# Q6B · Hobbs: Ravi and Suresh

Subject: Natural Language Processing

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/nlp/#tee/papers/final-2025/final-2025-q6b

Canonical: https://deep-learning-study-room.vercel.app/read/nlp/tee/papers/final-2025--final-2025-q6b/

Actual final examination · 2 December 2025 · 100 marks · 3 hours

Question. i. Explain Hobb’s Algorithm. Discuss the steps with help of example and state its limitation.  
ii. Sentence: Ravi met Suresh at the station after he arrived from Delhi. Using Hobbs’ algorithm, determine the most likely antecedent of the pronoun “he”. Show the steps of traversal according to the algorithm. [10 marks]

### Answer

### i. Method

**Hobbs’ algorithm** is a syntax-based pronoun-resolution procedure that searches constituency parse trees in a specified order. It proposes noun phrases as antecedents and filters incompatible candidates, for example by number and gender agreement.

**Read the tree labels first:** NP means noun phrase; S means sentence/clause; VP means verb phrase. X is the current NP or S search node, and p is the upward path used to reach it. Breadth-first means shallower nodes before deeper nodes; left-to-right fixes the order at the same depth. An N-bar (N′) node is the noun-centred part of an NP in a parse using that label.

### Complete ordered procedure

1. **Locate the pronoun.** Start at the NP immediately dominating the pronoun: the smallest NP containing it.
2. **Make the first climb.** Move upward to the first NP or S above the starting NP. Call it X and record the path p.
3. **Search initially to the left.** Traverse branches below X to the left of p, breadth-first and left-to-right. Propose an NP only if another NP or S lies between that candidate and X. Check agreement and other applicable constraints for each proposed candidate; accept the first eligible compatible one.
4. **Check the highest sentence.** If X is the highest S and no candidate has been accepted, search previous sentence trees, most recent first. In each, traverse breadth-first and left-to-right and propose NPs. Accept the first eligible compatible candidate; if the available context contains none, report no resolved antecedent.
5. **Otherwise climb again.** Move from X to the next NP or S above it. Make that node the new X and record the new upward path p.
6. **Test a higher NP.** If the new X is an NP, propose X only when p did not pass through its nominal N-bar child. Apply the same compatibility checks. Use the labels in the supplied parse; do not invent a missing N-bar branch.
7. **Search left again.** Traverse the new X’s branches to the left of p, breadth-first and left-to-right, proposing NPs. The extra intervening-NP/S requirement in Step 3 belongs to the initial search.
8. **Search permitted right branches.** If X is an S, traverse branches to the right of p, breadth-first and left-to-right, but do not descend below any NP or S encountered. Propose the encountered eligible NPs and test their compatibility.
9. **Repeat or finish.** Return to Step 4 until a compatible candidate is accepted or the available sentence trees are exhausted. Record the candidate order and why each candidate was accepted or rejected.

**Why the initial restriction matters:** in “John saw him”, the subject NP John is directly under the clause S, with no additional NP or S between them. Step 3 therefore does not propose John as the ordinary local antecedent of him. A reflexive such as himself is a different grammatical case. The worked question below shows the specific parse and traversal; selecting an antecedent is not simply choosing the nearest name.

### ii. Apply the method

**Given:** the pronoun he occurs as the subject of the subordinate clause “after he arrived from Delhi”. Assume that this clause is attached to the main verb phrase. The relevant main-clause NPs are Ravi, Suresh and the station.

**To find:** the first eligible singular-male antecedent in Hobbs’ syntactic search order.

```
S (main)
├── NP: Ravi                    ← first higher-left candidate
└── VP
    ├── VP: met [NP Suresh] at [NP the station]
    └── SBAR: after
        └── S (embedded)        ← first X
            ├── NP: he          ← start
            └── VP: arrived from Delhi
```

### Step 1. Start at the pronoun NP

Locate NP “he” inside the after-clause. Move to that clause’s S node. No earlier noun phrase in this small clause provides a candidate.

### Step 2. Move to the higher sentence

The embedded S is not the highest S, so Step 5 climbs through SBAR and VP to the main S. Step 7 now searches to the left of that new path. This is the later left search, so the extra initial-search restriction from Step 3 is not imposed again.

### Step 3. Examine candidates in structural order

The main subject NP Ravi is encountered before the deeper object NP Suresh in the relevant breadth-first search. Ravi is singular and compatible with “he”. “the station” would fail the ordinary person constraint.

### Step 4. Choose the first compatible candidate

Select Ravi under the assumed parse and agreement settings. Suresh remains a possible interpretation in ordinary language, but the syntactic preference chooses Ravi first.

### Limitations

- The result depends on the accuracy and attachment choices of the parse tree.
- Syntactic ordering alone does not model all semantic plausibility, discourse focus or world knowledge.
- Agreement can eliminate candidates but cannot distinguish two compatible male singular people.
- Long-distance references and sentences with multiple possible parses can remain ambiguous.

**Final answer:** Hobbs’ syntactic preference is **Ravi** for this analysis. This is an algorithmic selection, not proof of the speaker’s intended referent.

**Yaad rakho:** Nearest naam blindly nahi: tree ki search order aur agreement follow karo.