ISRO CSE 2018 | Question: 64

Question

A particular BNF definition for a "word is given by the following rules.

<word>      :: = <letter> | <letter> <charpair> | <letter> <intpair>
<charpair>  :: = <letter> <letter> | <charpair> <letter> <letter>
<intpair>   :: = <integer> <integer> | <intpair> <integer> <integer>
<letter>    :: = a | b | c | ...... | Y | Z
<integer>   :: = 0 | 1 | 2 | ...... | 9

Which of the following lexical entries can be derived from $\textsf{< word >}?$

1. pick
2. picks
3. $\textsf{c44}$

• A. I, II and III
• B. I and II only
• C. I and III only
• D. II and III only

Comments

The grammar is generating strings of odd length ... So option a can be eliminated in first place..

Answer 1

option d

Answer 2

pick can't be generated other are generated hence ans should be B

Comments

yes pick can't be generated.So D is correct choice.isn't it?

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yes option D is correct answer.

Answer 3

Ans: D.

What I thought, "word" is having only 4 char. Which can't be derived from the grammar.

"words" is having 5 char, which is odd, so can be derived from the grammar.

Also c22 can be drived.

So ans is II and III, implies option D.

Comments

Can you please explain the procedure to solve this type of problem?

Answer 4

Given grammar :  < word > :: =< letter > I < letter > < charpair > I < letter >< intpair >

Which means every word must start with a letter.

 

< charpair > :: =< letter >< letter > I < charpair >< letter >< letter >

Adds 2 letters at a time. 

Hence, Only odd number of letters are allowed.

 

< intpair > :: = < integer >< integer > I < intpair >< integer >< integer >

Adds 2 digits at a time.

Hence, Only even number of digits are allowed.

 

Hence, Ans  (D) II and III only