Transcript Lecture 3
Introduction to Computational Linguistics Eleni Miltsakaki AUTH Fall 2005-Lecture 3 1 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 2 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 3 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 4 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 5 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 6 How the conjunction test can help • Compare – I looked up John’s number – I looked up John’s chimney – *I looked up John’s number and in his cupboards – I looked up John’s chimney and in his cupboards 7 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 We’ll see examples in J. Allen’s Natural Language Understanding, Chapter 3 8 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 9 Chomsky’s hierarchy of languages • Regular – S a S1 • In the right-hand side of the rule you have one terminal followed by one nonterminal • YOU CAN’T COUNT • Context-free – SaSb – Number of a’s followed by equal number of b’s – YOU CAN’T COUNT MORE THAN 2 ELEMENTS • Context-sensitive – aAb aΨβ a and b can be any sequence of symbols • Type 0 grammars – Allow any arbitrary rewrite rules 10 What’s 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 11