04-AmbigPrecReduct

04-AmbigPrecReduct - Discussion#4 Chapter 10 Sections 1.6...

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Unformatted text preview: Discussion #4 Chapter 10, Sections 1.6 1/17 Discussion #4 Ambiguity and Precedence Grammars, Grammar Reductions Discussion #4 Chapter 10, Sections 1.6 2/17 Topics • Ambiguity • Precedence Grammars • Grammar Reductions Discussion #4 Chapter 10, Sections 1.6 3/17 Problems with Grammars • We are heading towards grammars that are reasonable , get the job done , and for which we can find efficient parsing algorithms . • Not all grammars are usable! – Ambiguous – “Silly” – Have unproductive non-terminals – Have unreachable rules Discussion #4 Chapter 10, Sections 1.6 4/17 Ambiguous Grammars • A grammar for which there are two different parse trees for the same terminal string is said to be ambiguous . • We can show that a grammar is ambiguous by giving two parse trees for the same terminal string. Discussion #4 Chapter 10, Sections 1.6 5/17 Φ = { E → D | ( E ) | E + E | E – E | E * E | E / E , D → 0 | 1 | … | 9 } E E E + E E * D D D 1 2 3 1 + 2 * 3 E E E * E E + D D D 1 2 3 1 + 2 * 3 An Ambiguous Grammar Discussion #4 Chapter 10, Sections 1.6 6/17 Ambiguity & Multiple Meanings • Precedence: (1+2)*3 ≠ 1+(2*3) • Associativity: (3- 2)- 1 ≠ 3- (2- 1) • Different if-then-else nestings • … Discussion #4...
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04-AmbigPrecReduct - Discussion#4 Chapter 10 Sections 1.6...

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