Keeping even numbered elements of an array ?

Answer 1

Assuming numeric keys:

foreach ($array as $key => $value) {
    if ($key % 2 != 0) {
        unset($array[$key]);
    }
}

---

EDIT

Here goes my slightly more insane solution which keeps the index continuous without re-indexing. ;o)

foreach ($array as $key => $value) {
    if (!($key%2)) {
        $array[$key/2] = $value;
    }
}
$array = array_slice($array, 0, ceil(count($array)/2));

Answer 2

Not necessarily the most efficient method, but since you mentioned that wasn't necessarily a requirement...

flip, filter, then flip back.

<?php
    function even($var)
    {
        return(!($var & 1));
    }

    $input = array('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', );
    $flipped = array_flip($input);
    $filtered = array_filter($flipped, 'even');
    $output = array_flip($filtered);
?>

Answer 3

  function dropHalf($a){
     $f=0;
     foreach($a as $k=>$v)
       if($f = ! $f)
         unset($a[$k]);
     return $a;
  }

That's the smallest version I could think off.

Answer 4

create a wrapper function

function getInput($i)
{
     global $input;
     return $input[$i*2];
}

The smallest and most efficient I guess.

Answer 5

<?php
$x = range('a', 'f');

$x = array_map('array_shift', 
       array_chunk($x, 2)
     );

var_dump($x);

or another one

<?php
class ArrayEvenIterator extends ArrayIterator {
    public function next() {
     parent::next();
     return parent::next();
    }
}

$x = range('a', 'f');
$x = iterator_to_array(new ArrayEvenIterator( $x ), false);

var_dump($x);

or with a php 5.3 closure (which isn't better than global in this case ;-) )

<?php
$x = range('a', 'f');

$x = array_filter( $x, function($e) use(&$c) { return 0===$c++%2; });

var_dump($x);

Answer 6

If you're using PHP 5.3 or later, or have the SPL extension installed (you will by default on PHP 5), you can use the FilterIterator and ArrayObject classes.

class EvenKeysFilter extends FilterIterator
{
    private function iseven($keyval)
    {
        return (($keyval % 2) == 0);
    }

    public function accept()
    {
        $keyval = $this->getInnerIterator()->key();
        return ($this->iseven($keyval));
    }
}

$input = array('a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', );
$inputobj = new ArrayObject($input);   

$evenFilterIter = new EvenKeysFilter($inputobj->getIterator());    
$output = iterator_to_array($evenFilterIter, false);

print_r($output);

(Props to VolkerK for pointing out iterator_to_array())

Which correctly outputs this:

Array
(
    [0] => a
    [1] => c
    [2] => e
    [3] => g
    [4] => i
)