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[ [[tutorial-en-001 | Intro]]    | [[tutorial-en-002 | Begin]] |
[[tutorial-en-003 | if/else]]    | [[tutorial-en-004 | Loops]] |
[[tutorial-en-005 | Arrays]]     | [[tutorial-en-006 | Graphics]] |
[[tutorial-en-007 | Animation]]  | [[tutorial-en-008 | Mouse]] |
[[tutorial-en-009 | Game]]       | [[tutorial-en-010 | Real]] |
[[tutorial-en-011 | Methods]]    | [[tutorial-en-012 | Class]] |
[[tutorial-en-013 | Class 2]]    | [[tutorial-en-014 | Applet]] |
[[tutorial-en-015 | MouseClick]] | [[tutorial-en-016 | Thread]] |
[[tutorial-en-017 | Button]]     ]
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====== Java For Kids tutorial 4 ======

===== Java LOOPs =====

=== WHILE loops ===
Try this:
<code java>
    void main() {
        // 1. declare and initialize variables
        int counter = 1;

        // 2. now do the while loop
        while (counter < 5)
        {
                printLine("counter  is equal to " + counter);
                // 3. add 1 to the value of counter
                counter = counter + 1;
        }

        // 4. the while loop has finished, so program continues on to here
        printLine("Goodbye");
    }
</code>

What was printed?
<code>
Counter is equal to 1
Counter is equal to 2
Counter is equal to 3
Counter is equal to 4
Goodbye
</code>

Notes on Above code:
  * Initialize the counter to 1.
  * The while loop (the statements inside the curly brackets after while) will repeat as long as the statement after it is true. If the statement is false (counter = 5 or greater) then the loop stops.
  * The counter adds one to itself. Each time the loop goes through, one is added to the counter until the condition (x<5) is false. I.e. x = 6. If the condition is false the loop is bypassed.
  * Goes on to the next line and prints "goodbye", and ends the program.

Loops are repeating sets of statements and in the case of the while loop,
here is the formula:
<code java>
    while (this statement is true)
    {
        // execute the statements or lines between these brackets
        // add 1 to the counter (usually)
    }
    // and continue the program with the following lines
</code>

E.g.
<code java>
    while (x <5)
    {
        printLine(" x = " + x);
        x = x + 1;
    }
</code>
What will the above code output?

{{java007.gif}}

** Java Exercises: **
  * Try the above example with the line x = x + 1; commented out.
<code java>
    void main() {
        while (x < 5)
        {
            printLine(" x = " + x);
            // x = x + 1;
        }
    }
</code>

  * Try the other examples of loops and comment out (start the line with a "<html>//</html>" so that line is not executed) the lines that add (increment) or minus (decrement) 1 to the condition variable (x or y).
  * What happens? Why?
  * Fix the errors in this code:
<code java>
    void main() {
        count = 0;
        while (count < 8)
        {
            Printline("Count is equal to " count);
    }
</code>

===== Java do ... while LOOPs =====
do while loops are similar to while loops except that the condition is tested
after the code to be repeated or looped.
The general form of a do ... while loop is as follows:
<code java>
    do {
        // execute code between these brackets
    }
    while (true or false (boolean) statement)
</code>

Note that the repeated code is always executed once, because the condition
is tested after the loop ends.
It may be better to use a plain old while loop in most cases,
because it tests the condition before the loop starts.

E.g.
<code java>
    int y = 10;
    printLine("Counting Down...");
    do {
	    printLine(".. " + y);
	    y = y - 1;
    }
    while (y > 1);
    printLine("Blast Off");
</code>
Try it! What did You get?

{{java008.gif}}

** Java Exericises **
  * Which loop always executes at least one pass or loop?
  * What is the output of the following code:
<code java>
    void main() {
        int counter = 10;

        while (counter > 1) {
            printLine(" " + counter +
                " green bottles hanging on the wall");
            printLine(" " + counter +
                " green bottles hanging on the wall");
            printLine("And if one green bottle should accidently fall, ");
            printLine("There'd be " + counter-1 +
                " green bottles hanging on the wall");
            counter = counter - 1;
        }
}
</code>
  * Sing what you have just printed out.
  * In a do ... while loop, when is the boolean statement tested?
  * Print out the set of even numbers from 20 to 30.
  * Print out the numbers between 27 and 35.

===== Java for LOOPs =====
Java for loops are a short hand way of;
  * initializing the counter,
  * setting the boolean condition, and
  * incrementing the counter,
All in one statement.

Before we carry on we have have to learn about the:

===== Java INCREMENT operator ( ++ ) =====
<code java>
    x++;
</code>
is the same as
<code java>
    x = x + 1;
</code>
X++ increments (or increases) the value of X by 1.
Here is an example that we could use in a while loop:
<code java>
    int x = 0;         // declare and initialize counter x
    while (x < 5)
    {
        printLine(" x = " + x);
        // add 1 to the value of x
        x++;        // same as (x = x + 1)
    }
</code>

Similarly, the:

===== DECREMENT operator is ( -- ) =====
y-- decrements (or decreases) y by 1.

<code java>
    y--;
</code>
is the same as
<code java>
    y = y - 1;
</code>

Try this:
<code java>
    int y = 10;
    while (y > 0)
    {
        printLine(" and " + y);
        y--;
    }
</code>

So, back to the for Loop.

A for Loop sets all the conditions of a loop inside the parantheses after
the for statement.

Example  A.
<code java>
    for (int x = 0; x < 10; x++)
    {
        printLine("x = " + x);
    }
</code>

The form of the for loop is:
<code java>
    for (INITIALIZE counter; boolean CONDITION; ITERATION)
    {
        // repeated code to be executed.
    }
</code>

Iteration means repetition, or repeating. In actual fact it is either
usually increased by 1 (x++) or decreased by 1 ( y--).

In example A above,
the Initialisation statement is "int x = 0;".

This statement declares x as an int and initializes x to be equal
to a value of 0 in one go.

The boolean condition is "x < 10;". This statement tests x and if it is true,
then the loop is allowed to execute, If it is false (x=10 or x > 10),
then the loop is bypassed and the program execution continues after
the { curly brackets } following the for statement.

Example B. Count down from 10 to 1:
<code java>
    for (int x = 10; x>0; x--)
    {
        printLine(x);
    }
</code>