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C'est la Z

Working with texts part 2 - bag of words

Following up on a previous post, we're going to continue to talk about playing with text. This time, building and working with a bag of words from a text.

A bag of words is a simple language processing model where you just consider individual words in a text. What they are and how many times they occur. This is a pretty simple model but you can still have a good bit of fun with your students with it.

For example, if you take a text like this:

This land is your land this land is my land

Once you clean the a data as per the previous post you get the following bag:


Not too interesting but it gets more interesting on a larger text. We used:

  • Act 1 Scene 1 from Macbeth (for testing - it's only 13 lines)
  • Cyrano de Bergerac (because it's my favorite play)
  • Moby Dick (chapter 1)
  • Moby Dick (full text)
  • The Book of Psalms

Assuming a cleaned string with all the text, building the bag is pretty easy:

1cleaned_text = "a string with all the cleaned text"
3for word in cleaned_text.split():
4  bag.setdefault(word,0)
5  bag[word] = bag[word] + 1

The setdefault in line three says that if word isn't a key in the dictionary then insert it with a value of 0, otherwise, if word is already a key in the dictionary do nothing. This saves us from constructs like:

1  if word not in bag:
2      bag[word]=1
3  else:
4      bag[word] = bag[word] + 1

Once we have our bag we can do some explorations. Take out all the values using bag.values() and sort them. Ask "what words are going to occur most frequently?" It's easy to guess that it will be words like the, of, and etc.. Words that don't really tell us anything about the text. It's a fun puzzle to figure out where the "interesting" words start and even the question of what might make a word in a text "interesting." Words like the, *and", etc. are known as stop words. Depending on the application, you might want ot remove them. Then again, you might not.

Usually we take a bit of time just to play with the bag, looking at the types of words in various count ranges. Sometimes we'll make bags for different chapters of books and compare them. This is also when I cover Python list comprehensions which make experimenting on bags of words much cleaner. Say, for instance you want to find all the words that occur between 50 and 100 times. Without list comprehensions you might write:

1result = []
2for word in bag:
3    if bag[word] >= 50 and bag[word] <= 100:
4        result.append(word) # or (word,count) or whatever

While with the comprehension it's:

result = [ word for word in bag if bag[word] >= 50 and bag[word] <= 100]

Besides basic exploring, what else can you do? You can make word vectors, that is a list or vector where an element is 1 if the word is present and 0 otherwise. Then you can do simple document comparisons. You can kick this up a notch if you calculate word frequencies based on the bag counts and use these.

You can also do basic sentiment analysis. Download a list of "positive" and "negative" words and then check bags against them. You can check to see how many negative or positive words are in a bag or you can work with frequencies. You can also use other categories.

While we were discussing this at our PD, some teachers thought that their classes might ahve a hard time doing all of this from scratch but thought that they could provide either the cleaning code or the bag building code and have the students build experiments off of the base. I thought it was a great idea.

At the end of our session we all agreed to think about how we might build a lesson or unit out of this bag of words idea. I'll leave you all with some web sites that use these ideas in what I think are interesting ways:


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