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Hector Miguel Chavez Western Michigan University

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Presentation Outline 2May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final steps Greibach Normal Form Algorithm (Example) Summary

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Introduction 3May 27, 2009 Grammar: G = (V, T, P, S) T = { a, b } Terminals V = A, B, C Variables S S Start Symbol P = S → A Production

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Introduction 4May 27, 2009 Grammar example S → aBSc S → abc Ba → aB Bb → bb L = { a n b n c n | n ≥ 1 } S aBSc aBabcc aaBbcc aabbcc

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Introduction 5May 27, 2009 Context free grammar The head of any production contains only one non-terminal symbol S → P P → aPb P → ε L = { a n b n | n ≥ 0 }

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Presentation Outline 6May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final simplification Greibach Normal Form Algorithm (Example) Summary

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Chomsky Normal Form 7May 27, 2009 A → BC A → α A context free grammar is said to be in Chomsky Normal Form if all productions are in the following form: A, B and C are non terminal symbols α is a terminal symbol

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Presentation Outline 8May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final steps Greibach Normal Form Algorithm (Example) Summary

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Preliminary Simplifications 9May 27, 2009 Eliminate Useless Symbols 1 Eliminate ε productions 2 Eliminate unit productions 3 There are three preliminary simplifications

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Preliminary Simplifications 10May 27, 2009 Eliminate Useless Symbols We need to determine if the symbol is useful by identifying if a symbol is generating and is reachable X is generating if X ω for some terminal string ω. X is reachable if there is a derivation X αXβ for some α and β * *

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Preliminary Simplifications 11May 27, 2009 Example: Removing non-generating symbols S → AB | a A → b S → AB | a A → b Initial CFL grammar S → AB | a A → b S → AB | a A → b Identify generating symbols S → a A → b S → a A → b Remove non-generating

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Preliminary Simplifications 12May 27, 2009 Example: Removing non-reachable symbols S → a Eliminate non-reachable S → a A → b S → a A → b Identify reachable symbols

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Preliminary Simplifications 13May 27, 2009 The order is important. S → AB | a A → b S → AB | a A → b Looking first for non-reachable symbols and then for non-generating symbols can still leave some useless symbols. S → a A → b S → a A → b

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Preliminary Simplifications 14May 27, 2009 Finding generating symbols If there is a production A → α, and every symbol of α is already known to be generating. Then A is generating S → AB | a A → b S → AB | a A → b We cannot use S → AB because B has not been established to be generating

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Preliminary Simplifications 15May 27, 2009 Finding reachable symbols S is surely reachable. All symbols in the body of a production with S in the head are reachable. S → AB | a A → b S → AB | a A → b In this example the symbols {S, A, B, a, b} are reachable.

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Preliminary Simplifications 16May 27, 2009 Eliminate Useless Symbols 1 Eliminate ε productions 2 Eliminate unit productions 3 There are three preliminary simplifications

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Preliminary Simplifications 17May 27, 2009 Eliminate ε Productions In a grammar ε productions are convenient but not essential If L has a CFG, then L – {ε} has a CFG Nullable variable A ε *

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Preliminary Simplifications 18May 27, 2009 If A is a nullable variable Whenever A appears on the body of a production A might or might not derive ε S → ASA | aB A → B | S B → b | ε Nullable: {A, B}

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Preliminary Simplifications 19May 27, 2009 Create two version of the production, one with the nullable variable and one without it Eliminate productions with ε bodies S → ASA | aB A → B | S B → b | ε S → ASA | aB | AS | SA | S | a A → B | S B → b Eliminate ε Productions

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Preliminary Simplifications 20May 27, 2009 Create two version of the production, one with the nullable variable and one without it Eliminate productions with ε bodies S → ASA | aB A → B | S B → b | ε S → ASA | aB | AS | SA | S | a A → B | S B → b Eliminate ε Productions

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Preliminary Simplifications 21May 27, 2009 Create two version of the production, one with the nullable variable and one without it Eliminate productions with ε bodies S → ASA | aB A → B | S B → b | ε S → ASA | aB | AS | SA | S | a A → B | S B → b Eliminate ε Productions

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Preliminary Simplifications 22May 27, 2009 Eliminate Useless Symbols 1 Eliminate ε productions 2 Eliminate unit productions 3 There are three preliminary simplifications

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Preliminary Simplifications 23May 27, 2009 Eliminate unit productions A unit production is one of the form A → B where both A and B are variables A B * A → B, B → ω, then A → ω Identify unit pairs

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Preliminary Simplifications 24May 27, 2009 Example: I → a | b | Ia | Ib | I0 | I1 F → I | (E) T → F | T * F E → T | E + T PairsProductions ( E, E )E → E + T ( E, T )E → T * F ( E, F )E → (E) ( E, I )E → a | b | Ia | Ib | I0 | I1 ( T, T )T → T * F ( T, F )T → (E) ( T, I )T → a | b | Ia |Ib | I0 | I1 ( F, F )F → (E) ( F, I )F → a | b | Ia | Ib | I0 | I1 ( I, I )I → a | b | Ia | Ib | I0 | I1 Basis: (A, A) is a unit pair of any variable A, if A A by 0 steps. * T = {*, +, (, ), a, b, 0, 1}

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Preliminary Simplifications 25May 27, 2009 Example: PairsProductions …… ( T, T )T → T * F ( T, F )T → (E) ( T, I )T → a | b | Ia |Ib | I0 | I1 …… I → a | b | Ia | Ib | I0 | I1 E → E + T | T * F | (E ) | a | b | la | lb | l0 | l1 T → T * F | (E) | a | b | Ia | Ib | I0 | I1 F → (E) | a | b | Ia | Ib | I0 | I1

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Presentation Outline 26May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final steps Greibach Normal Form Algorithm (Example) Summary

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Final Simplification 27May 27, 2009 Chomsky Normal Form (CNF) 1.Arrange that all bodies of length 2 or more to consists only of variables. 2.Break bodies of length 3 or more into a cascade of productions, each with a body consisting of two variables. Starting with a CFL grammar with the preliminary simplifications performed

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Final Simplification 28May 27, 2009 Step 1: For every terminal α that appears in a body of length 2 or more create a new variable that has only one production. E → E + T | T * F | (E ) | a | b | la | lb | l0 | l1 T → T * F | (E) | a | b | Ia | Ib | I0 | I1 F → (E) | a | b | Ia | Ib | I0 | I1 I → a | b | Ia | Ib | I0 | I1 E → EPT | TMF | LER | a | b | lA | lB | lZ | lO T → TMF | LER | a | b | IA | IB | IZ | IO F → LER | a | b | IA | IB | IZ | IO I → a | b | IA | IB | IZ | IO A → aB → bZ → 0O → 1 P → +M → *L → (R → )

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Final Simplification 29May 27, 2009 Step 2: Break bodies of length 3 or more adding more variables E → EPT | TMF | LER | a | b | lA | lB | lZ | lO T → TMF | LER | a | b | IA | IB | IZ | IO F → LER | a | b | IA | IB | IZ | IO I → a | b | IA | IB | IZ | IO A → aB → bZ → 0O → 1 P → +M → *L → (R → ) C 1 → PT C 2 → MF C 3 → ER

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Presentation Outline 30May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final steps Greibach Normal Form Algorithm (Example) Summary

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Greibach Normal Form 31May 27, 2009 A → αX A context free grammar is said to be in Greibach Normal Form if all productions are in the following form: A is a non terminal symbols α is a terminal symbol X is a sequence of non terminal symbols. It may be empty.

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Presentation Outline 32May 27, 2009 Introduction Chomsky normal form Preliminary simplifications Final steps Greibach Normal Form Algorithm (Example) Summary

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Greibach Normal Form 33May 27, 2009 Example: S → XA | BB B → b | SB X → b A → a CNF S = A 1 X = A 2 A = A 3 B = A 4 New Labels A 1 → A 2 A 3 | A 4 A 4 A 4 → b | A 1 A 4 A 2 → b A 3 → a Updated CNF

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Greibach Normal Form 34May 27, 2009 Example: A 1 → A 2 A 3 | A 4 A 4 A 4 → b | A 1 A 4 A 2 → b A 3 → a First Step X k is a string of zero or more variables A i → A j X k j > i A 4 → A 1 A 4

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Greibach Normal Form 35May 27, 2009 Example: A 1 → A 2 A 3 | A 4 A 4 A 4 → b | A 1 A 4 A 2 → b A 3 → a A 4 → A 1 A 4 A 4 → A 2 A 3 A 4 | A 4 A 4 A 4 | b A 4 → bA 3 A 4 | A 4 A 4 A 4 | b First Step A i → A j X k j > i

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Greibach Normal Form 36May 27, 2009 Example: Second Step Eliminate Left Recursions A 1 → A 2 A 3 | A 4 A 4 A 4 → bA 3 A 4 | A 4 A 4 A 4 | b A 2 → b A 3 → a A 4 → A 4 A 4 A 4

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Greibach Normal Form 37May 27, 2009 Example: Second Step Eliminate Left Recursions A 1 → A 2 A 3 | A 4 A 4 A 4 → bA 3 A 4 | A 4 A 4 A 4 | b A 2 → b A 3 → a A 4 → bA 3 A 4 | b| bA 3 A 4 Z | bZ Z → A 4 A 4 | A 4 A 4 Z

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Greibach Normal Form 38May 27, 2009 Example: A 1 → A 2 A 3 | A 4 A 4 A 4 → bA 3 A 4 | b | bA 3 A 4 Z | bZ Z → A 4 A 4 | A 4 A 4 Z A 2 → b A 3 → a A → αX GNF

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Greibach Normal Form 39May 27, 2009 Example: A 1 → A 2 A 3 | A 4 A 4 A 4 → bA 3 A 4 | b | bA 3 A 4 Z | bZ Z → A 4 A 4 | A 4 A 4 Z A 2 → b A 3 → a Z → bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4 | bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4 A 1 → bA 3 | bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4

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Greibach Normal Form 40May 27, 2009 Example: A 1 → bA 3 | bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4 A 4 → bA 3 A 4 | b | bA 3 A 4 Z | bZ Z → bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4 | bA 3 A 4 A 4 | bA 4 | bA 3 A 4 ZA 4 | bZA 4 A 2 → b A 3 → a Grammar in Greibach Normal Form

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Presentation Outline 41May 27, 2009 Summary (Some properties) Every CFG that doesn’t generate the empty string can be simplified to the Chomsky Normal Form and Greibach Normal Form The derivation tree in a grammar in CNF is a binary tree In the GNF, a string of length n has a derivation of exactly n steps Grammars in normal form can facilitate proofs CNF is used as starting point in the algorithm CYK

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Presentation Outline 42May 27, 2009 References [1] Introduction to Automata Theory, Languages and Computation, John E. Hopcroft, Rajeev Motwani and Jeffrey D. Ullman, 2nd edition, Addison Wesley 2001 (ISBN: 0-201-44124-1) [2] CS-311 HANDOUT, Greibach Normal Form (GNF), Humaira Kamal, http://suraj.lums.edu.pk/~cs311w02/GNF-handout.pdf [3] Conversion of a Chomsky Normal Form Grammar to Greibach Normal Form, Arup Guha, http://www.cs.ucf.edu/courses/cot4210/spring05/lectures/Lec14Greibac h.ppt

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Test Questions 43May 27, 2009 1.Convert the following grammar to the Chomsky Normal Form. 2.Is the following grammar context-free? 3.Prove that the language L = { a n b n c n | n ≥ 1 } is not context-free. 4.Convert the following grammar to the Greibach Normal Form. S → P P → aPb | ε S → aBSc | abc Ba → aB Bb → bb S -> a | CD | CS A -> a | b | SS C -> a D -> AS

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Presentation Outline 44May 27, 2009

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