Dec 19, 2007

Our Methods are Getting Loopy

Hello, the post may be a little messed up, blogger really doesn't like < pre >!

Today we are going to learn "loops". Loops are a way of repeating code over and over and over and so on... There are a couple types of loops in JAVA, "while", "do-while" and "for" loops. We will do "while" and the basic "for".

The while loop:
- while some condition is true the loop will run.

while (some condition) {
//code to run

An example of this:

while (x > 0) {
//code to run while x > 0

The for loop:
- this type of loop is for some code that you know needs to run a certain amount of times. These are basically a while loop with a sort of counter built in.

for (some variable; condition; increment/decrement) {
//code to run

An example of this:

for (int x = 0; x < 5 ; x = x + 1) {
//we declare a variable x and set it to zero
//more on declaring variables later
//this will run when x < 5
//x will go up by one each time the loop runs through

A couple of things we have to go over here:
1) The "//" indicates a comment. A comment is a piece of writing that java ignores. It just skips merrily along right over it. "//" is used for 1 line comments.

2) "/* your comments */" is for larger comments. All writing in between the /* */ is ignored.

3) Operators... the <, >, != and == are your basic operators. THERE ARE 2 EQUALS SIGNS THERE! This is on purpose, when comparing two values you use TWO EQUALS SIGNS! When setting something equal to another thing you use one equals sign. The != means not equal to.

4) You can also combine these operators, <= and >=, less than or equal to and greater than or equal to. For a complete list go here.

So now let's make a small program with some of the things we know. We'll make it display a message a certain amount of times.

public class Display {
public static void main(String[] args) {
for (int x = 0; x < 5; x = x + 1){
System.out.println("Counting: " + x);

The output to the command line for this will be:
Counting: 0
Counting: 1
Counting: 2
Counting: 3
Counting: 4

The x is incrementing so you will see it go up by one each time starting from 0. Notice the x = x + 1 only runs once the loop has gone through once and not immediatly.

The line:
System.out.println("Counting: " + x);
Notice that this is not:
System.out.print("Counting: " + x);

.print prints continuously on one line.
.println print to a new line each time.

Also in that line we have "Five times" + x
The "+" in this acts to combine the two for output. It concatenates it, joins them together.

The while loop itself is a bit simpler:

public class Display2{
public static void main(String[] args) {
int x = 0;
while (x < 5) {
x = x + 1;

This will give us an output of:

This is enough to soak in for one day. We'll start making some simple programs soon so we can get this deep in our heads.