1 600.465 Natural Language Processing Prof: Jason Eisner Webpage: http://cs.jhu.edu/~jason/465 syllabus, announcements, slides, homeworks 600.465 – Intro to NLP – J. Eisner 2 Goals of the field Computers would be a lot more useful if they could handle our email, do our library research, talk to us … But they are fazed by natural human language. How can we tell computers about language? (Or help them learn it as kids do?) 600.465 – Intro to NLP – J. Eisner 3 A few applications of NLP • Spelling correction, grammar checking … • Better search engines • Information extraction • Psychotherapy; Harlequin romances; etc. • New interfaces: – Speech recognition (and text-to-speech) – Dialogue systems (USS Enterprise onboard computer) – Machine translation (the Babel fish) 600.465 – Intro to NLP – J. Eisner 4 Goals of the course • Introduce you to NLP problems & solutions • Relation to linguistics & statistics • At the end you should: – Agree that language is subtle & interesting – Feel some ownership over the formal & statistical models – Understand research papers in the field 600.465 – Intro to NLP – J. Eisner 5 Ambiguity: Favorite Headlines • Iraqi Head Seeks Arms • Is There a Ring of Debris Around Uranus? • Juvenile Court to Try Shooting Defendant • Teacher Strikes Idle Kids • Stolen Painting Found by Tree • Kids Make Nutritious Snacks • Local HS Dropouts Cut in Half • Obesity Study Looks for Larger Test Group 600.465 – Intro to NLP – J. Eisner 6 Ambiguity: Favorite Headlines • British Left Waffles on Falkland Islands • Never Withhold Herpes Infection from Loved One • Red Tape Holds Up New Bridges • Man Struck by Lightning Faces Battery Charge • Clinton Wins on Budget, but More Lies Ahead • Hospitals Are Sued by 7 Foot Doctors600.465 – Intro to NLP – J. Eisner 7 Levels of Language • Phonetics/phonology/morphology: what words (or subwords) are we dealing with? • Syntax: What phrases are we dealing with? Which words modify one another? • Semantics: What’s the literal meaning? • Pragmatics: What should you conclude from the fact that I said something? How should you react? 600.465 – Intro to NLP – J. Eisner 8 Subtler Ambiguity • Q: Why does my high school give me a suspension for skipping class? • A: Administrative error. They’re supposed to give you a suspension for auto shop, and a jump rope for skipping class. (*rim shot*) 600.465 – Intro to NLP – J. Eisner 9 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. 600.465 – Intro to NLP – J. Eisner 10 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. To get a donut (spare tire) for his car? 600.465 – Intro to NLP – J. Eisner 11 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. store where donuts shop? or is run by donuts? or looks like a big donut? or made of donut? or has an emptiness at its core? 600.465 – Intro to NLP – J. Eisner 12 What’s hard about this story? I stopped smoking freshman year, but John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there.600.465 – Intro to NLP – J. Eisner 13 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. Describes where the store is? Or when he stopped? 600.465 – Intro to NLP – J. Eisner 14 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. Well, actually, he stopped there from hunger and exhaustion, not just from work. 600.465 – Intro to NLP – J. Eisner 15 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. At that moment, or habitually? (Similarly: Mozart composed music.) 600.465 – Intro to NLP – J. Eisner 16 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. That’s how often he thought it? 600.465 – Intro to NLP – J. Eisner 17 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. But actually, a coffee only stays good for about 10 minutes before it gets cold. 600.465 – Intro to NLP – J. Eisner 18 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. Similarly: In America a woman has a baby every 15 minutes. Our job is to find that woman and stop her.600.465 – Intro to NLP – J. Eisner 19 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. the particular coffee that was good every few hours? the donut store? the situation? 600.465 – Intro to NLP – J. Eisner 20 What’s hard about this story? John stopped at the donut store on his way home from work. He thought a coffee was good every few hours. But it turned out to be too expensive there. too expensive for what? what are we supposed to conclude about what John did? how do we connect “it” to “expensive”? 600.465 – Intro to NLP – J. Eisner 21 n-grams • Letter or word frequencies: 1-grams (= unigrams) – useful in solving cryptograms: ETAOINSHRDLU… • If you know the previous letter: 2-grams (= bigrams) – “h” is rare in English (4%; 4 points in Scrabble) – but “h” is common after “t” (20%) • If you know the