Construct DFA for Regular Expression a+aa*b in C

Design a Deterministic Finite Automata (DFA) for accepting the language L = (a+aa*b)* If the given string is accepted by DFA, then print “string is accepted”. Otherwise, print “string is rejected”.

Example 1

Input: Enter Input String
      aaaba
Output: String Accepted.

Explanation − The given string is of the form (a+aa*b)* as the first character is a and it is followed by a or ab.

Example 2

Input: Enter Input String
   baabaab
Output: String not Accepted.

The DFA for the given regular expression (a+aa*b) is −

Explanation −

• If the first character is always a, then traverse the remaining string and check if any of the characters is a or ab.

• If the first character in the string is ‘b’ then it prints “string is rejected”.

• If there exists any character other than a or b while traversing, then it prints “entered value is wrong”.

• Otherwise, print “string is accepted”.

Program

Following is the C program to construct DFA for Regular Expression (a+aa*b)* −

#include<stdio.h>
#include<conio.h>
#include<strings.h>
void main() {
   int table[2][2],i,j,l,status=0,success;
   char input[100];
   printf("To implementing DFA of language (a+aa*b)*
 Enter Input String:”);
   table[0][0]=1;
   table[0][1]=-1;
   table[1][0]=1;
   table[1][1]=0;
   scanf("%s",input);
   l=strlen(input);
   for (i=0;i<l;i++) {
      if(input[i]!='a'&&input[i]!='b') {
         printf("The entered Value is wrong");
         getch();
         exit(0);
      }
      if(input[i]=='a')
      status=table[status][0]; else
      status=table[status][1];
      if(status==-1) {
         printf("String not Accepted");
         break;
      }
   }
   if(i==l)
      printf("String Accepted");
   getch();
}

Output

When the above program is executed, it gives the following output −

Run 1:
To implementing DFA of language (a+aa*b)*
Enter Input String:cbsd
The entered Value is wrong.
Run 2:
To implementing DFA of language (a+aa*b)*
Enter Input String:abbababa
String not Accepted.
Run 3:
To implementing DFA of language (a+aa*b)*
Enter Input String:babbaab
String not Accepted.