21.3: Traversal with a for Loop

A lot of computations involve processing a string one character at a time. Often they start at the beginning, select each character in turn, do something to it, and continue until the end. This pattern of processing is called a traversal. One way to write a traversal is with a while loop:

index = 0
while index < len(fruit):
letter = fruit[index]
print letter
index = index + 1


This loop traverses the string and displays each letter on a line by itself. The loop condition is index < len(fruit), so when index is equal to the length of the string, the condition is false, and the body of the loop is not executed. The last character accessed is the one with the index len(fruit)-1, which is the last character in the string.

Exercise $$\PageIndex{1}$$

Write a function that takes a string as an argument and displays the letters backward, one per line.

Another way to write a traversal is with a for loop:

for char in fruit:
print char


Each time through the loop, the next character in the string is assigned to the variable char. The loop continues until no characters are left.

The following example shows how to use concatenation (string addition) and a for loop to generate an abecedarian series (that is, in alphabetical order). In Robert McCloskey’s book Make Way for Ducklings, the names of the ducklings are Jack, Kack, Lack, Mack, Nack, Ouack, Pack, and Quack. This loop outputs these names in order:

prefixes = 'JKLMNOPQ'
suffix = 'ack'

for letter in prefixes:
print letter + suffix


The output is:

Jack
Kack
Lack
Mack
Nack
Oack
Pack
Qack


Of course, that’s not quite right because “Ouack” and “Quack” are misspelled.

Exercise $$\PageIndex{2}$$

Modify the program to fix this error.