LEX Code that accepts string having even number’s of ‘a’ over input alphabet {a,b}
Prerequisite : Designing Finite Automata
Introduction :
In this article, we will discuss the DFA in LEX Code that accepts the string having even number’s of ‘a’ over input alphabet {a, b}. with the help of example. Let’s discuss it one y one.
Problem Overview :
LEX Code that accepts the string having even number’s of ‘a’ over input alphabet {a, b}.
Example –
Input : aba Output: Accepted Input : ababba Output: Not Accepted Input: 23ab Output:Invalid Input:ab345 Output:Invalid Input:aabababa Output:Not Accepted
Approach :
LEX provides us with an INITIAL state by default. So to make a DFA, use this as the initial state of the DFA.
We define two more states – A and DEAD, where the DEAD state would be used if encountering a wrong or invalid input. When the user inputs an invalid character, move to DEAD state, and then print “Invalid”.
If the input string ends at A then display the message “Not Accepted”. Else if the input string ends at state INITIAL, then displays the message “Accepted”.
Note :
To compile the lex program we need to have a Unix system that has flex installed into it. Then we need to save the file with the .l extension.
Example - filename.l
Then after saving the program closes the Lex file and then open the terminal and write the following commands as follows.
lex filename.l cc lex.yy.c ./a.out
LEX CODE :
%{ %} %s A DEAD %% <INITIAL>a BEGIN A; <INITIAL>b BEGIN INITIAL; <INITIAL>[^ab\n] BEGIN DEAD; <INITIAL>\n BEGIN INITIAL; {printf("Accepted\n");} <A>a BEGIN INITIAL; <A>b BEGIN A; <A>[^ab\n] BEGIN DEAD; <A>\n BEGIN INITIAL; {printf("Not Accepted\n");} <DEAD>[^\n] BEGIN DEAD; <DEAD>\n BEGIN INITIAL; {printf("Invalid\n");} %% int yywrap() { return 1; } int main() { printf("Enter String\n"); yylex(); return 0; }
OUTPUT –