\chapter{Case study: word play}

This chapter presents the second case study, which involves

solving word puzzles by searching for words that have certain

properties. For example, we'll find the longest palindromes

in English and search for words whose letters appear in

alphabetical order. And I will present another program development

plan: reduction to a previously solved problem.

\section{Reading word lists}

lexicon project (see \url{http://wikipedia.org/wiki/Moby_Project}). It

is a list of 113,809 official crosswords; that is, words that are

Moby collection, the filename is {\tt 113809of.fic}; you can download

a copy, with the simpler name {\tt words.txt}, from

\url{http://thinkpython2.com/code/words.txt}.

This file is in plain text, so you can open it with a text

editor, but you can also read it from Python. The built-in

function {\tt open} takes the name of the file as a parameter

and returns a {\bf file object} you can use to read the file.

{\tt fin} is a common name for a file object used for input. The file

object provides several methods for reading, including {\tt readline},

which reads characters from the file until it gets to a newline and

returns the result as a string: \index{readline method}

The first word in this particular list is ``aa'', which is a kind of

lava. The sequence \verb"\r\n" represents two whitespace characters,

a carriage return and a newline, that separate this word from the

next.

if you call {\tt readline} again, you get the next word:

The next word is ``aah'', which is a perfectly legitimate

word, so stop looking at me like that.

Or, if it's the whitespace that's bothering you,

we can get rid of it with the string method {\tt strip}:

You can also use a file object as part of a {\tt for} loop.

This program reads {\tt words.txt} and prints each word, one

per line:

You should at least attempt each one before you read the solutions.

\begin{exercise}

Write a program that reads {\tt words.txt} and prints only the

words with more than 20 characters (not counting whitespace).

In 1939 Ernest Vincent Wright published a 50,000 word novel called

{\em Gadsby} that does not contain the letter ``e''. Since ``e'' is

the most common letter in English, that's not easy to do.

In fact, it is difficult to construct a solitary thought without using

that most common symbol. It is slow going at first, but with caution

and hours of training you can gradually gain facility.

All right, I'll stop now.

Write a function called \verb"has_no_e" that returns {\tt True} if

the given word doesn't have the letter ``e'' in it.

Modify your program from the previous section to print only the words

that have no ``e'' and compute the percentage of the words in the list

that have no ``e''.

Write a function named {\tt avoids}

that takes a word and a string of forbidden letters, and

that returns {\tt True} if the word doesn't use any of the forbidden

letters.

Modify your program to prompt the user to enter a string

of forbidden letters and then print the number of words that

don't contain any of them.

Can you find a combination of 5 forbidden letters that

excludes the smallest number of words?

Write a function named \verb"uses_only" that takes a word and a

string of letters, and that returns {\tt True} if the word contains

letters {\tt acefhlo}? Other than ``Hoe alfalfa?''

Write a function named \verb"uses_all" that takes a word and a

string of required letters, and that returns {\tt True} if the word

uses all the required letters at least once. How many words are there

that use all the vowels {\tt aeiou}? How about {\tt aeiouy}?

Write a function called \verb"is_abecedarian" that returns

{\tt True} if the letters in a word appear in alphabetical order

(double letters are ok).

How many abecedarian words are there?

\section{Search}