DFA for Strings not ending with “THE”

Problem – Accept Strings that not ending with substring “THE”. Check if a given string is ending with “the” or not. The different forms of “the” which are avoided in the end of the string are:

"THE", "ThE", "THe", "tHE", "thE", "The", "tHe" and "the"

All those strings that are ending with any of the above mentioned forms of “the” are not accepted.

Determinitis finite automata (DFA) of strings that not ending with “THE” –
The initial and starting state in this dfa is Qo

Approach used in the program –
In this program, consider the 4 states to be 0, 1, 2 and 3. Now let us take a variable named DFA which will be initially 0. Whenever any transition takes place, it will update the value of DFA with the number associated with new state.



Example : If a transition occurs from state 0 to state 1 then the value of DFA will be updated to 1. If a transition occurs from state 2 to state 3 then the value of dfa will be updated to 3. In this way, apply this algorithm on entire string and if in the end, then reach state 0, 1 or 2 then our string will be accepted otherwise not.

Input : XYzabCthe
Output : NOT ACCEPTED

Input :  Themaliktth
Output :  ACCEPTED

C++

filter_none

edit
close

play_arrow

link
brightness_4
code

// CPP program to implement DFS that accepts
// all string that do not end with "THE"
#include <stdio.h>
#include <string.h>
  
// dfa tells the number associated
// with the present state
int dfa = 0;
  
// This function is for
// the starting state (zeroth) of DFA
void start(char c)
{
    // On receiving 'T' or 't' goto first state (1)
    if (c == 't' || c == 'T')
        dfa = 1;
}
  
// This function is for the first state of DFA
void state1(char c)
{
    // On receiving 'T' or 't' goto first state (1)
    if (c == 't' || c == 'T')
        dfa = 1;
  
    // On receiving 'H' or 'h' goto second state (2)
    else if (c == 'h' || c == 'H')
        dfa = 2;
  
    // else goto starting state (0)
    else
        dfa = 0;
}
  
// This function is for the second state of DFA
void state2(char c)
{
    // On receiving 'E' or 'e' goto third state (3)
    // else goto starting state (0)
    if (c == 'e' || c == 'E')
        dfa = 3;
    else
        dfa = 0;
}
  
// This function is for the third state of DFA
void state3(char c)
{
    // On receiving 'T' or 't' goto first state (1)
    // else goto starting state (0)
    if (c == 't' || c == 'T')
        dfa = 1;
    else
        dfa = 0;
}
  
bool isAccepted(char str[])
{
    // store length of string
    int len = strlen(str);
  
    for (int i=0; i < len; i++) {
            if (dfa == 0)
                start(str[i]);
  
            else if (dfa == 1)
                state1(str[i]);
  
            else if (dfa == 2)
                state2(str[i]);
  
            else
                state3(str[i]);        
    }
  
    return (dfa != 3);
}
  
// driver code
int main()
{
    char str[] = "forTHEgeeks";
    if (isAccepted(str) == true)
        printf("ACCEPTED\n");
    else
        printf("NOT ACCEPTED\n");
    return 0;
}

chevron_right






                        filter_none









Python3














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















# Python3 program to implement DFS that accepts  


# all stringing that do not end with "THE"  


  


# This function is for the starting 


# state (zeroth) of DFA  


def start(c): 


      


    # On receiving 'T' or 't' goto  


    # first state (1)  


    if (c == 't' or c == 'T'): 


        dfa=1


  


# This function is for the first state of DFA  


def state1(c):  


      


    # On receiving 'T' or 't' goto first state (1)  


    if (c == 't' or c == 'T'): 


        dfa = 1


  


    # On receiving 'H' or 'h' goto second state (2)  


    elif (c == 'h' or c == 'H'): 


        dfa = 2


  


    # else goto starting state (0)  


    else: 


        dfa = 0


  


# This function is for the second state of DFA  


def state2(c): 


      


    # On receiving 'E' or 'e' goto third  


    # state (3) else goto starting state (0)  


    if (c == 'e' or c == 'E'): 


        dfa = 3


    else: 


        dfa = 0


          


# This function is for the third state of DFA  


def state3(c): 


      


    # On receiving 'T' or 't' goto first  


    # state (1) else goto starting state (0)  


    if (c == 't' or c == 'T'): 


        dfa = 1


    else: 


        dfa = 0


  


def isAccepted(string): 


      


    # store length of stringing  


    length = len(string)  


  


    for i in range(length): 


        if (dfa == 0): 


            start(string[i]) 


        elif (dfa == 1): 


            state1(string[i]) 


        elif (dfa == 2): 


            state2(string[i]) 


        else: 


            state3(string[i])         


    return (dfa != 3)  


  


# Driver Code  


if __name__ == "__main__" : 


    string="forTHEgeeks"


      


    # dfa tells the number associated  


    # with the present state 


    dfa = 0


    if isAccepted(string): 


        print("ACCEPTED") 


    else: 


        print("NOT ACCEPTED") 


  


# This code is contributed by SHUBHAMSINGH10 



















                        chevron_right







                        filter_none









PHP














                                    filter_none




                                    edit

                                    close




                                    play_arrow




                                    link

                                    brightness_4

                                    code
                                




















<?php 


// PHP program to implement DFS 


// that accepts all string that 


// do not end with "THE" 


  


// dfa tells the number associated 


// with the present state 


$dfa = 0; 


  


// This function is for the 


// starting state (zeroth) of DFA 


function start($c) 


{ 


    global $dfa; 


    // On receiving 'T' or 't'  


    // goto first state (1) 


    if ($c == 't' || $c == 'T') 


        $dfa = 1; 


} 


  


// This function is for  


// the first state of DFA 


function state1($c) 


{ 


    global $dfa; 


    // On receiving 'T' or 't' 


    // goto first state (1) 


    if ($c == 't' || $c == 'T') 


        $dfa = 1; 


  


    // On receiving 'H' or 'h'  


    // goto second state (2) 


    else if ($c == 'h' || $c == 'H') 


        $dfa = 2; 


  


    // else goto starting state (0) 


    else


        $dfa = 0; 


} 


  


// This function is for 


// the second state of DFA 


function state2($c) 


{ 


    global $dfa; 


    // On receiving 'E' or 'e'  


    // goto third state (3) else 


    // goto starting state (0) 


    if ($c == 'e' || $c == 'E') 


        $dfa = 3; 


    else


        $dfa = 0; 


} 


  


// This function is for  


// the third state of DFA 


function state3($c) 


{ 


    global $dfa; 


    // On receiving 'T' or 't'  


    // goto first state (1) else 


    // goto starting state (0) 


    if ($c == 't' || $c == 'T') 


        $dfa = 1; 


    else


        $dfa = 0; 


} 


  


function isAccepted($str) 


{ 


    global $dfa; 


    // store length of string 


    $len = strlen($str); 


  


    for ($i=0; $i < $len; $i++)  


    { 


            if ($dfa == 0) 


                start($str[$i]); 


  


            else if ($dfa == 1) 


                state1($str[$i]); 


  


            else if ($dfa == 2) 


                state2($str[$i]); 


  


            else


                state3($str[$i]);  


    } 


  


    return ($dfa != 3); 


} 


  


// Driver Code 


$str = "forTHEgeeks"; 


if (isAccepted($str) == true) 


    echo "ACCEPTED\n"; 


else


    echo "NOT ACCEPTED\n"; 


  


// This code is contributed by m_kit 


?> 



















                        chevron_right







                        filter_none











Output :


ACCEPTED





The time complexity of this program is O(n)



          
          
          
          

            

    

        My Personal Notes
        arrow_drop_up
    

    

        
        

            
            
        

    


Recommended Posts:
RishabhMalik
Check out this Author's contributed articles.
If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please Improve this article if you find anything incorrect by clicking on  the "Improve Article" button below.


Improved By :  jit_t, SHUBHAMSINGH10