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Syntax: Introduction

Till now, we have discussed:

Language modeling — understanding ordering of words POS tagging — understanding roles of words

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Now we will discuss more complex notions, such as grammatical relations among words

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What is Syntax?

Pawan Goyal (IIT Kharagpur)

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Syntax

Refers to the way words are arranged together, and the relationship between then.

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Grammar

Compositionality

Problem:

- “Ambiguity”

Week 5: Lecture 1

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Syntax Tree: Example

Pawan Goyal (IIT Kharagpur)

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Syntax

Sentence: The man read this book

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We wish to understand how different words in a sentence are grouped, e.g., a Det and a Noun can together give a Noun Phrase, a Noun Phrase preceded by a verb can give a Verb phrase, etc.

Week 5: Lecture 1

Our final goal:

How to construct a syntax tree for a given sentence?

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Defining the notions: Constituency

Pawan Goyal (IIT Kharagpur)

Syntax

Constituent

A group of words acts as a single unit - phrases, clauses etc.

Week 5: Lecture 1

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the man from Amherst – NP (man: head word)

extremely clever – AP (clever: head word)

down the river – PP (down: head word)

killed the rabbit – VP (killed: head word)

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Defining the notions: Constituency

Pawan Goyal (IIT Kharagpur)

Syntax

Constituent

A group of words acts as a single unit - phrases, clauses etc.

Part of Speech - “Substitution Test"

Constituency: Noun Phrase

Kermit the frog they

December twenty-sixth

the reason he is running for president

Similarly, different word-sequences (phrases) can be used as a substitute for each other.

E.g., noun phrases

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Different words having the same POS (here, adjective)

The {sad, intelligent, green, fat, ...} one is in the corner. can be used as a substitute

for each other.

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Pawan Goyal (IIT Kharagpur)

Syntax

Evidence that constituency exists

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They appear in similar environments

Week 5: Lecture 1

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Pawan Goyal (IIT Kharagpur)

Syntax

Evidence that constituency exists

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They appear in similar environments

Week 5: Lecture 1

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Evidence that constituency exists

Pawan Goyal (IIT Kharagpur)

Syntax

They appear in similar environments

Can be placed in a number of different locations

Week 5: Lecture 1

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Evidence that constituency exists

Pawan Goyal (IIT Kharagpur)

Syntax

They appear in similar environments

Can be placed in a number of different locations

Week 5: Lecture 1

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Modeling Constituency: what tool do we need?

Pawan Goyal (IIT Kharagpur)

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Syntax

Basically, we want tools to understand which words (in a given text) come together as a unit.

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E.g., which words together make up a noun phrase?

Week 5: Lecture 1

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Modeling Constituency

Pawan Goyal (IIT Kharagpur)

Syntax

Context-free grammar

The most common way of modeling constituency

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Consists of production Rules

These rules express the ways in which the symbols of the language can be grouped and ordered together

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Modeling Constituency

Pawan Goyal (IIT Kharagpur)

Syntax

 

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CFG for Languages

Pawan Goyal (IIT Kharagpur)

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Syntax

 

Week 5: Lecture 1

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CFG for Languages

Pawan Goyal (IIT Kharagpur)

Syntax

Example

 

“Noun”, “ProperNoun”, “Det” are pre-terminals.

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But we cannot generate any string with these rules, since there is no terminal.

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CFG for Languages

Pawan Goyal (IIT Kharagpur)

Syntax

 

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NLP is highly context-sensitive BTW

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CFG as a generator

 

Pawan Goyal (IIT Kharagpur)

Syntax

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CFG as a generator

 

Pawan Goyal (IIT Kharagpur)

Syntax

Week 5: Lecture 1

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CFG as a generator

 

Pawan Goyal (IIT Kharagpur)

Syntax

Week 5: Lecture 1

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CFG as a generator

Pawan Goyal (IIT Kharagpur)

Syntax

 

Week 5: Lecture 1

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CFGs and Grammaticality

Pawan Goyal (IIT Kharagpur)

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Syntax

A CFG defines a formal language = set of all sentences (string of words) that can be derived by the grammar

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Sentences in this set are said to be grammatical

Sentences outside this set are said to be ungrammatical

Week 5: Lecture 1

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CFGs and Recursion

Pawan Goyal (IIT Kharagpur)

Syntax

 

Week 5: Lecture 1

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CFGs and Recursion

Pawan Goyal (IIT Kharagpur)

Syntax

 

Example Sentence

[SThe mailman ate his [NP lunch [PP with his friend [PP from the cleaning staff

[PP of the building [PP at the intersection [PP on the north end [PP of town]]]]]]].

start of a NP

start of a NP

start of a NP

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start of a NP

start of a NP