UGC NET CSE | December 2015 | Part 2 | Question: 43

Question

Which of the following statements is false?

• A. Top-down parsers are LL parsers where first L stands for left-to-right scan and second L stands for a leftmost derivation
• B. (000)* is a regular expression that matches only strings containing an odd number of zeroes, including the empty string
• C. Bottom-up parsers are in the LR family, where L stands for left-to-right scan and R stands for rightmost derivation
• D. The class of context-free languages is closed under reversal. That is, if L is any context-free language, then the language $L^R =\{w^R: w \varepsilon L\}$ is context-free

Answer 1

1. (000)* is a regular expression that matches only strings containing an odd number of zeroes, including the empty string also caontain 6, 12, 18, 24 which are even .

B is answer

Answer 2

• Top-down parsers are LL parsers where first L stands for left – to – right scan and second L stands for a leftmost derivation.Correct

 

• (000)* is a regular expression that matches only strings containing an odd number of zeroes, including the empty string.Incorrect (000)* will generate ε, 000, 000000, 000000000, …….. multiples of 3 which include odd and even strings, so this is incorrect.

 

• Bottom-up parsers are in the LR family, where L stands for left – to – right scan and R stands for rightmost derivation.Correct

 

• The class of context – free languages is closed under reversal. That is, if L is any context – free language, then the language L^R = {w^R: w∈L} is context – free.Correct

So, option (B) is correct.