Description
Parsing with Context Free Grammar (100 pts)
In this assignment you will implement the CKY algorithm for CFG and PCFG parsing. You
will also practice retrieving parse trees from a parse chart and working with such tree data
structures.
You can download the files for this project here: hw2.zip (or in the Files section on
Courseworks).
Python files:
To get you started, there are three Python files for this project.
1. cky.py will contain your parser and currently contains only scaffolding code.
2. grammar.py contains the class Pcfg which represents a PCFG grammar (explained below)
read in from a grammar file.
3. evaluate_parser.py contains a script that evaluates your parser against a test set.
Data files:
You will work with an existing PCFG grammar and a small test corpus.
The main data for this project has been extracted from the ATIS (Air Travel Information
Services) subsection of the Penn Treebank. ATIS is originally a spoken language corpus
containing user queries about air travel. These queries have been transcribed into text and
annotated with Penn-Treebank phrase structure syntax.
The data set contains sentences such as “what is the price of flights from indianapolis to
memphis .”
There were 576 sentences in total, out of which 518 were used for training (extracting the
grammar and probabilites) and 58 for test. The data set is obviously tiny compared to the
entire Penn Treebank and typically that would not be enough training data. However,
because the domain is so restricted, the extracted grammar is actually able to generalize
reasonably well to the test data.
There are 2 data files:
atis3.pcfg – contains the PCFG grammar (980 rules)
atis3_test.ptb – contains the test corpus (58 sentence).
Take a look at these files and make sure you understand the format. The tree structures are
atis3_test.ptb are a little different from what we have seen in class. Consider the following
example from the test file:
(TOP (S (NP i) (VP (WOULD would) (VP (LIKE like) (VP (TO to) (VP (TRAVEL travel) (PP (TO to) (NP westchester))))))) (
PUN .))
Note that there are no part of speech tags. In some cases phrases like NP directly project to
the terminal symbol (NP -> westchester). In other cases, nonterminals for a specific word
were added (TRAVEL -> travel).
The start-symbol for the grammar (and therefore the root for all trees) is “TOP”. This is the
result of an automatic conversion to make the tree structure compatible with the grammar in
Chomsky Normal Form.
While you are working on your parser, you might find it helpful to additionally create a
small toy grammar (for example, the one in the lecture slides) that you can try on some hand
written test cases, so that you can verify by hand that the output is correct.
Part 1 – reading the grammar and getting started (20 pts)
Take a look at grammar.py. The class Pcfg represents a PCFG grammar in chomsky normal
form. To instantiate a Pcfg object, you need to pass a file object to the constructor, which
contains the data. For example:
with open(‘atis3.pcfg’,’r’) as grammar_file:
grammar = Pcfg(grammar_file)
You can then access the instance variables of the Pcfg instance to get information about the
rules:
>>> grammar.startsymbol
‘TOP’
The dictionary lhs_to_rules maps left-hand-side (lhs) symbols to lists of rules. For example,
we will want to look up all rules of the form PP -> ??
>>> grammar.lhs_to_rules[‘PP’]
[(‘PP’, (‘ABOUT’, ‘NP’), 0.00133511348465), (‘PP’, (‘ADVP’, ‘PPBAR’), 0.00133511348465), (‘PP’, (‘AFTER’, ‘NP’), 0.025
3671562083), (‘PP’, (‘AROUND’, ‘NP’), 0.00667556742323), (‘PP’, (‘AS’, ‘ADJP’), 0.00133511348465), … ]
Each rule in the list is represented as (lhs, rhs, probability) triple. So the first rule in the list
would be
PP -> ABOUT NP with PCFG probability 0.00133511348465.
The rhs_to_rules dictionary contains the same rules as values, but indexed by right-handside. For example:
>>> grammar.rhs_to_rules[(‘ABOUT’,’NP’)]
[(‘PP’, (‘ABOUT’, ‘NP’), 0.00133511348465)]
>>> grammar.rhs_to_rules[(‘NP’,’VP’)]
[(‘NP’, (‘NP’, ‘VP’), 0.00602409638554), (‘S’, (‘NP’, ‘VP’), 0.694915254237), (‘SBAR’, (‘NP’, ‘VP’), 0.166666666667), (‘S
QBAR’, (‘NP’, ‘VP’), 0.289156626506)]
TODO:
• Write the method verify_grammar, that checks that the grammar is a valid PCFG in
CNF. You need to check that the rules have the right format (note all nonterminal
symbols are upper-case) and that all probabilities for the same lhs symbol sum to 1.0.
Hint: For improved numeric accuracy, use math.fsum to compute the sum.
• Then change the main section of grammar.py to read in the grammar, print out a
confirmation if the grammar is a valid PCFG in CNF or print an error message if it is not.
You should now be able to run grammar.py on grammars and verify that they are well
formed for the CKY parser.
Part 2 – Membership checking with CKY (30 pts)
The file cky.py already contains a class CkyParser. When a CkyParser instance is created a i
instance is passed to the constructor. The instance variable grammar can then be used to
access this Pcfg object.
TODO: Write the method is_in_language(self, tokens) by implementing the CKY algorithm.
Your method should read in a list of tokens and return True if the grammar can parse this
sentence and False otherwise. For example:
>>> parser = CkyParser(grammar)
>>> toks =[‘flights’, ‘from’, ‘miami’, ‘to’, ‘cleveland’, ‘.’] // Or: toks= ‘flights from miami to cleveland .’.split()
>>> parser.is_in_language(toks)
True
>>> toks =[‘miami’, ‘flights’,’cleveland’, ‘from’, ‘to’,’.’]
>>> parser.is_in_language(toks)
False
While parsing, you will need to access the dictionary self.grammar.rhs_to_rules. You can use
any data structure you want to represent the parse table (or read ahead to Part 3 of this
assignment, where a specific data structure is prescribed).
The ATIS grammar actually overgenerates a lot, so many unintuitive sentences can be
parsed. You might want to create a small test grammar and test cases. Also make sure that
this method works for grammar with different start symbols.
Part 3 – Parsing with backpointers (30 pts)
The parsing method in part 2 can identify if a string is in the language of the grammar, but it
does not produce a parse tree. It also does not take probabilities into account. You will now
extend the parser so that it retrieves the most probable parse for the input sentence, given
the PCFG probabilities in the grammar. The lecture slides on parsing with PCFG will be
helpful for this step.
TODO: Write the method parse_with_backpointers(self, tokens). You should modify your
CKY implementation from part 2, but use (and return) specific data structures. The method
should take a list of tokens as input and returns a) the parse table b) a probability table. Both
objects should be constructed during parsing. They replace whatever table data structure you
used in part 2.
The two data structures are somewhat complex. They will make sense once you understand
their purpose.
The first object is parse table containing backpointers, represented as a dictionary (this is
more convenient in Python than a 2D array). The keys of the dictionary are spans, for
example table[(0,3)] retrieves the entry for span 0 to 3 from the chart. The values of the
dictionary should be dictionaries that map nonterminal symbols to backpointers. For
example: table[(0,3)][‘NP’] returns the backpointers to the table entries that were used to
create the NP phrase over the span 0 and 3. For example, the value of table[(0,3)][‘NP’] could
be ((“NP”,0,2),(“FLIGHTS”,2,3)). This means that the parser has recognized an NP covering
the span 0 to 3, consisting of another NP from 0 to 2 and FLIGHTS from 2 to 3. The split
recorded in the table at table[(0,3)][‘NP’] is the one that results in the most probable parse for
the span [0,3] that is rooted in NP.
Terminal symbols in the table could just be represented as strings. For example the table
entry for table[(2,3)][“FLIGHTS”] should be “flights”.
The second object is similar, but records log probabilities instead of backpointers. For
example the value of probs[(0,3)][‘NP’] might be -12.1324. This value represents the log
probability of the best parse tree (according to the grammar) for the span 0,3 that results in
an NP.
Your parse_with_backpointers(self, tokens) method should be called like this:
>>> parser = CkyParser(grammar)
>>> toks =[‘flights’, ‘from’, ‘miami’, ‘to’, ‘cleveland’, ‘.’]
>>> table, probs = parser.parse_with_backpointers(toks)
During parsing, when you fill an entry on the backpointer parse table and iterate throught
the possible splits for a (span/nonterminal) combination, that entry on the table will contain
the back-pointers for the the current-best split you have found so far. For each new possible
split, you need to check if that split would produce a higher log probability. If so, you update
the entry in the backpointer table, as well as the entry in the probability table.
After parsing has finished, the table entry table[0,len(toks)][grammar.startsymbol] will
contain the best backpointers for the left and right subtree under the root
node. probs[0,len(toks)][grammar.startsymbol] will contain the total log-probability for the
best parse.
cky.py contains two test functions check_table_format(table)
and check_prob_format(probs) that you can use to make sure the two table data structures
are formatted correctly. Both functions should return True. Note that passing this test does
not guarantee that the content of the tables is correct, just that the data structures are
probably formatted correctly.
>>> check_table_format(table)
True
>>> check_prob_format(probs)
True
Part 4 – Retrieving a parse tree (20 pts)
You now have a working parser, but in order to evaluate its performance we still need to
reconstruct a parse tree from the backpointer table returned by parse_with_backpointers.
Write the function get_tree(chart, i,j, nt) which should return the parse-tree rooted in nonterminal nt and covering span i,j.
For example
>>> parser = CkyParser(grammar)
>>> toks =[‘flights’, ‘from’, ‘miami’, ‘to’, ‘cleveland’, ‘.’]
>>> table, probs = parser.parse_with_backpointers(toks)
>>> get_tree(table, 0, len(toks), grammar.startsymbol)
(‘TOP’, (‘NP’, (‘NP’, ‘flights’), (‘NPBAR’, (‘PP’, (‘FROM’, ‘from’), (‘NP’, ‘miami’)), (‘PP’, (‘TO’, ‘to’), (‘NP’, ‘cleveland’)))), (‘
PUN’, ‘.’))
Note that the intended format is the same as the data in the treebank. Each tree is
represented as tuple where the first element is the parent node and the remaining elements
are children. Each child is either a tree or a terminal string.
Hint: Recursively traverse the parse chart to assemble this tree.
Part 5 – Evaluating the Parser (0 pts, but necessary to know that yoru parser is working well)
The program evaluate_parser.py evaluates your parser by comparing the output trees to the
trees in the test data set.
You can run the program like this:
python evaluate_parser.py atis3.pcfg atis3_test.ptb
Even though this program has been written for you, it might be useful to take a look at how
it works.
The program imports your CKY parser class. It then reads in the test file line-by-line. For
each test tree, it extracts the yield of the tree (i.e. the list of leaf nodes). It then feeds this list
as input to your parser, obtains a parse chart, and then retrieves the predicted tree by calling
your get_tree method.
It then compares the predicted tree against the target tree in the following way (this is a
standard approach to evaluating constiuency parsers, called PARSEVAL):
First it obtains a set of the spans in each tree (including the nonterminal label). I then
computes precision, recall, and F-score (which is the harmonic mean between precision and
recall) between these two sets. The script finally reports the coverage (percentage of test
sentences that had anyparse), the average F-score for parsed sentences, and the average Fscore for all sentences (including 0 f-scores for unparsed sentences).
My parser implementation produces the following result on the atis3 test corpus:
Coverage: 67%, Average F-score (parsed sentences): 0.95, Average F-score (all sentences):
0.64
What you need to submit
cky.py
evaluate_parser.py
grammar.py
Do not submit the data files.
Pack these files together in a zip or tgz file as described on top of this page. Please follow the
submission instructions precisely!
