Part 4 - Loops and Arrays | JavaScript

JavaScript Loops

Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
In JavaScript, there are two different kind of loops:

• for - loops through a block of code a specified number of times
• while - loops through a block of code while a specified condition is true

While Loops

While loops are pieces of code which will repeat until the condition is met. This is very useful for things like passwords when you want to keep asking the user until they get it correct. For example this code will repeat until the user enters the word 'hello':

 var password = 'hello';

 var input = prompt('Please enter the password', '');

 while(input != password)

 {

 var input= prompt('Please enter the password''');

 }

This will continuously loop the code inside the { } until the test input does not equal password is false (the password is correct).

For Loops

For loops are used to do something a set number of times. For example:

 for(loop=0; loop < 11; loop++)

 {

 document.writeln(loop);

 }

This will start by setting the variable loop to 0, it will then loop, adding one to the value each time (using the loop++ code) as long as loop is less than 11. They take the form:

 for(starting value; test; increment)

These have many uses.

Arrays

What is an Array?

An array is a special variable, which can hold more than one value, at a time.
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:

$cars1="Saab"; $cars2="Volvo"; $cars3="BMW";

However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
The best solution here is to use an array!
An array can hold all your variable values under a single name. And you can access the values by referring to the array name.
Each element in the array has its own ID so that it can be easily accessed.

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Create an Array

The following code creates an Array object called myCars:

var myCars=new Array();

There are two ways of adding values to an array (you can add as many values as you need to define as many variables you require).
1:

var myCars=new Array(); myCars[0]="Saab"; myCars[1]="Volvo"; myCars[2]="BMW";

You could also pass an integer argument to control the array's size:

var myCars=new Array(3); myCars[0]="Saab"; myCars[1]="Volvo"; myCars[2]="BMW";

2:

var myCars=new Array("Saab","Volvo","BMW");

Note: If you specify numbers or true/false values inside the array then the type of variables will be numeric or Boolean instead of string.

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Access an Array

You can refer to a particular element in an array by referring to the name of the array and the index number. The index number starts at 0.
The following code line:

document.write(myCars[0]);

will result in the following output:

Saab

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Modify Values in an Array

To modify a value in an existing array, just add a new value to the array with a specified index number:

myCars[0]="Opel";

Now, the following code line:

document.write(myCars[0]);

will result in the following output:
Opel