21.7: Looping and Counting

The following program counts the number of times the letter a appears in a string:

word = 'banana'
count = 0
for letter in word:
    if letter == 'a':
        count = count + 1
print count

This program demonstrates another pattern of computation called a counter. The variable count is initialized to 0 and then incremented each time an a is found. When the loop exits, count contains the result—the total number of a’s.

Exercise \(\PageIndex{1}\)

Encapsulate this code in a function named count, and generalize it so that it accepts the string and the letter as arguments.

Exercise \(\PageIndex{2}\)

Rewrite this function so that instead of traversing the string, it uses the three-parameter version of find from the previous section.