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Computational Linguistics

Today, we explore this interstice

cartoon from xkcd.com

Computational Linguistics

(aka Natural Language Processing)

Christopher Manning

Ling 1 November 4, 2011

What is Computational Linguistics?

• Getting computers to perform useful tasks involving human languages whether for:

• Enabling human-machine communication • Improving human-human communication • Doing stuff with language data ... email, blogs, etc.

• Examples: • Machine Translation

• Automatic Question Answering • Speech Recognition • Text-to-Speech Synthesis • Text Understanding

CL vs. NLP

• Why say "Computational Linguistics (CL)" versus "Natural Language Processing" (NLP)? • Either choose either freely or ... • Computational Linguistics

• The science of computers dealing with language • Some interest in modeling what people do

• Natural Language Processing • Developing computer systems for processing and understanding human language text

TheVision

Language:theul?mateUI

NLP:Goalsofthefield

• FromtheloXy... • full-onnaturallanguageunderstanding

• par?cipa?oninspokendialogues• open-domainques?onanswering• real-?mebi-direc?onaltransla?on

• ...tothemundane • iden?fyingspam

• categorizingnewsstories• finding&comparingproductinforma?onontheweb• assessingsen?menttowardproducts,brands,stocks,...Predominantinrecentyears

NLPinthecommercialworld

Powerset

Currentmo?va?onsforNLP

• Theexplosionofmachine-readablenaturallanguagetext• Exabytes(10 18

bytes)oftext,doublingeveryyearortwo• Webpages,emails,IMs,SMSs,tweets,docs,PDFs,...• Opportunity - andincreasingnecessity - toextractmeaning

• Media?onofhumaninterac?onsbycomputers • Opportunityforthecomputerinthelooptodomuchmore • Growingroleoflanguageinhuman-computerinterac?on

What'sdrivingNLP?Threetrends:

Furthermo?va?onforCL

Onereasonforstudyinglanguage - andformepersonallythemostcompellingreason - isthatitistemp6ngtoregardlanguage,inthetradi6onalphrase,asa"mirrorofmind".Chomsky,1975Forthesamereason,computa?onallinguis?csisacompellingwaytostudyhumanlanguageacquisi?onandprocessing.Some?mes,thebestwaytounderstandsomethingistobuildamodelofit.WhatIcannotcreate,Idonotunderstand.Feynman,1988

Subfieldsandtasks

Spamdetec?on

SPORTSBUSINESS

YES

Seman?csearch

Whyiscomputa?onallinguis?cshard?

Humanlanguages:

• arehighlyambiguousatalllevels

• arecomplex,withrecursivestructuresandcoreference• subtlyexploitcontexttoconveymeaning• arefuzzyandvague• requirereasoningabouttheworldforunderstanding• arepartofasocialsystem:persuading,insul?ng,amusing,...

OK,whyelseisNLPhard?

Ohsomanyreasons!

non--andardEnglih

Greatjob@jus?nbieber!WereSOOPROUDofwhatyouveaccomplished!Utaughtus2#neversaynever&youyourselfshouldnevergiveupeither -

egmen-a1oniueidiom neologim unfriendretweetbromanceteabagger gardenpa-hen-ence

Themanwhohuntsducksoutonweekends.

Thecokonshirtsaremadefromgrowshere.

-rickyen1-yname ...amuta?onontheforgene... worldknowledge

MaryandSuearesisters.

MaryandSuearemothers.

proody

Ineversaidshestolemymoney.

lexicalpecifici-y

Butthat'swhatmakesitfun!

theNewYork-NewHavenRailroad theNewYork-NewHavenRailroad

Meaningsandexpressions

sodasoXdrinkpopbeverageCoke

Onemeaning,manyexpressions

RussiaincreasingpriceofgasforGeorgia

Consideraseman?csearchapplica?on:

NPNPVPSNPNPPPVPS

photosfromworth1000.com seman?cambiguitysyntac?cambiguity

Ambiguousheadlines

Teacher Strikes Idle Kids

China to Orbit Human on Oct. 15 Juvenile Court to Try Shooting Defendant Clinton Wins on Budget, but More Lies Ahead Local High School Dropouts Cut in Half Police: Crack Found in Man's Buttocks

Parsing

• Parsing is the process of taking a string and a grammar and returning a parse tree or trees for that string

the flight left

Sleft VP Verb NP Det N the flight

Parsing involves search

• As with most everything of interest, parsing involves a search which involves the making of choices

• We'll look at some basic methods to give you an idea of the problem

Top-Down Parsing

• Since we're trying to find trees rooted with an S (Sentence) start with the rules that give us an S.

• Then work your way down from there to the words.

Top-Down Space

S NP VP S VP S NP VP Aux NP VP S NP VP S NP VP Aux S VP NP Aux S NP VP S VP S S

Bottom-Up Parsing

• Of course, we also want trees that cover the input words. So start with trees that link up with the words in the right way.

• Then work your way up from there.

Bottom-Up Space

that flight Book flight that Book flight that Book flight that Book flight that Book flight that Book flight that Book

Control

• We need to keep track of the search space and have a strategy to make choices • We need to systematically explore everything to make sure we find the right parse for a sentence • Which node to try to expand next? • Which grammar rule to use to expand a node?

Top-Down, Depth-First, Left-to-Right Search

Example

Example

[flight] [flight]

Example

flight flight

Efficientparsing

• Thatshouldgiveyouthegeneralideaofhowaparsercanworkbyexploringhypothesessystema?cally • Butreallyweneedtodomuchmoretomake

How to choose between parses?

• Probabilistic methods! • Augment the grammar rules with probabilities • Modify the parser to keep only most probable parses • At the end, return the most probable parse

A statistical scientific revolution

• Computational Linguistics before 1990: • Hand-built parsers, hand-built dialogue systems • High precision, low coverage methods • Computational Linguistics after 1995: • Automatically trained parsers, unsupervised clustering, statistical machine translation • High coverage, low precision methods • Build models exploiting data

Demos!

IfyoumightlikeNLP/CompLing...

• learnJavaorPython(andplaywithJavaNLPorNLTK)

• getsomeexposuretolinguis?cs(LING1,...)• andtologic,probability,sta?s?cs,linearalgebra• studyAIandmachinelearning(CS121,CS221,CS229)• readJurafsky&Mar?norManning&Schütze• Take

• Ling180/CS124:FromLanguagestoInforma?on• Ling284/CS224N:NaturalLanguageProcessing• Ling281/CS224S:SpeechRecogni?on&Synthesis• Ling/CS224U:NaturalLanguageProcessing

Onemorefortheroad

cartoon from qwantz.comquotesdbs_dbs21.pdfusesText_27