1. Tutorial 05 -- CSC3130 : Formal Languages and Automata Theory Tu Shikui ( sktu@cse.cuhk.edu.hk ) SHB 905, Office hour: Thursday 2:30pm-3:30pm 2008-10-06

2. Outline Context-free Languages, Context-free grammars (CFG), Push-down Automata (PDA) Grammar Transformations Remove ε- production; Remove unit-production; Covert to Chomsky-Normal-Form (CNF) Cocke-Younger-Kasami (CYK) algorithm

3. Relations Context-free Languages L Context-free Grammars G Push-down Automata M L = L(G) L = L(M) L(G) = L(M)

4. Example (I) Given the following CFG S  X | Y X  aXb | aX | a Y  aYb | Yb | b (1) L(G) = ? (2) Design an equivalent PDA for it. Σ={a, b}

5. Example (I) --- solution: L(S) S  X | Y X  aXb | aX | a Y  aYb | Yb | b Try to write some strings generated by it: S  X  aXb  aaXbb  aaaXbb  aaaabb S  Y  aYb  aYbb  aaYbbb  aabbbb more a’s than b’s more b’s than a’s Observations: Start from S, we can enter two States X & Y, and X, Y are “independent”; In X state, always more a are generated; In Y state, always more b are generated. Ls = Lx U Ly Lx = { a i b j ; i>j } Lx = { a i b j ; i<j } L(S) = { a i b j ; i≠j }

6. Example (I) --- solution: PDA S  X | Y X  aXb | aX | a Y  aYb | Yb | b L(S) = { a i b j ; i≠j } PDA = NFA + a stack (infinite memory) = { a i b j ; i>j } U { a i b j ; i<j } A possible way: “divide and conquer” L x = { a i b j ; i>j } a,  /A q0q0 ,  /# ,/,/ q1q1 b,A/  b,#/# q2q2 q3q3 ,#/  L Y = { a i b j ; i<j } q’ 0 ,  /# ,/,/ q’ 1 ,A/  q’ 2 ,A/  q’ 3 ,#/  a,  /Ab,A/  ,  /  Combine both …

7. Example (II) Given the following language: (1) design a CFG for it; (2) design a PDA for it. L = {0 i 1 j : i ≤ j ≤ 2i, i=0,1,…},  = {0, 1}

8. Example (II) -- solution: CFG L = {0 i 1 j : i ≤ j ≤ 2i, i=0,1,…},  = {0, 1} Consider two extreme cases: (a). if j = i, then L 1 = { 0 i 1 j : i=j }; (b). if j = 2i, then L 2 = { 0 i 1 j : 2i=j }. S  0S1 S  ε S  0S11 S  ε If i ≤ j ≤ 2i, then randomly choose “red- rule” or “blue-rule” in the generation. “red-rule” “blue-rule” S  0S1 S  0S11 S  ε

9. Example (II) -- solution: CFG L = {0 i 1 j : i ≤ j ≤ 2i, i=0,1,…},  = {0, 1} S  0S1 S  0S11 S  ε Need to verify L = L(G) G = 1). L(G) is a subset of L: The “red-rule” and “blue-rule” guarantee that in each derivation, the number of 1s generated is one or two times larger than that of 0s. So, L(G) is a subset of L. 2). L is a subset of L(G): For any w = 0 i 1 j, i ≤ j ≤ 2i, we use “red-rule” ( 2i - j ) times and then “blue-rule” ( j - i ) times, i.e., S =*=> 0 2i-j S1 2i-j =*=> 0 2i-j 0 j-i S1 2(j-i) 1 2i-j ==> 0 i 1 j = w

10. Example (II) -- solution: PDA L = {0 i 1 j : i ≤ j ≤ 2i, i=0,1,…},  = {0, 1} Similar idea: “randomly choose two extreme cases” q0q0 ,  /# 0,  /X ,/,/ q1q1 q2q2 ,#/  1,X/  1,X/X 1,X/  q3q3

11. Example (III) Given the following language: (1). Design a CFG for it; (2). Design a PDA for it. L = { a i b j c k d l : i,j,k,l=0,1,…; i+k=j+l }, where the alphabet Σ= {a, b, c, d}

12. Example (III) – solution: CFG L = { a i b j c k d l : i,j,k,l=0,1,…; i+k=j+l }, Note that i + k = j + l ==> | i – j | = | l – k |. Assume n = i – j = l – k > 0, then i = n + j, l = n + k, and w = a i b j c k d l = a n a j b j c k d k d n w anan dndn ajbjajbj ckdkckdk ajaj bjbj ckck dkdk Three blocks come from the same template: N  x N x S  aSd | XY X  aXb | ε Y  cYd | ε S=*=>a n Sd n =*=> a n XYd n =*=>a n a j b j Yd n =*=>a n a j b j c k b k d n = a n+j b j c k b n+k (n+j) + k = j + (n+k)

13. Example (III) – solution: PDA L = { a i b j c k d l : i,j,k,l=0,1,…; i+k=j+l }, Main idea: (1)use X to record an a or c; use Y to record an b or d. (2)Compare #X and #Y: by cancellation. How to realize the comparison by cancellation? Action1 : Push an X, when a or c was read; Action2 : Pop an X (if any, otherwise push a Y), when b or d was read. Action3 : Pop an Y (if any, otherwise push an X), when a or c was read.

14. Example (III) – solution: PDA L = { a i b j c k d l : i,j,k,l=0,1,…; i+k=j+l }, Action1 : Push an X, when a or c was read; Action2 : Pop an X (if any, otherwise push a Y), when b or d was read. Action3 : Pop an Y (if any, otherwise push an X), when a or c was read. ,  /# q5q5 q1q1 a,  /X ,/,/ b,#/Y# q2q2 ,/,/ c,X/XX q3q3 ,/,/ q4q4 ,/,/ b,X/  b,Y/YY c,#/X# c,Y/  d,X/  d,#/Y# d,Y/YY

15. Outline Context-free Languages, Context-free grammars (CFG), Push-down Automata (PDA) Grammar Transformations Remove ε- production; Remove unit-production; Covert to Chomsky-Normal-Form (CNF) Cocke-Younger-Kasami (CYK) algorithm

16. Remove ε-production Example: Remove ε-productions of the following grammar G: S  ABaC A  BC B  b | ε C  D | ε D  d

17. Remove ε-production S  ABaC | BaC | AaC | aC | ABa | Ba | Aa | a A  BC | C | B B  b | ε C  D | ε D  d Nullable variables: A, B, C AddDelete For A:S  BaC For B:S  AaC | aC A  C B  ε For C:S  ABa | Ba | Aa | a A  B C  ε For i = 1 to k For every production of the form A →  N i , add another production A →  If N i →  is a production, remove it

18. Remove unit-productions Example: Remove the unit-productions of the following grammar: S  S + T | T T  T * F | F F  (S) | a

19. Remove unit-productions Add Delete S  T For T  T*F S  T*F For T  F  a S  a S  S+T | T | T*F | a | (S) T  T*F | F | a | (S) F  (S) | a For T  F  (S) S  (S) T  F For F  a S  a For F  (S) S  (S) A 1 → A 2 →... → A k → A 1 A 1 → A 2 →... → A k →  A 1 → ,..., A k →  delete it and replace everything with A 1 Rule 1: Rule 2:

20. Convert CFG to Chomsky-Normal- Form (CNF) Example: Convert the following CFG to CNF: S  0AB A  0D | 1AD B  0 D  1

21. Convert CFG to Chomsky-Normal- Form (CNF) A → BcDE replace terminals with new variables A → BCDE C → c break up sequences with new variables A → BX 1 X 1 → CX 2 X 2 → DE C → c S  0AB A  0D | 1AD B  0 D  1 S  XAB; X  0 A  XD | YAD; Y  1

22. Convert CFG to Chomsky-Normal- Form (CNF) A → BcDE replace terminals with new variables A → BCDE C → c break up sequences with new variables A → BX 1 X 1 → CX 2 X 2 → DE C → c S  XAB X  0 A  XD |YAD Y  1 B  0 D  1 S  XN 1 ; N 1  AB A  YN 2 ; N 2  AD

23. Convert CFG to Chomsky-Normal- Form (CNF) A → BcDE replace terminals with new variables A → BCDE C → c break up sequences with new variables A → BX 1 X 1 → CX 2 X 2 → DE C → c S  XN 1 N 1  AB A  XD A  YN 2 N 2  AD X  0 Y  1 B  0 D  1

24. Outline Context-free Languages, Context-free grammars (CFG), Push-down Automata (PDA) Grammar Transformations Remove ε- production; Remove unit-production; Covert to Chomsky-Normal-Form (CNF) Cocke-Younger-Kasami (CYK) algorithm

25. Constructing the parse table Input : w = a 1 a 2 …a n ∈ Σ+ Output : The parse table T for w such that t ij contains A ⇔ A + ⇒ a i a i+1 …a i+j-1 Steps: Step(1): cell ( i,1 ) = { A | A→a i ∈ P, 1≤i≤n } Step(2): for 1 ≤ k < j, cell ( i,j ) = { A | for some k, A→BC ∈ P, B is in cell ( i,k ), C is in cell ( i+k, j-k ) } Step(3): repeat Step(2) for 1≤ i≤ n, 1≤ j ≤ n-i+1

26. Cocke-Younger-Kasami (CYK) algorithm j=5 cell 11, cell 24 cell 12, cell 33 cell 13, cell 42 cell 14, cell 51 j=4 cell 11, cell 23 cell 12, cell 32 cell 13, cell 41 cell 21, cell 33 cell 22, cell 42 cell 23, cell 51 j=3 cell 11, cell 22 cell 12, cell 31 cell 21, cell 32 cell 22, cell 41 cell 31, cell 42 cell 32, cell 51 j=2 cell 11, cell 21 cell 21, cell 31 cell 31, cell 41 cell 41, cell 51 j=1 A1  x1A1  x1 A2  x2A2  x2 A3  x3A3  x3 A4  x4A4  x4 A5  x5A5  x5 i=1i=2i=3i=4i=5 x1x1 x2x2 x3x3 x4x4 x5x5 Cell (i,k), Cell (i+k, j-k) k = 1, …, j-1

27. Cocke-Younger-Kasami (CYK) algorithm j=5 cell 11, cell 24: S, A cell 12, cell 33: S, A cell 13, cell 42: S, A cell 14, cell 51: S, A j=4 cell 11, cell 23: S, A cell 12, cell 32: S, A cell 13, cell 41: S, A cell 21, cell 33: A cell 22, cell 42: S, A cell 23, cell 51: -- j=3 cell 11, cell 22 : S cell 12, cell 31: A, S cell 21, cell 32: -- cell 22, cell 41: S cell 31, cell 42: S,A cell 32, cell 51: -- j=2 cell 11, cell 21: S, A cell 21, cell 31: A cell 31, cell 41: S cell 41, cell 51: S,A j=1 A 1  x 1 : AA 2  x 2 : SA 3  x 3 : AA 4  x 4 :AA 5  x 5 :S i=1i=2i=3i=4i=5 abaab S → AA | AS | b A → SA | AS | a Test: w = abaab

28. Cocke-Younger-Kasami (CYK) algorithm j=5 cell 11, cell 24: S, A cell 12, cell 33: S, A cell 13, cell 42: S, A cell 14, cell 51: S, A j=4 cell 11, cell 23: S, A cell 12, cell 32: S, A cell 13, cell 41: S, A cell 21, cell 33: A cell 22, cell 42: S, A cell 23, cell 51: -- j=3 cell 11, cell 22 : S cell 12, cell 31: A, S cell 21, cell 32: -- cell 22, cell 41: S cell 31, cell 42: S,A cell 32, cell 51: -- j=2 cell 11, cell 21: S, A cell 21, cell 31: A cell 31, cell 41: S cell 41, cell 51: S,A j=1 A 1  x 1 : AA 2  x 2 : SA 3  x 3 : AA 4  x 4 :AA 5  x 5 :S i=1i=2i=3i=4i=5 abaab S → AA | AS | b A → SA | AS | a Test: w = abaab Trace back to find a parse tree

29. Cocke-Younger-Kasami (CYK) algorithm j=5 cell 11, cell 24: S, A cell 12, cell 33: S, A cell 13, cell 42: S, A cell 14, cell 51: S, A j=4 cell 11, cell 23: S, A cell 12, cell 32: S, A cell 13, cell 41: S, A cell 21, cell 33: A cell 22, cell 42: S, A cell 23, cell 51: -- j=3 cell 11, cell 22 : S cell 12, cell 31: A, S cell 21, cell 32: -- cell 22, cell 41: S cell 31, cell 42: S,A cell 32, cell 51: -- j=2 cell 11, cell 21: S, A cell 21, cell 31: A cell 31, cell 41: S cell 41, cell 51: S,A j=1 A 1  x 1 : AA 2  x 2 : SA 3  x 3 : AA 4  x 4 : AA 5  x 5 :S i=1i=2i=3i=4i=5 abaab S → AA | AS | b A → SA | AS | a Test: w = abaab We can find another parse tree.

30. End of this tutorial! Thanks for coming!