Filter array - odd even

How can a filter out the array entries with an odd or even index number?

Array
(
    [0] => string1
    [1] => string2
    [2] => string3
    [3] => string4
)

Like, i want it remove the [0] and [2] entries from the array. Or say i have 0,1,2,3,4,5,6,7,8,9 - i would need to remove 0,2,4,6,8.

From stackoverflow

• foreach($arr as $key => $value) if($key&1) unset($arr[$key]);

  Instead if($key&1) you can use if(!($key&1))

  mofle : This removes every odd number, like [1] and [3]. How can I remove every even number, like [0], [2]...?

  artlung : just change the if statement in Thinker's code to (!$key&1) ... foreach($arr as $key => $value) if(!$key&1) unset($arr[$key]);

  mofle : That's what I thought too, but it gives me [1],[2],[3], but not the [4], it should only give me [1] and [3]. Any thoughts?

  Thinker : If !$key&1 not works it is because operator precedence. I edited my code.

  mofle : Thanks, it works now ;)

• Here's a "hax" solution:

  Use array_filter in combination with an "isodd" function.

  array_filter seems only to work on values, so you can first array_flip and then use array_filter.

  array_flip(array_filter(array_flip($data), create_function('$a','return $a%2;')))
  

  Matt : create_function is not recommended as it has a memory leak.

  Ross : And they're hard to read as well :) Anonymous functions would be worth looking at for when 5.3.0 is released (http://uk2.php.net/manual/en/functions.anonymous.php)

  v3 : Awesome, anonymous functions! Didn't know that they were planned, thanks

• I'd do it like this...

  for($i = 0; $i < count($array); $i++)
  {
      if($i % 2) // OR if(!($i % 2))
      {
          unset($array[$i]);
      }
  }
  

  v3 : I think that should be a < instead of a <=

  Kieran Hall : Ah yes, my mistake.

• You could also use SPL FilterIterator like this:

  class IndexFilter extends FilterIterator {
      public function __construct (array $data) {
          parent::__construct(new ArrayIterator($data));
      }   
  
      public function accept () {
          # return even keys only
          return !($this->key() % 2);
      }     
  }
  
  $arr      = array('string1', 'string2', 'string3', 'string4');
  $filtered = array();
  
  foreach (new IndexFilter($arr) as $key => $value) {
      $filtered[$key] = $value;
  }
  
  print_r($filtered);