Calculating Probabilities

You now have counts for how often an email message appears in each category, so the next step is to convert these numbers into probabilities. A probability is a number between 0 and 1, indicating how likely an event is. In this case, you can calculate the probability that a word is in a particular category by dividing the number of times the word appears in a document in that category by the total number of documents in that category.

Add a method called fprob to the classifier class:

  def fprob(self,f,cat):
    if self.catcount(cat)==0: return 0
    # The total number of times this feature appeared in this
    # category divided by the total number of items in this category
    return self.fcount(f,cat)/self.catcount(cat)