1. MA/CSSE 474 Theory of Computation
Algorithms and Decision Procedures for Regular Languages
Intro to Context-free Grammars

2. Your parents will probably concur!…
Your Questions?
Your parents will probably concur!…
Previous class days' material
Reading Assignments
HW 7 or 8 problems
Exam 2
Anything else

3. Summary of Algorithms
The next few slides are here for reference. I do not expect to spend class time on them.
You should know how to do all of them, but
during today's exercises you may simply "call" any of them
as part of your decision procedures.
● Operate on FSMs without altering the language that is accepted:
● ndfsmtodfsm(M: NDFSM)
● minDFSM (M:DFSM)
● buildFSMcanonicalform(M:FSM)

4. Summary of Algorithms
● Compute functions of languages defined as FSMs:
● Given FSMs M1 and M2, construct a FSM M3 such that
L(M3) = L(M2)  L(M1).
● Given FSMs M1 and M2, construct a new FSM M3 such that
L(M3) = L(M2) L(M1).
● Given FSM M, construct an FSM M* such that
L(M*) = (L(M))*.
● Given a DFSM M, construct an FSM M* such that
L(M*) = L(M).
● Given two FSMs M1 and M2, construct an FSM M3 such that
L(M3) = L(M2)  L(M1).
L(M3) = L(M2) - L(M1).
● Given an FSM M, construct an FSM M* such that
L(M*) = (L(M))R.
● Given an FSM M, construct an FSM M* that accepts
letsub(L(M)).

5. Algorithms, Continued
● Converting between FSMs and regular expressions:
● Given a regular expression , construct an FSM M
such that:
L() = L(M)
● Given an FSM M, construct a regular expression 
● Algorithms that implement operations on languages defined by regular expressions: any operation that can be performed on languages defined by FSMs can be implemented by converting all regular expressions to equivalent FSMs and then executing the appropriate FSM algorithm.

6. Algorithms, Continued ● Converting between FSMs and regular grammars:
● Given a regular grammar G, construct an FSM M
such that:
L(G) = L(M)
● Given an FSM M, construct a regular grammar G
L(G) = L(M).
We have not yet discussed regular grammars. They are in the reading assignment for tomorrow. This is here for completeness.

7. Decision Procedures
A decision procedure is an algorithm whose result is a Boolean value (in this course's context, sometimes we will write "yes" or "no" instead of "true" or "false"). It must:
● Halt, no matter what (correctly typed) input it is given.
● Always give the correct answer
Usually a decision procedure has one or more parameters Zero-parameter decision procedures are not very interesting!

8. Decision Procedure Example
Given two regular expressions 1 and 2, is:
(L(1)  L(2)) – {}  ?
1. From 1, construct an FSM M1 such that L(1) = L(M1).
2. From 2, construct an FSM M2 such that L(2) = L(M2).
3. Construct M such that L(M ) = L(M1)  L(M2).
4. Construct M such that L(M) = {}.
5. Construct M such that L(M ) = L(M ) - L(M).
6. If L(M ) is empty return False; else return True.
For practice later: Given two regular expressions 1 and 2, are there at least 3 strings that are generated by both of them?

9. Decision Procedure Practice
Find a group of three students.
Work together to write decision procedures for as many of the problems on the following slide as you can do in the allotted time.

10. Decision Procedures for Regular Languages
● Given a FSM M and a string w, does M accept w?
● Given a regular expression  and a string w, does 
generate w?
● Given a FSM M, is L(M) empty?
● Given a FSM M, does L(M) = M*?
● Given a FSM M, is L(M) finite?
● Given a FSM M, is L(M) infinite?
● Given two FSMs M1 and M2, are they equivalent?
● Given an DFSM M, is M minimal?

11. Membership We can answer the membership question by running an FSM.
But we must be careful if it's an NDFSM:
These slide are all hidden, and the contents are on my Qiuiz18 key document

12. Membership decideFSM(M: FSM, w: string) =
If ndfsmsimulate(M, w) accepts
then return True
else return False.
decideregex(: regular expression, w: string) =
From , use regextofsm to construct an FSM M
such that L() = L(M).
Return decideFSM(M, w).
Recall that ndfsmsimulate takes epsilon-closure at every stage, so there is no danger of getting into an infinite loop.

13. Emptiness Given an FSM M, is L(M) empty? The graph analysis approach:
1. Mark all states that are reachable via some path from the
start state of M.
2. If at least one marked state is an accepting state, return False. Else return True.
The simulation approach:
1. Let M = ndfsmtodfsm(M).
2. For each string w in * such that |w| < |KM | do:
Run decideFSM(M, w).
3. If M accepts at least one such string, return False. Else return True.

14. Totality Given an FSM M, is L(M) = M*? Construct M to accept L(M).
2. Return emptyFSM(M ).

15. Finiteness Given an FSM M, is L(M) finite?
The graph analysis approach:
The simulation approach
The mere presence of a loop does not guarantee that L(M) is infinite. The loop might be:
labeled only with ,
unreachable from the start state, or
not on a path to an accepting state.
1. M = ndfsmtodfsm(M).
2. M = minDFSM(M).
3. Mark all states in M that are on a path to an accepting
state.
4. Considering only marked states, determine whether there
are any cycles in M.
5. If there are cycles, return True. Else return False.
The simulation approach:
M = ndfsmtodfsm(M).
2. For each string w in * such that ________________ do: [answer: |KM |  w  2|KM | - 1 ]
Run decideFSM(M, w).
3. If M accepts at least one such string, return False.
Else return True.

16. Equivalence other words, is L(M1) = L(M2)? We can describe two
● Given two FSMs M1 and M2, are they equivalent? In
other words, is L(M1) = L(M2)? We can describe two
different algorithms for answering this question.

17. Equivalence other words, is L(M1) = L(M2)?
● Given two FSMs M1 and M2, are they equivalent? In
other words, is L(M1) = L(M2)?
equalFSMs1(M1: FSM, M2: FSM) =
1. M1 = buildFSMcanonicalform(M1).
2. M2 = buildFSMcanonicalform(M2).
3. If M1 and M2 are equal, return True, else return False.

18. Equivalence other words, is L(M1) = L(M2)?
● Given two FSMs M1 and M2, are they equivalent? In
other words, is L(M1) = L(M2)?
Observe that M1 and M2 are equivalent iff:
(L(M1) - L(M2))  (L(M2) - L(M1)) = .
equalFSMs2(M1: FSM, M2: FSM) =
1. Construct MA to accept L(M1) - L(M2).
2. Construct MB to accept L(M2) - L(M1).
3. Construct MC to accept L(MA)  L(MB).
4. Return emptyFSM(MC).

19. Minimality ● Given DFSM M, is M minimal? M = minDFSM(M).
2. If |KM| = |KM | return True; else return False.

20. Context-Free Grammars
CFG ≡ BNF (mostly)
Chapter 11

21. Languages and Machines

22. Rewrite Systems and Grammars
A rewrite system (a.k.a. production system or rule-based system) is:
● a list of rules, and
● an algorithm for applying them.
Each rule has a left-hand side (lhs) and a right hand side (rhs)
Example rules:
S  aSb
aS  
aSb  bSabSa

23. Simple-rewrite algorithm
simple-rewrite(R: rewrite system, w: initial string) =
1. Set working-string to w.
2. Until told by R to halt do:
Match the lhs of some rule against some part of working-string.
Replace the matched part of working-string with the rhs of the rule that was matched.
3. Return working-string.
lhs means "left-hand side" rhs means "right-hand side"

24. An Example w = SaS Rules: [1] S  aSb [2] aS  
● What order to apply the rules?
● When to quit?
Do a couple of examples.

25. Rule Based Systems ● Expert systems ● Cognitive modeling
● Business practice modeling
● General models of computation
● Grammars

26. Grammars Define Languages
A CFG G=(V, , R, S) (Each part is finite)
 is the terminal alphabet; it contains the symbols that make up the strings in L(G), and
N is the nonterminal alphabet a set of working symbols that G uses to structure the language. These symbols disappear by the time the grammar finishes its job and generates a string. Note:  ∩ N =  Rule alphabet: V =  ∪ N
R: A set of productions of the form A  β, where A ∊ N and β ∊ V*.
G has a unique start symbol, S ∊ N

27. Generating Many Strings
Multiple rules may match.
Grammar: S  aSb, S  bSa, and S  
Derivation so far: S  aSb  aaSbb 
Three choices at the next step:
S  aSb  aaSbb  aaaSbbb (using rule 1),
S  aSb  aaSbb  aabSabb (using rule 2),
S  aSb  aaSbb  aabb (using rule 3).

28. Generating Many Strings
One rule may match in more than one way.
Grammar: S  aTTb, T  bTa, and T  
Derivation so far: S  aTTb 
Two choices at the next step:
S  aTTb  abTaTb 
S  aTTb  aTbTab 

29. When to Stop May stop when:
The working string no longer contains any nonterminal symbols (including, when it is ).
In this case, we say that the working string is generated by the grammar.
Example:
S  aSb  aaSbb  aabb

30. When to Stop Rules: S  aSb, S  bTa, and S  
May stop when:
There are nonterminal symbols in the working string but none of them is in a substring that is the left-hand side of any rule in the grammar.
In this case, we have a blocked or non-terminated derivation but no generated string.
Example:
Rules: S  aSb, S  bTa, and S  
Derivations: S  aSb  abTab  [blocked]

31. When to Stop It is possible that neither (1) nor (2) is achieved.
Example:
G contains only the rules S  Ba and B  bB, with S the start symbol.
Then all derivations proceed as:
S  Ba  bBa  bbBa  bbbBa  bbbbBa  ...

32. Context-free Grammars, Languages, and PDAs
Context-free Language L
Context-free Grammar
Accepts
PDA

33. Context-Free Grammars
No restrictions on the form of the right hand sides.
S  abDeFGab
But require single non-terminal on left hand side.
S 
but not ASB 

34. anbn Balanced Parentheses language ambn : m>= n
Find a grammar that generates each
S  
S  aSb

35. Context-Free Grammars
A context-free grammar G is a quadruple,
(V, , R, S), where:
● V is the rule alphabet, which contains nonterminals
and terminals.
●  (the set of terminals) is a subset of V,
● R (the set of rules) is a finite subset of (V - ) V*,
● S (the start symbol) is an element of V - .
Example:
({S, a, b}, {a, b}, {S  a S b, S  }, S)
Mention N = V - 
Rules are also known as productions.