Introduction to Computational Linguistics

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Introduction to Computational Linguistics

• Eleni Miltsakaki
• AUTH
• Fall 2005-Lecture 3

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Review exercise 1

• Identify every major phrase in the following
  sentences. Indicate the head and any complements
  of the head.
• The man played his guitar in the street
• The people dissatisfied with the verdict left
  the courtroom

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Review exercise 2

• Classify the following verbs as being
  intransitive, transitive or ditransitive. If the
  verb can be used in more than one of these forms,
  give each possible classification. Give an
  example sentence to demonstrate your analysis.
• Cry
• Sing
• Donate
• put

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Review exercise 3

• Parse the following sentences
• The old man put his violin in its case
• We had to call off the meeting until next Monday

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What makes a good grammar?

• Generality
• The range of sentences covered by the rules
• Selectivity
• The range of sentences that can be identified as
  ungrammatical
• Understandability
• How simple the grammar is

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Hint for making rules general

• Pay attention to constituents
• Diagnostic of constituency
• Conjunction
• Compare
• I ate a hamburger and a hot dog
• I will eat the hamburger and throw away the hot
  dog
• I ate a hamburger and John ate a hot dog
• I ate a hamburger and on the stove
• I ate a cold hot dog and well burned
• I ate the hot dog

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How the conjunction test can help

• Compare
• I looked up Johns number
• I looked up Johns chimney
• I looked up Johns number and in his cupboards
• I looked up Johns chimney and in his cupboards

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Parsing strategies

• Top-down
• A top down parser starts with S and attempts to
  rewrite it into a sequence of terminal symbols
  that matches the words in the input sentence
• Bottom-up
• You take a sequence of symbols and match it to
  the right hand side of the rule, i.e. start with
  Det N and match it to get the NP
• Bottom-up chart parsing
• To avoid unnecessary repetition of the matching
  process you use a data structure called chart
  that allows you to record partial results
• Well see examples in J. Allens Natural Language
  Understanding, Chapter 3

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What is generative capacity?

• The range of languages that a formalism can
  describe
• Formal languages allow a precise (mathematic)
  characterization
• Natural languages CANNOT be characterized
  precisely enough to define generative capacity

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Chomskys hierarchy of languages

• Regular
• S ? a S1
• In the right-hand side of the rule you have one
  terminal followed by one non-terminal
• YOU CANT COUNT
• Context-free
• S ? a S b
• Number of as followed by equal number of bs
• YOU CANT COUNT MORE THAN 2 ELEMENTS
• Context-sensitive
• aAb ? a?ß a and b can be any
  sequence of symbols
• Type 0 grammars
• Allow any arbitrary rewrite rules

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Whats next

• Lexicalized grammars
• Lexicalized Tree Adjoining Grammar (LTAG)
• Lexical Functional Grammar (LFG)
• Head Driven Phrase Structure Grammar (HPSG)
• Human parsing preferences
• Then move on to how to represent meaning
• Two basic logical languages
• Propositional logic
• Predicate logic