Array Manipulation With PHP4 - Flipping Out
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A number of other functions are available to help you perform miscellaneous array manipulation. For example, the array_flip() function can be used to interchange keys and values in an array,

<?
// create array
$music = array("pop" => "Britney Spears", "rock" => "Aerosmith", "jazz" =>
"Louis Armstrong");

// returns a new array ("Britney Spears" => "pop", "Aerosmith" => "rock",
"Louis Armstrong" => "jazz")
array_flip($music);
?>

while the array_merge() function combines multiple arrays into one single array.

<?
// create arrays
$good_guys = array("princess" => "Leia", "teacher" => "Yoda", "new hope" =>
"Luke");
$bad_guys = array("bad guy" => "Darth", "worse guy" => "The Emperor");

// returns a combined array("princess => Leia", "teacher => Yoda", "new
hope => Luke", "bad guy => Darth", "worse guy => The Emperor")
$starwars = array_merge($good_guys, $bad_guys);
?>

The array_diff() function accepts two or more arrays as arguments, and returns an array containing all those values of the first array which are absent in the remaining ones,

<?
// create arrays
$subset = array(7, 14, 21, 28);
$list = array(1, 4, 7, 8, 0, 23, 45, 15, 67, 29, 22);

// returns ("14", "21", "28")
array_diff($subset, $list);
?>

while the array_intersect() function does the opposite, calculating a list of those values of the first array which are present in all the remaining ones.

<?
// create arrays
$subset = array(7, 14, 21, 28);
$list = array(1, 4, 7, 8, 0, 23, 45, 15, 67, 29, 22);

// returns ("7")
array_intersect($subset, $list);
?>

The array_sum() function adds all the elements of an array and returns the total,

<?
// create an array
$subset = array(7, 14, 21, 28);

// returns 70
echo array_sum($subset);
?>

while the array_count_values() function calculates the frequency with which values appear within an array.

<?
// create array
$clones = array("Tom", "Tom", "Harry", "Tom", "Harry", "Harry", "Harry",
"Tom", "Frank");

// returns the hash ("Tom => 4", "Harry => 4", "Frank => 1")
array_count_values($clones);
?>

The array_rand() function randomly returns one or more keys from an array.

<?
// create array
$desserts = array("chocolate mousse", "tiramisu", "apple pie", "chocolate
fudge cake", "apricot fritters");

// returns the random array (1, 3)
array_rand($desserts, 2);

// create array
$starwars = array("princess" => "Leia", "teacher" => "Yoda", "new hope" =>
"Luke", "bad guy" => "Darth", "worse guy" => "The Emperor");

// returns the random array ("princess", "bad guy")
array_rand($starwars, 2);
?>

And finally, the very useful array_walk() function allows you to run a user-defined function on every element of an array. The following example uses it to apply a specific number format to all the numbers in an array:

<?
// create arrays
$numbers = array(1, 567, 1.6777777777777, 0.031, 100.1, -98.6);
$new_numbers = array();

// function to format numbers and add them to new array
function format($num)
{
	global $new_numbers;
	$new_numbers[] = sprintf("%1.2f", $num);
}

// runs the function format() on every element of $numbers
array_walk($numbers, "format");

// $new_numbers now contains ("1.00", "567.00", "1.68", "0.03", "100.10",
"-98.60")
?>

And that's about all I have. I hope you enjoyed this article, and that it offered you some insight into the massive amount of power at your disposal when it comes to manipulating arrays. Should you require more information, the best place to go is the PHP manual page on arrays at http://www.php.net/manual/en/ref.array.php

Take care, and I'll see you soon!

Note: All examples in this article have been tested on Linux/i586 with PHP 4.0.6. Examples are illustrative only, and are not meant for a production environment. YMMV!

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