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Implementing DFA in C

Deterministic Finite Automata (DFA) is good model for recognizing Regular Expression (RegEx). The result of a RegEx is a language. So, Simply we conclude that “DFA decides that is a specific string in a specific language or not”. From now on, I won’t get in details of DFA and RegEx any more. If you’d like to learn these, I recommend you to watch this video. Remaing part of the blogpost is about implementing a DFA in C.

Assume we have a DFA like below.

An Example of a DFA

I’d like to implement a algorithm for this specific DFA. Without any reflection, anyone could choke numerous loops while designing. Besides, you will only have a algorithm for this specific DFA. It is not the smart method. I want to show you a generic and wisely method.

Firstly, Make a transaction table for this DFA. Columns represents alphabet and Rows represent states. Each elements in matrix show state number to go.


In this table, We have clear information about where to go at initial state. For example, Next alphabet symbol is 1 while DFA is in S1. So, DFA have to go S1.

Let’s assume W means input string.

#include <stdio.h>
#include <string.h> //Used for only strlen() function

int main(void){
    int i; //Counter

    int TransactionTable[3][2] = { //Row=state, Column=alphabet
    {0, 1},
    {2, 0},
    {1, 2}

    int StartingState, AcceptingState, InitialState;

    StartingState = 0;
    AcceptingState = 0;
    InitialState = StartingState;

    char w[100] = "10001011101"; //String will be processed by DFA

    i = 0;
    while( i < strlen(w)){ // strlen(w) means size of string
        // w[i] is a char. for getting a value of a char, substracted by '0'
        InitialState = TransactionTable[InitialState][(w[i] - '0')];
        printf("InitialState: %d, SymbolToProcess: %d, #Transactions: %d\n", InitialState, w[i] - '0', i+1); //Only for information

    if( InitialState == AcceptingState ){
        printf("'%s' is Accepted!", w);
        printf("'%s' is Rejected.", w);

    return 0;

I passed the DFA as a Transaction Table Matrix. So, I have a generic solution for DFAs. That simple! If you’d like to make a implementation for different DFA, You have to just change TransactionTable matrix, StartingState and AcceptingState variables.