PHP: Associative Arrays I

Derek Bridge

Department of Computer Science,
University College Cork

PHP: Associative Arrays I

Aims:

• to learn about PHP's associative arrays (how to create, insert, modify, and visit each element)

Associative arrays

• An associative array (sometimes called a map) is a collection of storage locations, indexed by keys (strings)
• To create an associative array in PHP, e.g.:

  $fruit_prices = array('Apples' => 1.59, 'Pears' => 2.34,
    'Kumquats' => 4.05, 'Jujubes' => 2.34);

• Another way to create an associative array, e.g.

  $fruit_prices['Apples'] = 1.59;
$fruit_prices['Pears'] = 2.34;
$fruit_prices['Kumquats'] = 4.05;
$fruit_prices['Jujubes'] = 2.34;

Associative arrays

• More examples:

  $month_lengths = array('Jan' => 31, 'Feb' => 28, 
      'Mar' => 31, 'Apr' => 30, 'May' => 31, 'Jun' => 30, 'Jul' => 31,
      'Aug' => 31, 'Sep' => 30, 'Oct' => 31, 'Nov' => 31, 'Dec' => 31);
      
  $module_lecturers['AL1101'] = 'Hugh Jeegoh';
  $module_lecturers['WC1101'] = 'Ann O Domini';

Accessing the elements in an associative array

• To insert a new key and value into an existing associative array, e.g.:

  $fruit_prices['Mangoes'] = 5.26;

• To modify the value for an existing key in an associative array, e.g.

  $fruit_prices['Kumquats'] = 3.93;

• To access a value in an array, e.g.

  $pear_price = $fruit_prices['Pears'];
echo "Pears cost {$pear_price} ";
echo "but mangoes cost {$fruit_prices['Mangoes']}";

Doing things to each value in an associative array

• The foreach statement:

  foreach ($a as $k => $v)
{
    zero, one or more statements
}

• This is a loop
  • It visits each element in array $a in turn
  • For each element, it executes the statements in the curly braces
  • Within these statements, it refers to the key of each element as $k and its value as $v

Class exercise: what is the output of the following?

echo "<table>";
foreach ($module_lecturers as $module => $lecturer) 
{
    echo "<tr>";
    echo "<td>{$module}</td>";
    echo "<td>{$lecturer}</td>";
    echo "</tr>";
}
echo "</table>";

Class exercise

• Write a PHP script that outputs an unordered list of months that have exactly 31 days

Doing things to each value in an indexed and associative arrays

• The following is mostly for indexed arrays:

  foreach ($a as $v)
{
    zero, one or more statements
}

  However, it can be used on associative arrays too
• The following is mostly for associative arrays:

  foreach ($a as $k => $v)
{
    zero, one or more statements
}

  However, it can be used on indexed arrays too

Class exercise

Hence, what does this output?

$toppings = array('mushrooms', 'anchovies', 'chocolate', 'baked beans');
echo "<ul>";
foreach ( $toppings as $k => $v )
{
    echo "<li>{$k}: {$v}</li>";
}
echo "</ul>";

And this?

$month_lengths = array('Jan' => 31, 'Feb' => 28, 
    'Mar' => 31, 'Apr' => 30, 'May' => 31, 'Jun' => 30, 'Jul' => 31,
    'Aug' => 31, 'Sep' => 30, 'Oct' => 31, 'Nov' => 31, 'Dec' => 31);
echo "<ul>";
foreach ( $month_lengths as $v )
{
    echo "<li>{$v}</li>";
}
echo "</ul>";