Natural Language Processing Week 5 NPTEL Assignment Answers 2025

NPTEL Natural Language Processing Week 5 Assignment Answers 2024

1. Which of the following are true?

• A) Given a CFG and its corresponding CNF, they produce different languages.
• B) It requires ‘2n-1’ productions or steps in CNF to generate a string w of length “n”.
• C) For a given grammar, there can be more than one CNF.
• D) None of the above

Answer: B, C
Explanation:

• CNF preserves the language of the original CFG, so A is false.
• In CNF, deriving a string of length n requires 2n−1 steps because of the binary tree structure of CNF productions — B is correct.
• Multiple equivalent CNF representations can exist for the same grammar — C is correct.

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2. Consider the CFG given below:
S → xSylV
V → Vz | ε

How many non-terminals should be added to convert the CFG into CNF?

• A) 2
• B) 4
• C) 5
• D) 3

Answer: C
Explanation:
To convert this grammar into CNF, we need to replace mixed terminals and non-terminals, break down RHS with more than 2 symbols, and handle nullable productions. This usually introduces around 5 additional non-terminals.

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3. In the above Q. 2, how many different null productions are there in the CFG to CNF converted form?

• A) 0
• B) 1
• C) 2
• D) 3

Answer: A
Explanation:
In CNF, all null (ε) productions are removed except possibly for the start symbol. So, after converting, null productions are typically eliminated — answer is 0.

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4. In the above Q. 2, how many different production rules/steps are there in the CFG to CNF converted form?

• A) 2
• B) 4
• C) 7
• D) 10

Answer: C
Explanation:
The original grammar is broken down into binary rules and terminals are separated, which increases the number of rules. This results in approximately 7 rules.

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5. Using CKY algorithm, find the probability score for the most probable tree for the sentence S₁ = “students play football with friends”.

• A) 6.06 × 10⁻⁴
• B) 1.62 × 10⁻⁶
• C) 2.73 × 10⁻³
• D) 4.33 × 10⁻⁶

Answer: B
Explanation:
The CKY algorithm uses dynamic programming to compute the highest scoring parse tree. From the precomputed probabilities, option B is the correct value.

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6. Using CKY algorithm, find the number of parse trees for the sentence S₂ = “students like painting” and the probability score for at least one of the probable trees.

• A) 1, 4.95 × 10⁻³
• B) 3, 0.36 × 10⁻³
• C) 2, 0.99 × 10⁻³
• D) 2, 0.54 × 10⁻³

Answer: C
Explanation:
The sentence “students like painting” has 2 valid parse trees, and one of them has a score of 0.99 × 10⁻³ based on the CKY table.

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7.

Answer: B

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8. Which of the following grammars are valid CNF?

• a) A → B; A → BCD; A → BC
• b) B → CD; B → b; B → ε
• c) C → c; C → c; C → c
• d) A → BC; A → a

Answer: D
Explanation:
CNF rules must be of the form: A → BC or A → a (where B and C are non-terminals and a is a terminal).

• a) has invalid rules (BCD — too many non-terminals).
• b) includes ε which is not allowed unless it’s the start symbol.
• c) is repetitive and doesn’t follow CNF strictly.
• d) follows CNF perfectly.