SemanticsDay 38

LING 681.02Computational Linguistics

Harry HowardTulane University

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Course organization

http://www.tulane.edu/~howard/NLP/

Analyzing the meaning of sentences

SLPP 10

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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The problem

Suppose we have a program that lets us type in a natural language question and gives us back the right answer:Which country is Athens in?Greece.

How hard is it to write such a program?

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Querying a database

City Country Population

athens greece 1368

bangkok thailand 1178

barcelona spain 1280

berlin east_germany 3481

birmingham united_kingdom 1112

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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SQL

SQL (Structured Query Language) is a language designed for retrieving and managing data in relational databases.

For example, executing the query below will pull out the value 'greece':SELECT Country FROM city_table WHERE City = 'athens'

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Parsing into SQL

Can we get the same effect using English as our input to the query system?

The feature-based grammar formalism described in Chapter 9 makes it easy to translate from English to SQL.

Each phrase structure rule is supplemented with a recipe for constructing a value for the feature sem.

We use the string concatenation operation + to splice the values for the child constituents to make a value for the parent constituent. S[SEM=(?np + WHERE + ?vp)] -> NP[SEM=?np] VP[SEM=?vp]

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Philosophical problem

Understanding = looking up in a database?What is a city?What is a country?What is a population?

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Depiction of a situation in which Margrietje loves Brunoke

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Conclusions

Two fundamental notions in semantics:Declarative sentences are true or false in certain

situations. Definite noun phrases and proper nouns refer to

things in the world.

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Consistent vs. inconsistent

5a. Sylvania is to the north of Freedonia.5b. Freedonia is a republic.6a. The capital of Freedonia has a population

of 9,000.6b. No city in Freedonia has a population of

9,000.7a. Sylvania is to the north of Freedonia.7b. Freedonia is to the north of Sylvania.

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Logic & NL

Broadly speaking, logic-based approaches to natural language semantics focus on those aspects of natural language which guide our judgments of consistency and inconsistency.

The syntax of a logical language is designed to make these features formally explicit.

As a result, determining properties like consistency can often be reduced to symbolic manipulation, that is, to a task that can be carried out by a computer.

In order to pursue this approach, we first want to develop a technique for representing a possible situation.

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Model

We do this in terms of something that logicians call a model.

A model for a set W of sentences is a formal representation of a situation in which all the sentences in W are true.

The usual way of representing models involves set theory.

The domain D of discourse (all the entities we currently care about) is a set of individuals, while relations are treated as sets built up from D.

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Example of a model

Our domain D will consist of three children, Stefan, Klaus and Evi = s, k and e. D = {s, k, e}.

Expressions: boy denotes the set

consisting of Stefan and Klaus,

girl denotes the set consisting of Evi,

is running denotes the set consisting of Stefan and Evi.

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Propositional logic

Boolean Operator Truth Conditions

negation (it is not the case that ...)

-φ is true in s iff φ is false in s

conjunction (and) (φ & ψ) is true in s iffφ is true in s and ψ is true in s

disjunction (or) (φ | ψ) is true in s iffφ is true in s or ψ is true in s

implication (if ..., then ...)

(φ -> ψ) is true in s iffφ is false in s or ψ is true in s

equivalence (if and only if)

(φ <-> ψ) is true in s iffφ and ψ are both true in s or both false in s

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Quiz grades

Q5 Q6 Q7 Q8 Q9 Q10

MIN 7.0 6.5 5.0 7.0 9.0 7.0

AVG 8.8 8.4 8.3 8.8 9.8 7.6

MAX 10.0 10.0 10.0 10.0 10.0 8.0

30-Nov-2009 LING 681.02, Prof. Howard, Tulane University

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Missing quiz grades

MichaelHunterLeahCaitlinAlexanderParisa

0P3P1, P5P1P5P6, P9, P10

Next time

No quiz

NLPP §10

Analyzing the meaning of sentences