Program to build DFA that accepts the languages ending with “01” over the characters {0, 1}

Given a binary string str, the task is to build a DFA that accepts the languages ending with “01” over the characters {0, 1}.

Examples:

Input: str = “00011101”
Output: String Accepted
Explanation:
As the given string ends with “01”, therefore, it is accepted.

Input: str = “00001111”
Output: String Rejected
Explanation:
As the given string ends with “11”, therefore, it is rejected.

Approach: To solve the above problem, below are the class state and member function needed:

• Pair 1: It deals with the first type of input i.e. 0 and links it with an object pointer to the next state.
• Pair 2: It deals with the second type of input i.e. 1 and links it with an object pointer to the next state.
• m_x: It defines whether a particular state is initial or final.

Class member functions are defined below:

• initialize(): This function takes the two pairs(used for two kinds of inputs) as parameters along with the state defining character(i or f).
• transition(): This acts as a transition table for the automata and with a particular input it goes from one state to next state.
• traverse(): This function takes the input string and passes it through the automata.
• check(): This function checks whether after the input has ended the end state is a final state or not. If it is final then the string is accepted otherwise rejected.

For this problem three states are needed. Hence, create three-class objects and initialize them with required values as:

• State 1: On input 0 it goes to State 2 and on input 1 it goes to itself.
• State 2: On input 0 it goes to itself and on input 1 it goes to State 3.
• State 3: On input 0 it goes to State 2 and on input 1 goes to State 1. Also, this is our final state.

Below is the implementation of the above approach:

C++14

// C++ program of a DFA that accepts
// all string ending with "01"

#include <iostream>
#include <string>
using namespace std;

// Class for automata
class State {
private:
    // Data members to store the input
    pair<int, State*> Pair1;
    pair<int, State*> Pair2;
    char m_x;

public:
    // Pointer to the state of automata
    static State* m_ptr;

    // Constructor to initialize state
    State()
    {
        Pair1.first = 0;
        Pair1.second = nullptr;
        Pair2.first = 0;
        Pair2.second = nullptr;
        m_x = ' ';
    }

    // Initialise pair1 and pair2
    // with state x
    void initialize(
        pair<int, State*> pair1,
        pair<int, State*> pair2, char x)
    {
        Pair1 = pair1;
        Pair2 = pair2;
        m_x = x;
    }

    // Passes a string through automata
    static void transition(int input);
    static void traverse(string& str, int n);

    // Checks if the last state
    // is final or not
    static void check();
};

// Pointer to the current
// state of automata
State* State::m_ptr{ nullptr };

// Function to provide state
// transition of automata
void State::transition(int input)
{
    if ((*m_ptr).Pair1.first == input)
        m_ptr = (*m_ptr).Pair1.second;
    else
        m_ptr = (*m_ptr).Pair2.second;
}

// Checks if the last state
// is final or not
void State::check()
{
    if ((*m_ptr).m_x == 'f')
        cout << "String Accepted\n"
             << endl;
    else
        cout << "String Rejected\n"
             << endl;
}

// Passes a string through automata
void State::traverse(string& str, int n)
{
    for (int i = 0; i < n; i++) {
        int x{ (int)str[i] - (int)'0' };
        transition(x);
    }
}

// Function to check if the given
// is accepted in DFA or not
void isAccepted(string str)
{
    // States of the automata
    State one, two, three;

    // Transition table for required
    // automata
    one.initialize({ 0, &two },
                   { 1, &one }, 'i');
    two.initialize({ 0, &two },
                   { 1, &three }, 'i');
    three.initialize({ 0, &two },
                     { 1, &one }, 'f');

    int length{ static_cast<int>(str.length()) };
    State::m_ptr = &one;

    // Function call
    State::traverse(str, length);
    State::check();
}

// Driver Code
int main()
{
    string str{ "00111101" };

    isAccepted(str);

    return 0;
}

Output:

String Accepted

Time Complexity: O(N)
Auxiliary Space: O(1)