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Numerically Indexed Arrays

Numerically indexed arrays are supported in most programming languages. In PHP, the indices start at zero by default, although you can alter this value.

Initializing Numerically Indexed Arrays

To create the array shown in Figure 3.1, use the following line of PHP code:

$products = array( 'Tires', 'Oil', 'Spark Plugs' );

This code creates an array called $products containing the three values given: 'Tires', 'Oil', and 'Spark Plugs'. Note that, like echo, array() is actually a language construct rather than a function.

Depending on the contents you need in your array, you might not need to manually initialize them as in the preceding example. If you have the data you need in another array, you can simply copy one array to another using the = operator.

If you want an ascending sequence of numbers stored in an array, you can use the range() function to automatically create the array for you. The following statement creates an array called numbers with elements ranging from 1 to 10:

$numbers = range(1,10);

The range() function has an optional third parameter that allows you to set the step size between values. For instance, if you want an array of the odd numbers between 1 and 10, you could create it as follows:

$odds = range(1, 10, 2);

The range() function can also be used with characters, as in this example:

$letters = range('a', 'z');

If you have information stored in a file on disk, you can load the array contents directly from the file. We look at this topic later in this chapter under the heading "Loading Arrays from Files."

If you have the data for your array stored in a database, you can load the array contents directly from the database. This process is covered in Chapter 11, "Accessing Your MySQL Database from the Web with PHP."

You can also use various functions to extract part of an array or to reorder an array. We look at some of these functions later in this chapter under the heading "Performing Other Array Manipulations."

Accessing Array Contents

To access the contents of a variable, you use its name. If the variable is an array, you access the contents using the variable name and a key or index. The key or index indicates which of the values in the array you access. The index is placed in square brackets after the name.

Type $products[0], $products[1], and $products[2] to use the contents of the $products array.

By default, element zero is the first element in the array. The same numbering scheme is used in C, C++, Java, and a number of other languages, but it might take some getting used to if you are not familiar with it.

As with other variables, you change array elements' contents by using the = operator. The following line replaces the first element in the array 'Tires' with 'Fuses':

$products[0] = 'Fuses';

You can use the following line to add a new element—'Fuses'—to the end of the array, giving a total of four elements:

$products[3] = 'Fuses';

To display the contents, you could type this line:

echo "$products[0] $products[1] $products[2] $products[3]";

Note that although PHP's string parsing is pretty clever, you can confuse it. If you are having trouble with array or other variables not being interpreted correctly when embedded in a double-quoted string, you can either put them outside quotes or look up complex syntax in Chapter 4, "String Manipulation and Regular Expressions." The preceding echo statement works correctly, but in many of the more complex examples later in this chapter, you will notice that the variables are outside the quoted strings.

Like other PHP variables, arrays do not need to be initialized or created in advance. They are automatically created the first time you use them.

The following code creates the same $products array created previously with the array() statement:

$products[0] = 'Tires';
$products[1] = 'Oil';
$products[2] = 'Spark Plugs';

If $products does not already exist, the first line will create a new array with just one element. The subsequent lines add values to the array. The array is dynamically resized as you add elements to it. This resizing capability is not present in most other programming languages.

Using Loops to Access the Array

Because the array is indexed by a sequence of numbers, you can use a for loop to more easily display its contents:

for ( $i = 0; $i<3; $i++ )
 echo "$products[$i] ";

This loop provides similar output to the preceding code but requires less typing than manually writing code to work with each element in a large array. The ability to use a simple loop to access each element is a nice feature of arrays. You can also use the foreach loop, specially designed for use with arrays. In this example, you could use it as follows:

foreach ($products as $current)
 echo $current.' ';

This code stores each element in turn in the variable $current and prints it out.

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