Think Python, 3rd Edition

Chapter 7. Iteration and Search

In 1939, Ernest Vincent Wright published a 50,000-word novel called Gadsby that does not contain the letter “e.” Since “e” is the most common letter in English, writing even a few words without using it is difficult. To get a sense of how difficult, in this chapter we’ll compute the fraction of English words have at least one “e.”

For that, we’ll use for statements to loop through the letters in a string and the words in a file, and we’ll update variables in a loop to count the number of words that contain an “e.” We’ll use the in operator to check whether a letter appears in a word, and you’ll learn a programming pattern called a “linear search.”

As an exercise, you’ll use these tools to solve a word puzzle called “Spelling Bee.”

Loops and Strings

In Chapter 3 we saw a for loop that uses the range function to display a sequence of numbers:

for i in range(3):
    print(i, end=' ')
       

0 1 2 
       

This version uses the keyword argument end, so the print function puts a space after each number rather than a newline.

We can also use a for loop to display the letters in a string:

for letter in 'Gadsby':
    print(letter, end=' ')
       

G a d s b y 
       

Notice that I changed the name of the variable from i to letter, which provides more information about the value it refers to. The variable defined in a for loop is called the loop variable.

Now that we can loop through the letters in a word, we can check whether it contains the letter “e”:

for letter in "Gadsby":
    if