NFA to DFA conversion algorithm with solved example


Finite automata include two different classes Deterministic Finite Automata (DFA) and the Non deterministic Finite Automata (NFA). These are converted from one to another i.e. Non-Deterministic Finite Automata to Deterministic Finite Automata (NFA to DFA) conversion and Deterministic Finite Automata to Non-Deterministic Finite Automata (DFA to NFA) conversion. Some more enhancement of the NFA machine is NFA with ε moves i.e. (ε-NFA) can be converted into DFA and vice versa. A DFA is directly converted into a Regular expression. The below diagram explains all conversions of Finite Automata conversion.

NFA to DFA & DFA to NFA conversion block diagram

Figure (1) : (NFA→ DFA) and (DFA→ NFA) conversion block diagram

Non Deterministic Finite Automata to Deterministic Finite Automata (NFA to DFA conversion) :

The goal is to construct a Deterministic Finite Automata (DFA) from a given Non-Deterministic Finite Automata (DFA) machine which is much faster in recognition of an input string. The main idea behind this conversion is to construct a DFA machine in which each state will correspond to a set of NFA states.

Worst Case complexity of (NFA→ DFA) conversion:

The worst-case complexity of NFA to DFA conversion is O(2N) of sets of states. The DFA can have exponentially many more states than the NFA in rare cases.

NFA to DFA conversion Algorithm:

A NFA                         S = States = { s0, s1, …, sN} = SNFA
                                  δ = Move function = MoveNFA
                                  Move’(S, a) → Set of states

A DFA                         S = States = {?, ?, …, ?} = SDFA
                                  δ = Move function = MoveDFA
                                  Move(s, a) → Single state from SDFA

Main idea:
Each state in SDFA will be a set of states from the NFA SDFA = { {…}, {…}, …, {…} }
(The name of the states in constructed DFA is arbitrary and can be changed later if desired.)


Construct a transition table TransitionD. Each DFA state is a set of NFA states. TransitionD simulates in parallel all possible moves N can make on a given string.

Operations to keep track of sets of NFA states:

ε Closure (S):
Set of states reachable from state S via epsilon.

ε Closure (T):
Set of states reachable from any state in set T via epsilon.

move (T, a):
Set of states to which there is an NFA transition from states in T on a symbol a.


initially, ε-Closure (S0) in TransitionD.
While unmarked state T in TransitionD
    mark T
for each input symbol ‘a’
do u = ε Closure (T, a)
If u is not in TransitionD
then add u to TransitionD
Transition [T, a] = U

The following algorithm shows a computation of ε -Closure function.
Push all states in T onto the stack.
initialize ε-Closure (T) to T
while the stack is not empty
do pop top element t
for each state u with ε-edge
t to u
do If u is not in ε-Closure(T)
do add u ε Closure (T)
push u onto the stack

Remarkable Steps to convert NFA→ DFA:

  1. Construct the transition table of the given NFA machine.
  2. Scan the next states column in the transition table from the initial state to the final state.
  3. If any of the next states consists of more than one state on the single input alphabet. Then merge them and make it a new state. Place this newly constructed state in the DFA transition table as the present state.
  4. The next state of this newly constructed state on the input alphabet will be the summation of each next state which parts in the NFA transition table.
  5. Repeat steps 2 to step 4 until all the states in the NFA transition table will be scanned completely.
  6. The final transition table must have a single next state at a single input alphabet.


NFA to DFA conversion

Figure (1): NFA

Construct the equivalent DFA of the NFA given in figure (1).
Step 1: Transition Table of NFA from Figure (1):

Present State Next State
0 1
→q0 {q2} Φ
q1 Φ {q0, q2}
q2* {q0, q1} {q0}

Step 2: Transition Table of DFA:

Present State Next State
0 1
→{q0} {q2} Φ
{q2} {q0, q1} {q0}
{q0, q1} {q2} {q0, q2}
{q0, q2} {q0, q1, q2} {q0}
{q0, q1, q2} {q0, q1, q2} {q0}

NFA to DFA conversion

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Hello I am Er Parag Verma. I am tech blogger, Professor and Entrepreneur. I am on the mission to change the pattern of learning to make it easy, valuable and advance.



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