Lecture 3 : DFA AND NFA

Question 1

What is regular expression

Answer 1

Regular expressions (REs) are a language for expressing patterns

Question 2

What is L(s)?

Answer 2

L(s) is known as the “language of s”.

Question 3

What is s*?

Answer 3

s* is a RE such that L(s*) = L(s) concatenated zero or more times.

Question 4

What is s?

Answer 4

S? indicates that s is optional.
S? can be written as (S|e).

Question 5

What is s+

Answer 5

s+ indicates that s is repeated one or more times
s+ can be written as ss*

Question 6

What is [a-z]

Answer 6

[a-z] indicates any character in that range.
[a-z] can be written as (a|b|…|z)

Question 7

What is r1|r2 and r1.r2?

Answer 7

r1|r2 means either r1 or r2
r1.r2 means r1 followed by r2

Question 8

What is [^x]?

Answer 8

[ˆx] indicates any character except one.[ˆx] can be written as Ʃ- x

Question 9

A DFA has 5 tuples?

Answer 9

{Q,E,q0,F,&}
Q: Set of all states
E: Finite set of input symbols
q0: initial state
F: Set of all final state
&: transition function

Question 10

Common ways to solve ambiguity in nfa

Answer 10

Crystal ball interpretation
Many-worlds interpretation

Question 11

Describe Crystal ball interpretation

Answer 11

The Crystal ball interpretation suggests that the NFA somehow “knows” what the best choice is, by some means external to the NFA itself.