ARRAYS - INTRODUCTION

Every high-level programming language supports arrays of one sort or another: a variable that can contain not just a simple character string or a numeric value but a matrix of many data elements. PHP provides rich support for arrays and techniques for processing them.

This article begins with a basic introduction to arrays and then looks at how they can be manipulated in PHP - It is broken down into pages which deal with individual topics and there is an index to this article at the bottom of this page. This page - the introduction - discusses arrays in quite general terms. There are side panels which give more formal definitions of terminology together with examples of different types of array.

At its most basic level an array can hold a simple list of values, maybe a shopping list like

	bread
	lemonade
	apples
	grapes
	coffee

If you think about writing code without arrays, every value would need to be stored using a simple variable. Each item in the shopping list would need to be assigned to a string variable using code like this

$item1 = "Bread";
$item2 = "Lemonade";
$item3 = "Apples";
$item4 = "Grapes";
$item5 = "Coffee";

which is perfectly good PHP code but printing the entire list would necessitate knowing - and coding - every variable name, for instance

print("<BR>$item1<BR>$item2<BR>$item3<BR>$item4<BR>$item5");

which, again, is perfectly good PHP code. But what happens when you add Eggs to the list? If Eggs goes into $item6 you'll need to change all the code that handles the shopping list, adding an assignment for $item6

$item6 = "Eggs";

and changing the print function to add a further HTML line break (<BR>) and the additional variable, $item6.

If, on the other hand, the shopping list was stored in an array, the list could hold one or more entries. The entries could be accessed by their index and we could use the following construct

$shoppinglist = array("Bread", "Lemonade",
   "Apples", "Grapes", "Coffee", "Eggs");

then code to implement this

for each element in array $shoppinglist
   print one value from $shoppinglist
loop for next element in $shoppinglist

This will print every value from the list whether there is one, ten, a hundred or a thousand. We'll return to the real PHP code to do this after we've looked at creating and filling arrays.

In PHP arrays are named in exactly the same way as other variables: A dollar sign followed by either an underscore character, an alphabetical character or a numeric character followed by any combination of underscores, aplha characters or numerics.

Each element of a PHP array has an index - in PHP this is also known as a key and in some older programming texts (and some not-so-old!) the index is referred to as a subscript. The index is used to access a particular element in the array. In many programming languages array elements are referenced by a numeric index but in PHP the index (or key) can be an index number, a more complex (not necessarily sequential) numeric value or a character string. We'll be returning to accessing elements using their keys but, as part of this introduction, suffice it to say that the index value is placed inside square brackets following the array name, as in $shoppinglist[2] $items[44] $score["Home team"] $score["Away team"] which gives huge flexibility.

How arrays are indexed Simple, numerically indexed list Index Value 0 Bread 1 Lemonade 2 Apples 3 Grapes 4 Coffee 5 Eggs More complex, keyed example of an associative array Key Value default.htm Home page map.htm Site map weather.htm Weather map shopping.htm On-line store front