NLP

advertisement
CS 4100 Artificial Intelligence
Prof. C. Hafner
Class Notes April 3and5, 2012
Why Natural Language Processing ?
• Huge amounts of data
– Internet = at least 20
billion pages
– Intranet
• Applications for
processing large
amounts of texts
require NLP expertise
•
•
•
•
Classify text into categories
Index and search large texts
Automatic translation
Speech understanding
– Understand phone conversations
• Information extraction
– Extract useful information from
resumes
• Automatic summarization
– Condense 1 book into 1 page
• Question answering
• Knowledge acquisition
• Text generation / dialogues
Natural?
• Natural Language?
– Refers to the language spoken by people, e.g.
English, Japanese, Swahili, as opposed to artificial
languages, like C++, Java, etc.
• Natural Language Processing
– Applications that deal with natural language in a
useful way (beyond token/string matching)
• Computational Linguistics
– Doing linguistics on computers
– More on the linguistic side than NLP, but closely
related ]
Why Natural Language Processing?
•
•
•
•
•
kJfmmfj mmmvvv nnnffn333
Uj iheale eleee mnster vensi credur
Baboi oi cestnitze
Coovoel2^ ekk; ldsllk lkdf vnnjfj?
Fgmflmllk mlfm kfre xnnn!
Computers Lack Knowledge!
• Computers “see” text in English the same you
have seen the previous text!
• People naturally have
– “Common sense”
– Reasoning capacity
– Years of life experience
• Computers naturally have
– No common sense
– No reasoning capacity
– No life experience
Where does it fit in the CS taxonomy?
Computers
Databases
Artificial Intelligence
Robotics
Information
Retrieval
Algorithms
Networking
Search
Natural Language Processing
Machine
Translation
Language
Analysis
Semantics
Parsing
Linguistics Levels of Analysis
• Speech and text (and sign language)
• Levels
–
–
–
–
–
Phonology: sounds / letters / pronunciation
Morphology: the structure of words
Syntax: how these sequences are structured
Semantics: meaning of the strings
Pragmatics: what we use language to accomplish
• Interaction between levels
Issues in Syntax
• Shallow parsing:
“the dog chased the bear”
“the dog” “chased the bear”
subject - predicate
Identify basic structures
NP-[the dog] VP-[chased the bear]
• Deeper analysis: “the dog ate my homework”
– Who did what? (literal meaning) - semantics
– The meaning in context - pragmatics
Issues in Syntax
• Full parsing: John loves Mary
Help figuring out (automatically) questions like: Who did what
and when?
More Issues
• Anaphora Resolution: discourse
“The dog entered my room. It scared me”
• Preposition Attachment (syntax & semantics)
“I saw the man in the park with a telescope”
Issues in Semantics
•
•
•
•
•
Understand language! How?
“plant” = industrial plant
“plant” = living organism
Words are ambiguous
Importance of semantics?
– Machine Translation: wrong translations
– Information Retrieval: wrong information
– Anaphora Resolution: wrong referents
Why Semantics?
• The sea is home to million of plants and animals
• English  French [commercial MT system]
• Le mer est a la maison de billion des usines et
des animaux
• French  English
• The sea is at the home for billions of factories
and animals
Issues in Semantics
• How to learn the meaning of words?
• From dictionaries: word senses
plant, works, industrial plant -- (buildings for carrying on
industrial labor; "they built a large plant to manufacture
automobiles")
plant, flora, plant life -- (a living organism lacking the
power of locomotion)
They are producing about 1,000 automobiles in the new
plant
The sea flora consists in 1,000 different plant species
The plant was close to the farm.
Issues in Semantics
• Learn from annotated examples:
– Assume 100 examples containing “plant” previously
tagged by a human
– Train a learning algorithm
– How to choose the learning algorithm?
– How to obtain the 100 tagged examples?
Issues in Pragmatics
Why?
To modify the beliefs of other agents
Why?
To change the actions of other agents
Issues in Learning Semantics
• Learning?
– Assume a (large) amount of annotated data = training
– Assume a new text not annotated = test
• Learn from previous experience (training) to classify
new data (test)
• Bayes nets, decision trees, memory based learning
(e.g. nearest neighbor), neural networks
Issues in Information Extraction
• “There was a group of about 8-9 people close to
the entrance on Highway 75”
• Who? “8-9 people”
• Where? “highway 75”
• Extract information
• Detect new patterns:
– Detect hacking / hidden information / etc.
• Gov./mil. puts lots of money put into IE research
Issues in Information Retrieval
• General model:
– A huge collection of texts
– A query
• Task: find documents that are relevant to the
given query
• How? Create an index, like the index in a book
• More …
– Vector-space models
– Boolean models
• Examples: Google, Yahoo, etc.
Issues in Information Retrieval
•
•
•
•
Retrieve specific information
Question Answering
“What is the height of mount Everest?”
11,000 feet
Issues in Information Retrieval
• Find information across languages!
• Cross Language Information Retrieval
• “What is the minimum age requirement for car
rental in Italy?”
• Search also Italian texts for “eta minima per
noleggio macchine”
• Integrate large number of languages
• Integrate into performant IR engines
Issues in Machine Translation
• Text to Text Machine Translations
• Speech to Speech Machine Translations
• Most of the work has addressed pairs of widely
spread languages like English-French, EnglishChinese
Issues in Machine Translations
• How to translate text?
– Learn from previously translated data
•  Need parallel corpora
• French-English, Chinese-English have the
Hansards
• Reasonable translations
• Chinese-Hindi – no such tools available today!
Speech Act Theory
“I pronounce you husband & wife” “I sentence you to 5 years”
Natural languages are NOT context free – but almost!
About 40% of words in NY Times are not in a (large) dictionary –
Natural language is “productive”
Example: “I SAW A MAN IN THE PARK WITH A TELESCOPE”
Parsing
• Parsing with CFGs refers to the task of assigning correct
trees to input strings
• Correct here means a tree that covers all and only the
elements of the input and has an S at the top
• It doesn’t actually mean that the system can select the
correct tree from among the possible trees
• As with everything of interest, parsing involves a search
that involves the making of choices
• Example: “I SAW A MAN IN THE PARK WITH A
TELESCOPE”
The problem of “scaling up” – the same as in
knowledge representation, planning, etc but even more difficult
Sentence-Types
• Declaratives: A plane left
– S -> NP VP
• Imperatives: Leave!
– S -> VP
• Yes-No Questions: Did the plane leave?
– S -> Aux NP VP
• WH Questions: When did the plane leave?
– S -> WH Aux NP VP
Potential Problems in CFG
• Agreement
• Subcategorization
• Movement
Agreement
•This dog
•Those dogs
•*This dogs
•*Those dog
•This dog eats
•Those dogs eat
•*This dog eat
•*Those dogs eats
Subcategorization
•
•
•
•
•
•
Sneeze: John sneezed
Find: Please find [a flight to NY]NP
Give: Give [me]NP[a cheaper fare]NP
Help: Can you help [me]NP[with a flight]PP
Prefer: I prefer [to leave earlier]TO-VP
Told: I was told [United has a flight]S
• *John sneezed the book
• *I prefer United has a flight
• *Give with a flight
• Subcat expresses the constraints that a predicate (verb for now)
places on the number and type of the argument it wants to take
So?
• So the various rules for VPs overgenerate.
– They permit the presence of strings containing verbs
and arguments that don’t go together
– For example
– VP -> V NP therefore
– Sneezed the book is a VP since “sneeze” is a verb
and “the book” is a valid NP
– Subcategorization frames can help with this problem
(“slow down” overgeneration)
Movement
• Core example
– [[My travel agent]NP [booked [the flight]NP]VP]S
• I.e. “book” is a straightforward transitive verb. It
expects a single NP arg within the VP as an
argument, and a single NP arg as the subject.
Movement
• What about?
• Which flight did the travel agent book ?
(“Which flight” is the object of the verb “book”. It
was “moved” to the front of the sentence!!)
– Which flight do you want me to have the travel
agent book?
• The direct object argument to “book” can be a
long way from where its supposed to appear.
• Here it is separated from its verb by 2 other
verbs.
• Therefore NL cannot be a finite state
language
Semantics: Fillmore’s Case Grammar
• “Cases” are semantically based not grammatical
ones like in Latin or German
• Charles Fillmore, “The Case for Case”, 1968
• Produced more than one version
Case Grammar Semantics
Case grammar semantics:
• Treats the verb as a predicate and the subject, objects,
and other subordinate clauses as “arguments”.
• Labels the arguments with their relationship to the verbpredicate (called “cases”) [uses subcategorization info]
• Ex: John sold his car – agent and object cases
• Ex: John sold his car to Mary – agent, object and
recipient cases
Fillmore’s list of cases
• Agentive (A): the case of the typically animate
perceived instigator of the action identified by
the verb.
• Instrumental (I) the case of the inanimate force
or object causally involved in the action of state
identified by the verb.
Fillmore’s list of cases
• Dative (D) - later Experiencer (E): the case of
the animate being affected by the state or action
identified by the verb.
• Factitive (F) - later Goal (G): the case of the
object or being resulting from the action or state
identified by the verb, or understood as a part of
the meaning of the verb.
Fillmore’s list of cases
• Locative (L): the case which identified the
location or spatial orientation of the state or
action identified by the verb.
• Objective (O): the semantically most neutral
case, the case of anything representable by a
noun whose role in the action or state
identified by the verb is identified by the
semantic interpretation of the verb itself.
Case grammar semantics
Semantic (case) roles don’t depend simply on syntactic
roles
• Ex: John sold his car to Mary – agent, object and
recipient cases
• Ex: John sold Mary his car
• Ex: John broke the window
• Ex: John broke the window with a hammer – agent,
object and instrument cases
• Ex: A hammer broke the window
Informal quiz: Consider these sentences:
1.
2.
3.
4.
5.
6.
The burglar opened the door.
The door was opened by the burglar.
The burglar opened the door with a crowbar.
The door was opened by a crowbar.
The crowbar opened the door.
The door opened.
Case analysis :
1.
2.
3.
4.
5.
6.
The burglar opened the door.
S
O
The door was opened by the burglar.
S
A
The burglar opened the door with a crowbar.
S
O
A
The door was opened by a crowbar.
S
A
The crowbar opened the door.
S
O
The door opened.
S
Strengths
• Only one Noun Phrase occupies each case
role in relation to a particular verb
• Therefore one could classify verbs in terms of
which case roles they took. e.g.:
o “open” - O, {A} {I}
o “shout” - A, O, {E}
{} denotes optional elements
• This model has been used in Artificial
Intelligence, along with the sub-categorization
of verbs (described earlier)
Weaknesses
• Researchers could not agree on a standard
set of cases.
• Not always easy in practice to allocate
particular Noun Phrases to cases.
• When it gets difficult there is a temptation to
use the Objective (O) as a kind of “dustbin
case” for all the NPs that don’t seem to fit
anywhere else.
Download
Related flashcards

Cryptography

26 cards

Create Flashcards