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1 CS2 Language Processing note 3 CS2Ah 5..4 CS2 Language Processing note 3 Nondeterministic finite automata In this lecture we look at nondeterministic finite automata and prove the Conversion Theorem, which shows how to turn a nondeterministic automaton into a deterministic one that recognises the same language. Nondeterministic finite automata Recall that the essential restriction we made when defining deterministic finite automata was to require that in each state there is exactly one possible transition for each input symbol. We now drop this restriction, obtaining a larger class of automata called nondeterministic automata (NFAs). It may happen that during a computation an NFA has several choices where to go next, or that it has no choice at all and gets stuck. Non-determinism gives us greater flexibility when modelling reactive systems; we can use it to describe systems whose behaviour we cannot completely predict. We pay a price for this flexibility NFAs do not share the convenient feature of DFAs that for each input string there is exactly one computation path that can be entirely predicted. In this course, we shall mainly use NFAs as a technical tool that turns out to be very useful in our investigation of regular languages. We start by giving a formal definition: Definition 3.. A nondeterministic finite automaton (or NFA) is a tuple consisting of:. a finite set of states, 2. a finite alphabet, 3. a distinguished starting state, 4. a set of final states (the ones that indicate acceptance), and 5. a description of all the possible transitions. must be given by a table that answers the following question: Given a state and an input symbol a, what are the possible next states? Thus the entries in the table are sets of states, as opposed to single states for a DFA. Remark 3.2. In mathematical terms, is a function from the set to the powerset of. We can also view as a subset of, that is, a ternary relation, where a tuple!" #$ %'&( is in if a transition from to '& is possible on input #.

2 > 4 CS2 Language Processing note 3 CS2Ah 5..4 Therefore, is often referred to as the transition relation of the NFA. Example 3.3. Figure shows an NFA, which can formally be specified as follows: a,b a b a 2 3 where the transition relation Figure : The NFA of Example 3.3 ) +*,'-" /. 2" 3"45, a b ,'3"45 98: is given by the following table: a b -,'-" /.:4,'-"4. ;,<=4,'3"4 ; 3 ; ; Recall that we use the symbol ; to denote the empty set. If it appears in a transition table, this means that no transitions are possible in the corresponding situation. We are now faced with the issue of how to define the notion of accepting a string for an NFA. The problem is that many different computations are possible on a given input string. An NFA is said to accept a string > if there is at least one computation of on > that does not get stuck and that ends in a state belonging to. Just as for DFAs, the language recognised (or accepted) by an NFA?@ A, B DCFEG accepts > Example 3.3 continued. The language accepted by the NFA?@ HAI KJ > aba E > L, a b 4'C<MN of Figure is is defined to be: that is, the language consisting of all strings over the alphabet, a b 4 ending in aba. Example 3.4: An enhanced combination lock. Recall the combination lock of Example.. We modelled it by a DFA accepting all strings over the alphabet, /O/O/O7 9 4 that ended with 2. In Exercise 3 of Lecture Note you were asked to enhance the lock in such a way that different key combinations can be used to open it. Specifically, you were asked to design a DFA whose language consists of all strings over, /O/OO7 9 4 ending in either 2, 2, or. If you did that exercise, you will have realized that constructing such a DFA is not so easy. It is much easier to construct an NFA accepting the same language Figure 2 shows one. 2

3 CS2 Language Processing note 3 CS2Ah Figure 2: An NFA for the enhanced combination lock NFAs with PPP -transitions In some situations it can be useful to enhance NFAs by allowing transitions that are not triggered by any input symbol; such transitions are called P -transitions. If in a state an P -transition to a state & is possible, then whenever the automaton reaches state in a computation it can immediately proceed to state & without needing any further input. Formally, an NFA with P -transitions is a tuple satisfying the same specifications as an NFA (cf. Definition 3.), except that the table describing the transition relation has an additional column that for each state gives the set of all states reachable from by an P -transition. Example 3.5. Figure 3 shows an NFA with P -transitions, which can formally be speci- a b a c Figure 3: An NFA with P -transitions fied as follows: Q*%,'-6 /.: 2" 36 %R645, 3 a b c 45-6,SR645 98:

4 P Z CS2 Language Processing note 3 CS2Ah 5..4 where the transition relation is given by the following table: a b c P -,5.:4 ; ;,5. " 36 R"4. ;,<=4 ; ;,'3"4 3 ; ;,SR64 ; R ; The language accepted by consists of the string abac and all its suffixes, that is,?@ HTU V, c ac bac abac 45O The Powerset Construction It is clear that every regular language, that is, every language recognised by some DFA, can be recognised by an NFA, as DFAs are just special cases of NFAs in which the possibility of nondeterminism is not exploited. Furthermore, every language recognised by some NFA can also be recognised by an NFA with P -transitions, because any NFA can be viewed as an NFA with P -transitions in which no P -transitions occur. The very surprising fact is that the converse of these observations is also true any language recognised by an NFA, with or without P -transitions, is regular. Theorem 3.6. If a language? is accepted by some NFA with P -transitions, then there is also a DFA that accepts?. Therefore, every language accepted by an NFA, with or without P -transitions, is regular. Proof: Let W + H 9 2 be an NFA with P -transitions that accepts?. We shall build Q HYG 2 2Z// [&\ < a DFA X that accepts the same language. The idea is to keep track of all possible computations of using the states of X. This is done by identifying each state of X with a certain subset of, namely all the possible states that could be in at that point. Hence we let Y^ _` a, the power set of (sometimes written cb ), that is, the set of all subsets of. The initial state ZS of X is the set of all states of that are reachable in without reading any input ( and anything reachable from / using just P -transitions). For a state B and a letter a, we write dfe a if Z is reachable from in after reading the input symbol a, again allowing for possible P -transitions. For Zg and a we can then define " hz I jï kml/n,oes a dqe p o4. In other words, " hz a is the set of all possible states that can be reached from some state in Z on reading the symbol a (allowing for any number of P -transitions). Finally we take &,<Z9r )E=ZsAut V;64. We can prove that this DFA X does indeed accept the same language as by induction on the length of strings. (Exercise: Do so!) Since set of states of the DFA X we construct in the proof of Theorem 3.6 is the powerset of the set of states of the NFA we started with, the construction is often referred to as the powerset construction. Actually, we can slightly simplify X by discarding all states that cannot be reached from the start state ZS by any possible sequence of transitions obviously such states are useless anyway. Then we can construct X in a systematic way by the following conversion algorithm: 4

5 p CS2 Language Processing note 3 CS2Ah Construct the start state Z consisting of and all states of that can be reached from by one or several P -transitions. Mark Z/ as unfinished. 2. While there still are unfinished states: (a) Take an unfinished state Z. (b) For each a v, let " hz #=w xi9kmlyn,oze{ d e. If 6 HZ #= is neither finished nor unfinished yet, then mark 6 HZc #" as unfinished. (c) Mark Z as finished. 3. Mark all states that contain a final state from as final states of X. o4 An example conversion It is easiest to understand the proof by seeing a worked example in action. convert the NFA in Figure 4 into a DFA X recognising the same language. We shall Figure 4: An NFA with P -transitions We first construct the start state ZS of X. It consists of the start state - of and all states reachable from - by P -transitions. Thus we have Z'} V,'-6 /. =4. Now we construct the transition function using the conversion algorithm described in the previous section. We represent the current status of by an incomplete table; unfinished states are those whose rows are incomplete and finished states are those whose rows are complete. We start with:,'-" /. 2=4 5

6 CS2 Language Processing note 3 CS2Ah 5..4 Now we take the only unfinished state,'-6. 2=4 and compute its transitions. We add the two states reachable from,'-" /. 2=4 by reading and, respectively, as new unfinished states to our table:,'-6 /. =4,'3" RN ~"4 ;,'36 RN ~"4 ; We proceed with state,'3" RN ~"4 :,'-" /. 2=4,'36 RN ~"4 ;,'3" RN ~"4,'~6 2=4,<" 36 2 = ~"4 ;,'~6 =4,<" 36 2 = ~"4 And state ; :,'-" /. 2=4,'36 RN ~"4 ;,'3" RN ~"4,'~6 2=4,<" 36 2 = ~"4,'~6 =4,<" 36 2 = ~"4 And so on:,'-6 /. =4,'36 %RN ~"4 ;,'36 RN ~"4,'~6 254,<" 3" 2 " ~"4,'~" 2=4,'~6 254 ;,<" 3" 2 " ~"4,'-6 /.: 2=4,'36 RN ~=4 ;,'36 R6 ~"4,'~6 =4,<" 36 2 " ~=4,'~6 2=4,'~6 =4 ;,<" 36 2 " ~=4,SRN ~6 254,'36 ~"4,SRN ~" 2=4,'36 ~"4,'-6 /. =4,'36 %RN ~"4 ;,'36 RN ~"4,'~6 254,<" 3" 2 " ~"4,'~" 2=4,'~6 254 ;,<" 3" 2 " ~"4,SRN ~6 2=4,'3" ~"4,SRN ~6 =4,'~6 254,<" 2 " ~=4,'3" ~"4,<" 2 " ~"4,'-6 /.: 2=4,'36 RN ~=4 ;,'36 R6 ~"4,'~6 =4,<" 36 2 " ~=4,'~6 2=4,'~6 =4 ;,<" 36 2 " ~=4,SRN ~6 254,'36 ~"4,SRN ~" 2=4,'~6 =4,<" 2 = ~"4,'36 ~"4,'~6 =4,'36 ~"4,<" 2 = ~"4 6

7 CS2 Language Processing note 3 CS2Ah 5..4 Finally, we obtain the complete transition function :,'-" /. 2=4,'36 RN ~"4 ;,'3" RN ~"4,'~6 2=4,<" 36 2 = ~"4,'~6 =4,'~6 2=4 ;,<" 36 2 = ~"4,SRN ~6 =4,'36 ~=4,SR6 ~6 2=4,'~6 2=4,<= 2 " ~"4,'36 ~"4,'~6 2=4,'36 ~=4,<= 2 " ~"4,SRN ~6 =4 ; This table also gives us the state set of X : J,'-6 /. =45,'36 R6 ~"45 ;N,'~" 2=45,< ~"45 7,SR6 ~" ,'36 ~545,<= " ~=4 M O The alphabet is the same as that of,, 4, and we already know the start state,,'-6 /.: 2=4. It remains to determine the set of final states. It consists of all states of X that contain a final state of, i.e., of all states that contain. Therefore, the set of final states is J,'~6 2545,SRN ~6 =4 M O The DFA X is displayed in Figure 5., {,,2} φ {3,4,6} {6,7} {2,5,6} {2,3,5,6} {3,6} {4,6.7} Figure 5: A DFA equivalent to the NFA of Figure 4 7

8 CS2 Language Processing note 3 CS2Ah 5..4 DFAs versus NFAs We have seen that NFAs and DFAs recognise exactly the same class of languages, namely the regular languages. It is natural to ask what the relative merits and demerits of the two forms of automata are. More to the point, why introduce the class of NFAs at all, given that we already have a class of automata that do the same job? The determinism of a DFA is important when implementing a program (or physical machine) for recognising a regular language. NFAs are not as well suited to language recognition because nondeterminism gives the machine the possibility of making incorrect choices. In general, when an NFA is used for language recognition, it is necessary to backtrack and explore all possible choices: a time consuming activity. On the other hand, NFAs are very convenient for demonstrating that languages are regular. Often it is much easier to provide an NFA for a language than a DFA think of Example 3.4. In particular, one of the problems with finding a DFA for a regular language is that, in general, a DFA for a language might have to be significantly larger than a corresponding NFA. Quantitatively, we see from the proof of the Conversion Theorem that, given an NFA with states, the corresponding DFA X might have as many as c states, making the conversion computationally explosive. (The example in Figure 4, worked out above, happens to be particularly well behaved!) Sometimes, but not always, it is possible to improve on the DFA produced by the Conversion Theorem, by finding an equivalent DFA with fewer states. Nevertheless, in general, even the smallest DFA for a language grows exponentially in the size of the NFA recognising the language. Exercises. List all possible computations for the NFA in Figure 2 when given the input string...-6.:.-c. 2. Describe the language recognised by the NFA in Figure Consider the NFA where *,'-6 /.: 2" 3"45 7, is given by the following table: a a b 45-6,'-"45 8 b -,'-" /. 2" 3"4 ;. ;,'-"4 ;,5.:4 3 ;,<=4 (a) Give a string beginning with a that is not accepted by. (b) Construct a DFA that accepts the same language as. 4. Let be an alphabet. Prove that every finite language?ƒ C is regular. Don Sannella 8

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