The following program counts the number of times the letter a appears in a string:
word = 'banana'count = 0for letter in word: if letter == 'a': count = count + 1print 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 6.3 Encapsulate this code in a function named count, and generalize it so that it accepts the string and the letter as arguments.