What does the following function do?
def find(word, letter): index = 0 while index < len(word): if word[index] == letter: return index index = index + 1 return -1
In a sense, find is the opposite of the  operator. Instead of taking an index and extracting the corresponding character, it takes a character and ﬁnds the index where that character appears. If the character is not found, the function returns -1.
This is the ﬁrst example we have seen of a return statement inside a loop. If word[index] == letter, the function breaks out of the loop and returns immediately.
If the character doesn’t appear in the string, the program exits the loop normally and returns -1.
This pattern of computation—traversing a sequence and returning when we ﬁnd what we are looking for—is called a search.
Exercise 8.4.Modify findso that it has a third parameter, the index in wordwhere it should start looking.