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Strings and Things

Looping and Counting

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

String fruit = "banana";
int length = fruit.length();
int count = 0;

int index = 0;
while (index < length) {
if (fruit.charAt(index) == 'a') {
count = count + 1;
}
index = index + 1;
}
System.out.println (count);

This program demonstrates a common idiom, called a counter. The variable count is initialized to zero and then incremented each time we find an 'a' (to increment is to increase by one; it is the opposite of decrement, and unrelated to excrement, which is a noun). When we exit the loop, count contains the result: the total number of a's.

As an exercise, encapsulate this code in a method named countLetters, and generalize it so that it accepts the string and the letter as arguments.

As a second exercise, rewrite the method so that it uses indexOf to locate the a's, rather than checking the characters one by one.

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Strings and Things

Looping and Counting

Last Update: 2011-01-24