme.

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Lecture # 17

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  1. Lecture # 17 Syntax Directed Definition

  2. Translation Schemes • A translation scheme is a CF grammar embedded with semantic actions rest +term { print(“+”) } rest Embeddedsemantic action rest + term { print(“+”) } rest

  3. Example Translation Scheme expr expr+termexpr  expr-termexpr  termterm  0term  1…term  9 { print(“+”) }{ print(“-”) }{ print(“0”) }{ print(“1”) }…{ print(“9”) }

  4. Example Translation Scheme (cont’d) expr { print(“+”) } + expr term { print(“2”) } { print(“-”) } - 2 expr term { print(“5”) } 5 term { print(“9”) } 9 Translates 9-5+2 into postfix 95-2+

  5. Attributes • Attribute values may represent • Numbers (literal constants) • Strings (literal constants) • Memory locations, such as a frame index of a local variable or function argument • A data type for type checking of expressions • Scoping information for local declarations • Intermediate program representations

  6. Synthesized Versus Inherited Attributes • Given a productionAthen each semantic rule is of the formb := f(c1,c2,…,ck)where f is a function and ci are attributes of A and , and either • b is a synthesized attribute of A • b is an inherited attribute of one of the grammar symbols in 

  7. Synthesized Versus Inherited Attributes (cont’d) inherited Production Semantic Rule D T LT  int…L  id L.in:= T.typeT.type:=‘integer’…… := L.in synthesized

  8. S-Attributed Definitions • A syntax-directed definition that uses synthesized attributes exclusively is called an S-attributed definition (or S-attributed grammar) • A parse tree of an S-attributed definition is annotated with a single bottom-up traversal

  9. Example Attribute Grammar with Synthesized+Inherited Attributes Production Semantic Rule L.in:= T.typeT.type:= ‘integer’T.type:= ‘real’ L1.in:= L.in; addtype(id.entry, L.in)addtype(id.entry, L.in) D T LT  intT realL  L1,idL id Synthesized: T.type, id.entryInherited: L.in

  10. Example • Write Syntax Directed Definitions to convert a binary string to decimal value • Solution: • First we would think of synthesized and inherited attributes required. We identified three variables namely “var” for holding binary vale, “decval” for holding decimal value and “pos” for the place value

  11. Example (contd) Productions Semantic Rules S.pos = 0 S.pos=S.pos+1 S.decval= A.decval+S.decval A.val=0 A.pos=S.pos A.decval= A.val *2pos A.val=1 A.pos=S.pos A.decval= A.val *2pos S.pos =S.pos • S A S • A 0 • A 1 • S €

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