1. Semantics Day 38 LING 681.02 Computational Linguistics Harry Howard Tulane University

2. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University2 Course organization  http://www.tulane.edu/~howard/NLP/ http://www.tulane.edu/~howard/NLP/

3. Analyzing the meaning of sentences SLPP 10

4. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University4 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?

5. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University5 Querying a database CityCountryPopulation athensgreece1368 bangkokthailand1178 barcelonaspain1280 berlineast_germany3481 birminghamunited_kingdom1112

6. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University6 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'

7. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University7 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]

8. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University8 Philosophical problem  Understanding = looking up in a database?  What is a city?  What is a country?  What is a population?

9. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University9 Depiction of a situation in which Margrietje loves Brunoke

10. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University10 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.

11. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University11 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.

12. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University12 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.

13. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University13 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.

14. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University14 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.

15. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University15 Propositional logic Boolean OperatorTruth Conditions negation (it is not the case that...) -φ is true in siffφ is false in s conjunction (and)(φ & ψ) is true in siff φ is true in s and ψ is true in s disjunction (or)(φ | ψ) is true in siff φ is true in s or ψ is true in s implication (if..., then...) (φ -> ψ) is true in siff φ is false in s or ψ is true in s equivalence (if and only if) (φ ψ) is true in siff φ and ψ are both true in s or both false in s

16. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University16 Quiz grades Q5Q6Q7Q8Q9Q10 MIN 7.06.55.07.09.07.0 AVG 8.88.48.38.89.87.6 MAX 10.0 8.0

17. 30-Nov-2009LING 681.02, Prof. Howard, Tulane University17 Missing quiz grades  Michael  Hunter  Leah  Caitlin  Alexander  Parisa  0  P3  P1, P5  P1  P5  P6, P9, P10

18. Next time No quiz NLPP §10 Analyzing the meaning of sentences