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Ambiguous Grammars - ByteLABS

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Ambiguity

Removing the ambiguity

• Must rewrite the grammar to avoid generating the problem

• Match each else to innermost unmatched if (common sense rule)

1

Stmt

→

if Expr then Stmt

2

⏐

if Expr then WithElse else Stmt

3

⏐

Other Statements

4

WithElse

→

if Expr then WithElse else WithElse

5

⏐

Other Statements

With this grammar, example has only one rightmost derivation

Intuition: once into WithElse, we cannot generate an unmatched else

… a final if without an else can only come through rule 2 …

COMP 412, Fall 2002 18

Comp 412 Fall 2005

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Ambiguity This sentential form has two derivations if Expr 1 then if Expr 2 then Stmt 1 else Stmt 2 if if E 1 then else E 1 then if S 2 E 2 then if E 2 then else S 1 S 1 S 2 production 2, then production 1 production 1, then production 2 COMP 412, Fall 2002 17 Comp 412 Fall 2005

Ambiguity Removing the ambiguity • Must rewrite the grammar to avoid generating the problem • Match each else to innermost unmatched if (common sense rule) 1 Stmt → if Expr then Stmt 2 ⏐ if Expr then WithElse else Stmt 3 ⏐ Other Statements 4 WithElse → if Expr then WithElse else WithElse 5 ⏐ Other Statements With this grammar, example has only one rightmost derivation Intuition: once into WithElse, we cannot generate an unmatched else … a final if without an else can only come through rule 2 … COMP 412, Fall 2002 18 Comp 412 Fall 2005

Page 1: Ambiguous Grammars Definitions •

Ambiguous Grammars - ByteLABS

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