(5/5)

# DFA deterministic finite-state automaton

INSTRUCTIONS TO CANDIDATES
###### ANSWER ALL QUESTIONS

The following abbreviations are used in this paper.

DFA deterministic finite-state automaton

NDFA non-deterministic finite-state automaton

FSA finite-state automaton (either DFA or NDFA) NPDA non-deterministic pushdown stack automaton CFG context-free grammar

ID instantaneous description

TM Turing machine

1 (5 marks)

State the five components of the NDFA given below:

2 (10 marks)

Using a systematic procedure, convert the following NDFA into an equivalent DFA, and draw the resulting automaton. Show clearly all the steps involved.

3 (10 marks)

By constructing a tree, identify the distinguishable states for the DFA below. Hence draw a DFA which is equivalent to this one, but which has a minimal number of states. Note that this DFA is complete; you should ensure that your answer is also a complete DFA.

4 (5 marks)

Consider the DFA in Question 3 and convert it into a regular expression using a systematic procedure. [Hint: It may be easier to calculate the regular expression for the minimal DFA.]

5 (5 marks)

The company you are working for has a number of systems that exchange information in binary format. The company wishes to add a primitive error-detection scheme where prior to communicating a binary string, a system must attach either a 0 or a 1 to the end of the original string according to the following conditions:

The value attached is 0 if the number of 0s in the original string is greater than the number of 1s.

The value attached is 1 otherwise; that is if the number of 1s is greater than or equal to the number of 0s.

Your manager has asked you to implement a DFA that checks whether a binary string received by a system contains an error or not.

Your task here is to either produce such a DFA or prove to your boss that a DFA cannot be created to solve the task.

[Hint 1: If it is easier, you may first construct an NDFA and then convert it into a DFA.]

[Hint 2: You may use the pumping lemma to prove that the language is not regular, and therefore no DFA can be created.]

6 (5 marks)

Now, the company has decided to modify the error-detection schema. Prior to communicating a binary string, a system now must attach either a 0 or a 1 to the end of the original string according to the following conditions:

The value attached is 0 if the number of 1s in the original string is even;

The value attached is 1 otherwise; that is if the number of 1s in the original string is odd.

Your manager has asked you to implement a DFA that checks whether a binary string received by a system contains an error or not.

Your task here is to either produce such a DFA or prove to your boss that a DFA cannot be created to solve the task.

[Hint 1: If it is easier, you may first construct an NDFA and then convert it into a DFA.]

[Hint 2: You may use the pumping lemma to prove that the language is not regular, and therefore no DFA can be created.]

(5/5)

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