TRANSITION DIAGRAM BASED LEXICAL ANALYZER and FINITE AUTOMATA Class date : 12 August, 2013 Prepared by : Karimgailiu R Panmei Roll no. : 11CS10020 GROUP NO. : 9

Stream of input characters (source) State Transition Machine 1 State Transition Machine 2 State Transition Machine 3 Token Lex Compiler pattern1 pattern 2 pattern 3

Simulating the transition machines : Input string : x 1, x 2,x 3 …… x n Scan the input using forward pointer Machine accepts or rejects based on the transition function Match the longest prefix of the input If accepted, token is produced

Two approach to simulate the machine : Approach 1 : Sequential simulation Sequentially simulate the transition machines If the previous machine fails, reset the forward pointer Start the next transition machine

Approach 2 : Parallel simulation Simulate all transition diagrams in parallel The machine stops when no match is found

Pattern : Define based on regular expression Lex compiler C Compiler a.out Pattern a.out Source code tokens generate state transition machines NFA : Advantage DFA : advantage

FINITE AUTOMATA Recognize regular languages Accepts or rejects a possible input string Two types : 1. Non deterministic finite automata(NFA) 2. Deterministic finite automata(DFA)

NFA Definition : N={ Σ, S, s o, F, Δ } where Σ : set of input symbol S : finite set of states s o : start state F : finite set of final states Δ : transition function (S × Σ → P(S)) where P(S) is the power set of S

More about NFA Given an input a, NFA allows to go from one state to a multiple state a ϵ-transition is allowed ϵ

DFA Definition : D={ Σ, S, s o, f, δ } where Σ : set of input symbol S : finite set of states s o : start state F : finite set of final states δ : transition function (δ : S × Σ → S)

More on DFA No ϵ-transition is allowed For a given input a, DFA allows to moves from one state to a single state a

Simulating DFA Input : - a string x - DFA with start state s o, accepting states F and transition function detect Output : “yes” if accepts and “no” if rejects

Algorithm : s= s o ; c= read_next_char(); //read_next_char() returns the next //character while(c!=‘\0’) { s=detect(s,c); c=read_next_char(); } if(s ϵ F) return yes; else return no;

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