Java Notes

'for' Loop

Purpose

The for statement is similar to the while statement, but it is often easier to use if you are counting or indexing because it combines three elements of many loops: initialization, testing, and incrementing.

General Form

The for and equivalent while statements have these forms.

for (init-stmt; condition; next-stmt) {
    body
}

init-stmt;
while (condition) {
    body
    next-stmt;
}

There are three clauses in the for statement.

1. The init-stmt statement is done before the loop is started, usually to initialize an iteration variable.
2. The condition expression is tested before each time the loop is done. The loop isn't executed if the boolean expression is false (the same as the while loop).
3. The next-stmt statement is done after the body is executed. It typically increments an iteration variable.

Example - Printing a table of squares

Here is a loop written as both a while loop and a for loop. First using while:

int number = 1;
while (number <= 12) {
    System.out.println(number + " squared is " + (number * number));
    number++;
}

And here is the same loop using for.

for (int number = 1; number <= 12; number++) {
    System.out.println(number + " squared is " + (number * number));
}

Example - Counting doubled characters

This code will look at each character in a string, sentence, and count the number of times any character occurs doubled.

String sentence = ...;
int doubleCount = 0;     // Number of doubled characters.

// Start at second char (index 1).
for (int pos = 1; pos < sentence.length(); pos++) (
    // Compare each character to the previous character.
    if (sentence.charAt(pos) == sentence.charAt(pos-1)) {
        doubleCount++;
    }
}

Summary

The for loop is shorter, and combining the intialization, test, and increment in one statement makes it easier to read and verify that it's doing what you expect. The for loop is better when you are counting something. If you are doing something an indefinite number of times, while loop may be the better choice.