Natural Language Processing with Python

Back to Python: Making Decisions and Taking Control

So far, our little programs have had some interesting qualities: the ability to work with language, and the potential to save human effort through automation. A key feature of programming is the ability of machines to make decisions on our behalf, executing instructions when certain conditions are met, or repeatedly looping through text data until some condition is satisfied. This feature is known as control, and is the focus of this section.

Conditionals

Python supports a wide range of operators, such as < and >=, for testing the relationship between values. The full set of these relational operators are shown in Table 1-3.

Table 1-3. Numerical comparison operators

Operator	Relationship
<	Less than
<=	Less than or equal to
==	Equal to (note this is two “=”signs, not one)
!=	Not equal to
>	Greater than
>=	Greater than or equal to

We can use these to select different words from a sentence of news text. Here are some examples—notice only the operator is changed from one line to the next. They all use sent7, the first sentence from text7 (Wall Street Journal). As before, if you get an error saying that sent7 is undefined, you need to first type: from nltk.book import *.

>>> sent7 ['Pierre', 'Vinken', ',', '61', 'years', 'old', ',', 'will', 'join', 'the', 'board', 'as', 'a', 'nonexecutive', 'director', 'Nov.', '29', '.'] >>> [w for w in sent7 if len(w) < 4] [',', '61', 'old', ',', 'the', 'as', 'a', '29', '.'] >>> [w for w in sent7 if len(w) <= 4] [',', ...