imclab / pcfg

A Python implementation of Probabilistic Context-Free Grammar Parser.

Probabilistic Context-Free Grammar Parser

It is an implementation of Probabilistic Context-Free Grammar Parser. pcfg_parser.py reads lines from stdin, and outputs parsed trees.

Usage

Here is an example usage of this parser.

echo "the man saw the dog with the telescope" | python pcfg_parser.py counts_file.sample
["S", ["NP", ["DET", "the"], ["NOUN", "man"]], ["VP", ["VERB", "saw"], ["NP", ["NP", ["DET", "the"], ["NOUN", "dog"]], ["PP", ["ADP", "with"], ["NP", ["DET", "the"], ["NOUN", "telescope"]]]]]]

This parser outputs the parsed tree under the probabilistic model derived from the passed count file. A parsed tree is represented as a nested list in JSON format.

Counts File

If you want to train your own model, you need to prepare a counts file which contains the frequency of each word and rule in your training corpus. The format of counts file is as follows:

1 NONTERMINAL ADVP+DET
14 UNARYRULE NP+NOUN Stocks
37 BINARYRULE VP VERB NP+PRON

The first and second columns represents the number of counts and the type of entry, respectively. There are three types of entry and three formats for each type.

NONTERMINAL

This is a type for nonterminals such as S, VP, NP and so on. This type of entry has one more column for a specific nonterminal symbol.

UNARYRULE

This is a type for unary rules, which means assignments of nonterminals for terminal words. e.g. VERB -> said, VERB -> look, ADP -> after and NOUN -> point. This type of entry has two more columns: nonterminal rule and terminal symbol.

BINARYRULE

This is a type for binary rules such as S -> NP + VP. This type of entry has tree more columns: the leftmost item can be composed of the next item followed by the last item.