Home > Articles > Programming > PHP

  • Print
  • + Share This
This chapter is from the book

Multidimensional Arrays

Arrays do not have to be a simple list of keys and values; each location in the array can hold another array. This way, you can create a two-dimensional array. You can think of a two-dimensional array as a matrix, or grid, with width and height or rows and columns.

If you want to store more than one piece of data about each of Bob's products, you could use a two-dimensional array. Figure 3.3 shows Bob's products represented as a two-dimensional array with each row representing an individual product and each column representing a stored product attribute.

Figure 3.3Figure 3.3 You can store more information about Bob's products in a two- dimensional array.

Using PHP, you would write the following code to set up the data in the array shown in Figure 3.3:

$products = array( array( 'TIR', 'Tires', 100 ),
          array( 'OIL', 'Oil', 10 ),
          array( 'SPK', 'Spark Plugs', 4 ) );

You can see from this definition that the $products array now contains three arrays.

To access the data in a one-dimensional array, recall that you need the name of the array and the index of the element. A two-dimensional array is similar, except that each element has two indices: a row and a column. (The top row is row 0, and the far-left column is column 0.)

To display the contents of this array, you could manually access each element in order like this:

echo '|'.$products[0][0].'|'.$products[0][1].'|'.$products[0][2].'|<br />';
echo '|'.$products[1][0].'|'.$products[1][1].'|'.$products[1][2].'|<br />';
echo '|'.$products[2][0].'|'.$products[2][1].'|'.$products[2][2].'|<br />';

Alternatively, you could place a for loop inside another for loop to achieve the same result:

for ( $row = 0; $row < 3; $row++ )
{
 for ( $column = 0; $column < 3; $column++ )
 {
  echo '|'.$products[$row][$column];
 }
 echo '|<br />';
}

Both versions of this code produce the same output in the browser:

|TIR|Tires|100|
|OIL|Oil|10|
|SPK|Spark Plugs|4|

The only difference between the two examples is that your code will be shorter if you use the second version with a large array.

You might prefer to create column names instead of numbers, as shown in Figure 3.3. To store the same set of products, with the columns named as they are in Figure 3.3, you would use the following code:

$products = array( array( 'Code' => 'TIR',
             'Description' => 'Tires',
             'Price' => 100
            ),
          array( 'Code' => 'OIL',
             'Description' => 'Oil',
             'Price' => 10
            ),
          array( 'Code' => 'SPK',
             'Description' => 'Spark Plugs',
             'Price' =>4
            )
         );

This array is easier to work with if you want to retrieve a single value. Remembering that the description is stored in the Description column is easier than remembering it is stored in column 1. Using descriptive indices, you do not need to remember that an item is stored at [x][y]. You can easily find your data by referring to a location with meaningful row and column names.

You do, however, lose the ability to use a simple for loop to step through each column in turn. Here is one way to write code to display this array:

for ( $row = 0; $row < 3; $row++ )
{
 echo '|'.$products[$row]['Code'].'|'.$products[$row]['Description'].
    '|'.$products[$row]['Price'].'|<br />';
}

Using a for loop, you can step through the outer, numerically indexed $products array. Each row in the $products array is an array with descriptive indices. Using the each() and list() functions in a while loop, you can step through these inner arrays. Therefore, you need a while loop inside a for loop:

for ( $row = 0; $row < 3; $row++ )
{
 while ( list( $key, $value ) = each( $products[ $row ] ) )
 {
  echo "|$value";
 }
 echo '|<br />';
}

You do not need to stop at two dimensions. In the same way that array elements can hold new arrays, those new arrays, in turn, can hold more arrays.

A three-dimensional array has height, width, and depth. If you are comfortable thinking of a two-dimensional array as a table with rows and columns, imagine a pile or deck of those tables. Each element is referenced by its layer, row, and column.

If Bob divided his products into categories, you could use a three-dimensional array to store them. Figure 3.4 shows Bob's products in a three-dimensional array.

Figure 3.4Figure 3.4 This three-dimensional array allows you to divide products into categories.

From the code that defines this array, you can see that a three-dimensional array is an array containing arrays of arrays:

$categories = array( array ( array( 'CAR_TIR', 'Tires', 100 ),
               array( 'CAR_OIL', 'Oil', 10 ),
               array( 'CAR_SPK', 'Spark Plugs', 4 )
              ),
           array ( array( 'VAN_TIR', 'Tires', 120 ),
               array( 'VAN_OIL', 'Oil', 12 ),
               array( 'VAN_SPK', 'Spark Plugs', 5 )
              ),
           array ( array( 'TRK_TIR', 'Tires', 150 ),
               array( 'TRK_OIL', 'Oil', 15 ),
               array( 'TRK_SPK', 'Spark Plugs', 6 )
              )
          );

Because this array has only numeric indices, you can use nested for loops to display its contents:

for ( $layer = 0; $layer < 3; $layer++ )
{
 echo "Layer $layer<br />";
 for ( $row = 0; $row < 3; $row++ )
 {
  for ( $column = 0; $column < 3; $column++ )
  {
   echo '|'.$categories[$layer][$row][$column];
  }
  echo '|<br />';
 }
}

Because of the way multidimensional arrays are created, you could create four-, five-, or even six-dimensional arrays. There is no language limit to the number of dimensions, but it is difficult for people to visualize constructs with more than three dimensions. Most real-world problems match logically with constructs of three or fewer dimensions.

  • + Share This
  • 🔖 Save To Your Account