Programming Note

In Lisp or Scheme language, the anonymous function (which is called lambda-style function) is common used. These functions are special because it doesn’t have a name. So it can be only used once. Usually, these functions are very small and simple.

What these strange functions can be used for? Suppose there is an array that contains some integers. For example, $a=array(34,21,58,32,19,6). And we want to use “array_filter” function to retrieve the odd integers. The usage of the array_filter is:
array array_filter(array $input [,callback $callback])
The function iterates each element in $input. The $callback is a function which returns true or false. If it returns true, the current element is pushed into the result array. Using array_filter, it is easy to write the following code:

<?php
function is_odd($n){
	if ($n%2==1)
		return true;
	else
		return false;
}
$a=array(34,21,58,32,19,6);
$result=array_filter($a,'is_odd');
print_r($result);
?>

Its output is:

Array
(
    [1] => 21
    [4] => 19
)

Obviously, there is a small function “is_odd” to check if an integer is an odd number or not. However, it is used only in array_filter function. So if we can define the small function in array_filter’s second argument’s place, it is a better style.

PHP provides “create_function” that can create an anonymous function. The usage:
string create_function(string $args, string $code)
Now use this function to rewrite the example:

<?php
$a=array(34,21,58,32,19,6);
$result=array_filter($a,create_function('$n','return ($n%2==1)?true:false;'));
print_r($result);
?>

Tags: PHP, Programming

This entry was posted on Saturday, October 25th, 2008 at 1:16 pm and is filed under PHP. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.