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.

Deterministic 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++

// C++ program to implement DFS that accepts
// all string that do not end with "THE"
#include <iostream>
using namespace std;

// 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 if (c == 't' || c == 'T')
        dfa = 1;
      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(string str)
{

    // Store length of string
    int len = str.length();

    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()
{
    string str = "forTHEgeeks";
    if (isAccepted(str) == true)
        cout << "ACCEPTED\n";
    else
        cout << "NOT ACCEPTED\n";

    return 0;
}

// This code is contributed by ShubhamSingh10

C

// C 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 if (c == 't' || c == 'T')
        dfa = 1;
      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;
}

Java

// Java program to implement DFS that accepts
// all string that do not end with "THE"
import java.util.*;

class GFG
{

// dfa tells the number associated
// with the present state
static int dfa = 0;

// This function is for
// the starting state (zeroth) of DFA
static 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
static 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
static 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
static 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;
}

static boolean isAccepted(char str[])
{
    // store length of string
    int len = str.length;

    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
public static void main(String[] args)
{
    char str[] = "forTHEgeeks".toCharArray();
    if (isAccepted(str) == true)
        System.out.println("ACCEPTED\n");
    else
        System.out.println("NOT ACCEPTED\n");
}
}

/* This code is contributed by PrinciRaj1992 */

Python3

# Python3 program to implement DFS that accepts
# all string 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 string
    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

C#

// C# program to implement DFS that accepts
// all string that do not end with "THE"
using System;

class GFG
{

// dfa tells the number associated
// with the present state
static int dfa = 0;

// This function is for
// the starting state (zeroth) of DFA
static 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
static 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
static 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
static 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;
}

static bool isAccepted(char []str)
{
    // store length of string
    int len = str.Length;

    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
static public void Main ()
{
    char []str = "forTHEgeeks".ToCharArray();
    if (isAccepted(str) == true)
        Console.WriteLine("ACCEPTED\n");
    else
        Console.WriteLine("NOT ACCEPTED\n");
}
}

/* This code is contributed by ajit. */

PHP

<?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
?>

Javascript

<script>

    // Javascript program to
    // implement DFS that accepts
    // all string that do not end
    // with "THE"

    // dfa tells the number associated
    // with the present state
    let dfa = 0;

    // This function is for
    // the starting state (zeroth) of DFA
    function start(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
    function state1(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
    function state2(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
    function state3(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;
    }

    function isAccepted(str)
    {
        // store length of string
        let len = str.length;

        for (let 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);
    }

    let str = "forTHEgeeks".split('');
    if (isAccepted(str) == true)
        document.write("ACCEPTED");
    else
        document.write("NOT ACCEPTED");

</script>

Output :

ACCEPTED

Time Complexity: O(n)
Auxiliary Space: O(1) since it is using constant space