====== Lab 6 ======

**Try to complete as many problems as you can. Hand in a python code file (''fullName_lab6.py'') with what you have finished in MySchool before midnight today (24 September). **

If you can't manage to complete a particular problem please hand in your incomplete code -- comment it out if it produces an error.

===== 1. Parse CFG =====

Lets start with the CFG grammar from [[http://www.nltk.org/book/ch08.html#recursion-in-syntactic-structure|section 3.3 in chapter 8]] and expand upon it.

<code python>
grammar = nltk.CFG.fromstring("""
  S  -> NP VP
  NP -> Det Nom | PropN
  Nom -> Adj Nom | N
  VP -> V Adj | V NP | V S | V NP PP
  PP -> P NP
  PropN -> 'Buster' | 'Chatterer' | 'Joe'
  Det -> 'the' | 'a'
  N -> 'bear' | 'squirrel' | 'tree' | 'fish' | 'log'
  Adj  -> 'angry' | 'frightened' |  'little' | 'tall'
  V ->  'chased'  | 'saw' | 'said' | 'thought' | 'was' | 'put'
  P -> 'on'
  """)

parser = nltk.ChartParser(grammar)
sent = 'Joe saw a squirrel'.split()
for tree in parser.parse(sent):
    print(tree)
    #tree.draw()
</code>

==== 1.1 Understand and Expand  ====

Play around with the grammar and try to understand the production rules. Add some new words (terminals) to the productions and make up some sentences to parse.

You can use the ''draw'' method to render parse-trees.

==== 1.2 Add Personal Pronouns ====

Add some [[https://en.wikipedia.org/wiki/Pronoun#Personal|personal pronouns]] to the grammar.

Examples: 'I saw a bear', 'he frightened me'

==== 1.3 Transitive vs. Intransitive Verbs ====

Add some verbs and alter the grammar to make it differentiate between [[https://en.wikipedia.org/wiki/Transitive_verb|transitive]] and [[https://en.wikipedia.org/wiki/Intransitive_verb|intransitive verbs]].

Examples: 'the bear died', 'Joe saw the bear was dead', 'Buster gave Joe a fish'

==== 1.4 Add Number Agreement ====

Alter the grammar to add [[https://en.wikipedia.org/wiki/Agreement_(linguistics)#Number|number agreement]], start with something like ''S -> NPsg VPsg | NPpl VPpl'' and continue down the productions. The terminals could for example be something like: ''Nsg -> 'bear' | ...'', ''Vtsg -> 'eat' | 'eats' | ...''. Add plural forms of nouns.

Examples: 'a fish dies', 'the bears frightened a little squirrel'

**What to hand in: Your CFG grammar with all the modifications. Code to print out a couple of sentence parses for each subsection here above.**

===== 2. Parse PCFG =====

Now create a probabilistic version of your grammar (see [[http://www.nltk.org/book/ch08.html#weighted-grammar|section 3.6 in chapter 8]]) by adding made up, but reasonable probabilities to your productions.

<code python>
pgrammar = nltk.PCFG.fromstring("""
  S  -> NPsg VPsg [0.5] | NPpl VPpl [0.5]
  ...
  """)

viterbi_parser = nltk.ViterbiParser(pgrammar)
for tree in viterbi_parser.parse(['Buster', 'saw', 'a', 'bear']):
    print(tree)
</code>

Try to find ambiguous sentences to parse with the grammar. Then alter the production probabilities to change which is the most probable parsing.

**What to hand in: A PCFG grammar. Code to print out a couple of sentence parses.**