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Ambiguous Grammars - ByteLABS

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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

E 1

then

else

E 1

then

if

S 2

E 2

then

if

E 2

then else

S 1

S 2

production 2,

then production 1

production 1,

then production 2

COMP 412, Fall 2002 17

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

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Ambiguous Grammars - ByteLABS

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