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Syntax
Announcements
Natural Language Syntax
English Syntax
Programming languages and natural languages both have compositional syntax.
Is that a big bug or a little bug? I’ve never seen such a cute kangaroo. SQ S
VP
. NP VP
? PRPAUXADVP VP
. .
COPNP is NNP
NP
CC NP
NP
that DT JJ NN or DT JJ NN
I ‘ve RB
never seen
NP
DT DT JJ
Utterances from the Suppes subject in the “Child Language Data Exchange System (CHILDES)” project
VBN
a big bug a little bug
NN such a cute kangaroo
MacWhinney, B. (2000). The CHILDES Project: Tools for analyzing talk. Third Edition. Mahwah, NJ: Associates.
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Data Abstraction for Syntax
Representing English Syntax
The tree data abstraction can represent the structure of a sentence.
SQ VP
NP
NP
The label of a phrase is its tag: ‘SQ’
. The label of a word (a leaf) contains
COP is
NP
NP
?
both its tag and text: [‘JJ’, ‘big’]
(Demo)
NNP that
CC or
DT a
JJ little
NN bug
DT a
JJ big
NN bug
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Parsing
Files, Strings, and Lists
Some files are plain text and can be read into Python as either:
• One string containing the whole contents of the file: open(‘/some/file.txt’).read() • A list of strings, each containing one line: open(‘/some/file.txt’).readlines()
Useful string methods for processing the contents of a file:
.strip() returns a string without whitespace (spaces, tabs, etc.) on the ends
>>> ‘ hello ‘.strip()
‘hello’
.split() returns a list of strings that were separated by whitespace >>> ‘hi there’.split()
[‘hi’, ‘there’]
.replace(a, b) returns a string with all instances of string a replaced by string b
>>> ‘2+2’.replace(‘+’, ‘ + ‘)
‘2 + 2’
(Demo)
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Converting a List Describing a Constituent (Phrase or Word) to a Tree
[‘(‘, ‘ROOT’, ‘(‘, ‘SQ’, ‘(‘, ‘VP’, ‘(‘, ‘COP’, ‘is’, ‘)’, ‘(‘, ‘NP’, ‘(‘, ‘NN’, ‘that’, ‘)’,
‘)’, ‘(‘, ‘NP’, ‘(‘, ‘NP’, ‘(‘, ‘DT’, ‘a’, ‘)’, ‘(‘, ‘JJ’, ‘big’, ‘)’, ‘(‘, ‘NN’, ‘bug’, ‘)’,
‘)’, ‘(‘, ‘CC’, ‘or’, ‘)’, ‘(‘, ‘NP’, ‘(‘, ‘DT’, ‘a’, ‘)’, ‘(‘, ‘JJ’, ‘little’, ‘)’, ‘(‘,
‘NN’, ‘bug’, ‘)’, ‘)’, ‘)’, ‘)’, ‘(‘, ‘.’, ‘?’, ‘)’, ‘)’, ‘)’]
[…, ‘(‘, ‘NP’, ‘(‘, ‘DT’, ‘a’, ‘)’, ‘(‘, ‘JJ’, ‘big’, ‘)’, ‘(‘, ‘NN’, ‘bug’, ‘)’, ‘)’, …] i
def read_parse_tree(tokens, i):
Read the tag, which is tokens[i], then advance i.
While the current item is a ‘(‘, call read_parse_tree to construct a branch. Once the current item is a ‘)’, return a phrase from the tag and branches. Base case: there is no ‘(‘ or ‘)’ because there is just text after the tag.
read_parse_tree needs to return the tree it read and what to read next. (Demo)
9
Generating Language
Language Models
A statistical (or probabilistic) language model describes how likely some text would be.
What word do you think appears at the end of this ____?
Sampling from a statistical language model uses that description to generate language. A useful language model needs to generalize from examples.
E.g., Substitute any phrase in an utterance with any other phrase that has the same tag.
(S (NP (DT the) (NN dog)) (VP (VBD ran)))
(S (NP (DT the) (NN water)) (VP (VBD evaporated)))
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Sampling Strategy
Starting with the branches of the root node, flip a coin for each branch. If it comes up
heads, swap that branch for another constituent (phrase or word) that has the same tag.
Then, apply this procedure to all of the branches.
def gen_tree(t, flip):
“””Return a version of t in which constituents are randomly replaced.”””
if is_leaf(t):
return t
new_branches = []
for b in branches(t):
if flip():
b = random.choice(all_constituents_with_the_same_tag)
new_branches.append(gen_tree(b, flip))
return phrase(tag(t), new_branches)
(Demo)
First create a
dictionary from
tags to lists of
constituents
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Changing the Data Representation
Data Abstraction Makes It Possible to Change the Data Representation
>>> lines
[‘(ROOT (S (NP (NN this)) (VP (COP is) (NP (DT a) (NN book)))’, ‘ (. ?)))’]
>>> read_sentences(lines)[0]
[‘(‘, ‘ROOT’, ‘(‘, ‘S’, ‘(‘, ‘NP’, ‘(‘, ‘NN’, ‘this’, ‘)’, ‘)’, … , ‘)’]
>>> tokens_to_parse_tree(read_sentences(lines)[0])
[‘ROOT’, [‘S’, [‘NP’, [[‘NN’, ‘this’]]], [‘VP’, [[‘COP’, ‘is’]], … ]]]]
(Demo)
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