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Design a DFA that accepts a string containing 3 a’s and 3 b’s
  • Last Updated : 14 Dec, 2020

Problem Statement: Design a Definite Finite Automata for accepting the permutation of Three a’s and Three b’s over the input {a, b} 

Input: S = “aaabbb” 
Output: Accepted 
Explanation
The input has three a and three b.

Input: S = “abababa” 
Output: Accepted 
Explanation
The input has three a and three b. 

To design a DFA we need to check the input character by character. These are a few steps that should be kept in mind while designing a DFA. 

  • Think of all most of the possible input that can be accepted (like, in this example, aaabbb, bbbaaa, ababab, abaabb…etc).
  • Create an initial transition state.
  • Do transition on each input alphabet to other transition states.
  • Reach a final state such that in each step the language used was accept(like, in this example, an equal number of a and b).

Designing steps: 
Step 1: Create an initial state “A” which is indicated by —>. 
Step 2: Think of the possible string the can aaabbb or bbbaaa which can occur by transition like,  



  • The transition of input ‘a’ from state “A” to state “B”
  • The transition of input ‘a’ from state “B” to state “C”
  • The transition of input ‘a’ from state “C” to state “D”
  • The transition of input ‘a’ from state “D” to dead state “Q2”
  • The transition of input ‘b’ from state “D” to state “E”
  • The transition of input ‘b’ from state “E” to state “F”
  • The transition of input ‘b’ from state “F” to state “G”
  • The transition of input ‘b’ from state “G” to dead state “Q2”

Step 3: Now work on a possibility like b’s comes after two a’s for example aababb, aabbab. 

  • The transition of ‘b’ from state “C” to state “J”
  • The transition of ‘a’ from state “J” to state “E”
  • The transition of ‘b’ from state “J” to state “M”
  • The transition of ‘a’ from state “M” to state “F”
  • The transition of ‘b’ from state “M” to state “P”

The transition is shown in the diagram. 

Step-4: Now thinking of the possibility that starts with ‘aba’ and then fulfills the condition of the given language. For this following transitions to be done: 

  • The transition of ‘b’ from state “B” to state “I”
  • The transition of ‘a’ from state “I” to state “J”

Step-5: Now thinking of the possibility that starts with ‘abb’ and then fulfills the condition of the given language. For this following transitions to be done:

  • The transition of ‘b’ from state “I” to state “L”
  • The transition of ‘a’ from state “L” to state “M”



Step-6: Now there is a possibility that after reading one ‘a’ it reads all 3 b’s and then end execution by reading the remaining 2 a’s. For the transition of ‘b’ from state “L” to “O”. 

Step-7: there is a possibility that it reads ‘a’ after reading one ‘b’ and then it reads any combination of ‘a’ and ‘b’ so that string contains 3 a’s and 3 b’s.For this transition of ‘a’ is done from state”H” to state “I”. 

Step-8: Now, think about the strings starts with two b’s and the go for any combination of string to get the desired result for this we do a transition of ‘a’ from stare “K” to state “L”. 

Step-9: Now input alphabet ‘a’ of states D, E, F, G, and ‘b’ of states G, N, O, P are left for transitions.AS we have done all possibilities of the above language so the remaining transition will go to dead states “Q1″ and Q2”. 

  • The transition of input ‘a’ from state “D” to dead state “Q1”
  • The transition of input ‘a’ from state “E” to dead state “Q1”
  • The transition of input ‘a’ from state “F” to dead state “Q1”
  • The transition of input ‘a’ from state “G” to dead state “Q1”
  • The transition of input ‘b’ from state “G” to dead state “Q2”
  • The transition of input ‘b’ from state “N” to dead state “Q2”
  • The transition of input ‘b’ from state “O” to dead state “Q2”
  • The transition of input ‘b’ from state “P” to dead state “Q2”

Transition table of above DFA 

STATES INPUT (a) INPUT (b)
—> A (initial state) B H
B C I
C D J
D Q2 (dead state) E
E Q2 (dead state) F
F Q2 (dead state) G*(final state)
G* (final state) Q2 (dead state) Q1 (dead state)
H I K
I J L
J E M
K L N
L M O
M F P
N O Q1 (dead state)
O P Q1 (dead state)
P G* (final state) Q1 (dead state)

Below is the implementation of the DFA:

Java

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// Java implementation of the
// DFA of permutation of three
// a's and three b's
import java.util.*;
 
class GFG{
   
// State A
static void stateA(String n)
{
  if (n.charAt(0) == 'a')
    stateB(n.substring(1));
  else if (n.charAt(0) == 'b')
  {
    stateH(n.substring(1));
  }
 }
   
// State B
static void stateB(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateC(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateI(n.substring(1));
  }
}
    
// State C
static void stateC(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {
    if (n.charAt(0) == 'a')
      stateD(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateJ(n.substring(1));
  }
}
    
// State D
static void stateD(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else  
  {
    if (n.charAt(0) == 'a')
      stateQ2(n);
    else if (n.charAt(0) == 'b')
      stateE(n.substring(1));
  }
}   
       
// State E
static void stateE(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateQ2(n);
    else if (n.charAt(0) == 'b')
      stateF(n.substring(1));
  }
}   
    
// State F
static void stateF(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {  
    if (n.charAt(0) == 'a')
      stateQ2(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateG(n.substring(1)); 
  }
    
// State G
static void stateG(String n)
{
  if (n.length() == 0)
    System.out.print("String Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateQ2(n);
    else if (n.charAt(0) == 'b')
      stateQ2(n); 
  }
}
    
// State H
static void stateH(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateI(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateK(n.substring(1));
  }
    
// State I
static void stateI(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateJ(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateL(n.substring(1));
  }
}
   
// State J
static void stateJ(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {
    if (n.charAt(0) == 'a')
      stateE(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateM(n.substring(1));        
  }
}
               
// State K
static void stateK(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else  
  {
    if (n.charAt(0) == 'a')
      stateL(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateN(n.substring(1)); 
  }
}
              
// State L
static void stateL(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {  
    if (n.charAt(0) == 'a')
      stateM(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateO(n.substring(1)); 
  }
}
  
// State M
static void stateM(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {
    if (n.charAt(0) == 'a')
      stateF(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateP(n.substring(1));           
  }
}
  
// State N
static void stateN(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else  
    if (n.charAt(0) =='a')
      stateO(n.substring(1));
  else if (n.charAt(0) == 'b')
    stateQ1(n);
}
  
// State Q
static void stateO(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {   
    if (n.charAt(0) == 'a')
      stateP(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateQ1(n);
  }
}
           
// State P
static void stateP(String n)
{
  if (n.length() == 0)
    System.out.print("String Not Accepted");
  else
  {
    if (n.charAt(0) == 'a')
      stateG(n.substring(1));
    else if (n.charAt(0) == 'b')
      stateQ1(n.substring(1));    
  }
}
               
// State Q1
static void stateQ1(String n)
{
  System.out.print("String Not Accepted");
}
   
// State Q2
static void stateQ2(String n)
{
  System.out.print("String Not Accepted");
}
       
// Driver code
public static void main(String[] args)
{
   
  // Take String input
  String n = "abaabb";
  
  // Call stateA
  // to check the input
  stateA(n);
}
}
 
// This code is contributed by pratham76

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Python3

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# Python3 implementation of the
# DFA of permutation of three
# a's and three b's
 
# State A
def stateA(n):
    if(n[0]=='a'):
        stateB(n[1:])
    elif (n[0]=='b'):
        stateH(n[1:])
 
# State B
def stateB(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateC(n[1:])
        elif (n[0]=='b'):
            stateI(n[1:])
 
# State C
def stateC(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateD(n[1:])
        elif (n[0]=='b'):
            stateJ(n[1:])
 
# State D
def stateD(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateQ2(n)
        elif (n[0]=='b'):
            stateE(n[1:])
         
# State E
def stateE(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateQ2(n)
        elif (n[0]=='b'):
            stateF(n[1:])  
     
# State F
def stateF(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateQ2(n[1:])
        elif (n[0]=='b'):
            stateG(n[1:])    
            
# State G
def stateG(n):
    if(len(n)== 0):
        print("String Accepted")
    else:   
        if(n[0]=='a'):
            stateQ2(n)
        elif (n[0]=='b'):
            stateQ2(n) 
             
# State H
def stateH(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateI(n[1:])
        elif (n[0]=='b'):
            stateK(n[1:])
            
# State I
def stateI(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateJ(n[1:])
        elif (n[0]=='b'):
            stateL(n[1:])
 
# State J
def stateJ(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateE(n[1:])
        elif (n[0]=='b'):
            stateM(n[1:])        
             
# State K
def stateK(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateL(n[1:])
        elif (n[0]=='b'):
            stateN(n[1:]) 
             
# State L
def stateL(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateM(n[1:])
        elif (n[0]=='b'):
            stateO(n[1:])  
             
# State M
def stateM(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateF(n[1:])
        elif (n[0]=='b'):
            stateP(n[1:])           
 
# State N
def stateN(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateO(n[1:])
        elif (n[0]=='b'):
            stateQ1(n)
             
# State Q
def stateO(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateP(n[1:])
        elif (n[0]=='b'):
            stateQ1(n)
             
# State P
def stateP(n):
    if(len(n)== 0):
        print("String Not Accepted")
    else:   
        if(n[0]=='a'):
            stateG(n[1:])
        elif (n[0]=='b'):
            stateQ1(n[1:])    
             
# State Q1
def stateQ1(n):
    print("String Not Accepted")
 
# State Q2
def stateQ2(n):
    print("String Not Accepted")
 
# take string input
n = "abaabb"
 
# call stateA
# to check the input
stateA(n)

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C#

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// C# implementation of the
// DFA of permutation of three
// a's and three b's
using System;
using System.Collections;
class GFG{
  
// State A
static void stateA(string n)
{
  if(n[0] == 'a')
    stateB(n.Substring(1));
  else if (n[0] == 'b')
  {
    stateH(n.Substring(1));
  }
 }
  
// State B
static void stateB(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {   
    if(n[0] == 'a')
      stateC(n.Substring(1));
    else if (n[0] == 'b')
      stateI(n.Substring(1));
  }
}
   
// State C
static void stateC(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {
    if(n[0] == 'a')
      stateD(n.Substring(1));
    else if (n[0] == 'b')
      stateJ(n.Substring(1));
  }
}
   
// State D
static void stateD(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else   
  {
    if(n[0] == 'a')
      stateQ2(n);
    else if (n[0] == 'b')
      stateE(n.Substring(1));
  }
}   
      
// State E
static void stateE(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {   
    if(n[0] == 'a')
      stateQ2(n);
    else if (n[0] == 'b')
      stateF(n.Substring(1));
  }
}   
   
// State F
static void stateF(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {  
    if(n[0] == 'a')
      stateQ2(n.Substring(1));
    else if (n[0] == 'b')
      stateG(n.Substring(1)); 
  }
   
// State G
static void stateG(string n)
{
  if(n.Length == 0)
    Console.Write("String Accepted");
  else
  {   
    if(n[0] == 'a')
      stateQ2(n);
    else if (n[0] == 'b')
      stateQ2(n); 
  }
}
   
// State H
static void stateH(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {   
    if(n[0] == 'a')
      stateI(n.Substring(1));
    else if (n[0] == 'b')
      stateK(n.Substring(1));
  }
   
// State I
static void stateI(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {   
    if(n[0] == 'a')
      stateJ(n.Substring(1));
    else if (n[0] == 'b')
      stateL(n.Substring(1));
  }
}
  
// State J
static void stateJ(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {
    if(n[0] == 'a')
      stateE(n.Substring(1));
    else if (n[0] == 'b')
      stateM(n.Substring(1));        
  }
}
              
// State K
static void stateK(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else   
  {
    if(n[0] == 'a')
      stateL(n.Substring(1));
    else if (n[0] == 'b')
      stateN(n.Substring(1)); 
  }
}
             
// State L
static void stateL(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {  
    if(n[0] == 'a')
      stateM(n.Substring(1));
    else if (n[0] == 'b')
      stateO(n.Substring(1)); 
  }
}
 
// State M
static void stateM(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else{
    if(n[0] == 'a')
      stateF(n.Substring(1));
    else if (n[0] == 'b')
      stateP(n.Substring(1));           
  }
}
 
// State N
static void stateN(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else   
    if(n[0] =='a')
      stateO(n.Substring(1));
  else if (n[0] == 'b')
    stateQ1(n);
}
 
// State Q
static void stateO(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {   
    if(n[0] == 'a')
      stateP(n.Substring(1));
    else if(n[0] == 'b')
      stateQ1(n);
  }
}
          
// State P
static void stateP(string n)
{
  if(n.Length == 0)
    Console.Write("String Not Accepted");
  else
  {
    if(n[0] == 'a')
      stateG(n.Substring(1));
    else if (n[0] == 'b')
      stateQ1(n.Substring(1));    
  }
}
              
// State Q1
static void stateQ1(string n)
{
  Console.Write("String Not Accepted");
}
  
// State Q2
static void stateQ2(string n)
{
  Console.Write("String Not Accepted");
}
      
// Driver code
public static void Main (string[] args)
{
  // Take string input
  string n = "abaabb";
 
  // Call stateA
  // to check the input
  stateA(n);
}
}
 
// This code is contributed by rutvik_56

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Output: 

String Accepted

 

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