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Tim Strehle’s links and thoughts on Web apps, managing software development and Digital Asset Management, since 2002.
2015-09-03

Counting word frequency using NLTK FreqDist()

A pretty simple programming task: Find the most-used words in a text and count how often they’re used. (With the goal of later creating a pretty Wordle-like word cloud from this data.)

I assumed there would be some existing tool or code, and Roger Howard said NLTK’s FreqDist() was “easy as pie”.

So today I wrote the first Python program of my life, using NLTK, the Natural Language Toolkit. With the help of the NLTK tutorial and StackOverflow. I’m sure it’s terrible Python and bad use of NLTK. Sorry, I’m a total newbie.

In my example, the whole text is read into memory. I’d like to process a lot of text (say, a 300 MB text file) – can NLTK do this? My 30 MB file took 40 seconds to process. Anyway, I’m happy I got it working at all.

#!/usr/bin/env python
# coding=UTF-8
#
# Output the 50 most-used words from a text file, using NLTK FreqDist()
# (The text file must be in UTF-8 encoding.)
#
# Usage:
#
# ./freqdist_top_words.py input.txt
#
# Sample output:
#
# et;8
# dolorem;5
# est;4
# aut;4
# sint;4
# dolor;4
# laborum;3
# ...
#
# Requires NLTK. Official installation docs: http://www.nltk.org/install.html
#
# I installed it on my Debian box like this:
#
# sudo apt-get install python-pip
# sudo pip install -U nltk
# python
# >>> import nltk
# >>> nltk.download('stopwords')
# >>> nltk.download('punkt')
# >>> exit()

import sys
import codecs
import nltk
from nltk.corpus import stopwords

# NLTK's default German stopwords
default_stopwords = set(nltk.corpus.stopwords.words('german'))

# We're adding some on our own - could be done inline like this...
# custom_stopwords = set((u'–', u'dass', u'mehr'))
# ... but let's read them from a file instead (one stopword per line, UTF-8)
stopwords_file = './stopwords.txt'
custom_stopwords = set(codecs.open(stopwords_file, 'r', 'utf-8').read().splitlines())

all_stopwords = default_stopwords | custom_stopwords

input_file = sys.argv[1]

fp = codecs.open(input_file, 'r', 'utf-8')

words = nltk.word_tokenize(fp.read())

# Remove single-character tokens (mostly punctuation)
words = [word for word in words if len(word) > 1]

# Remove numbers
words = [word for word in words if not word.isnumeric()]

# Lowercase all words (default_stopwords are lowercase too)
words = [word.lower() for word in words]

# Stemming words seems to make matters worse, disabled
# stemmer = nltk.stem.snowball.SnowballStemmer('german')
# words = [stemmer.stem(word) for word in words]

# Remove stopwords
words = [word for word in words if word not in all_stopwords]

# Calculate frequency distribution
fdist = nltk.FreqDist(words)

# Output top 50 words

for word, frequency in fdist.most_common(50):
print(u'{};{}'.format(word, frequency))

If you want to teach me better Python, I’m open to suggestions for improvement :-)

Update: You can also find that script on GitHub. Many thanks to Roger Howard for improving it!

Thu, 03 Sep 2015 19:53:00 +0000