Split array into two arrays by index even or odd

Question

I have this array:

$array = array(a, b, c, d, e, f, g);

I want to split it in two arrays depending if the index is even or odd, like this:<

Answer 1

I am not sure if this is the most elegant way, but it should work a charm:

$odd=array();
$even=array();
$count=1;
foreach($array as $val)
{
    if($count%2==1)
    {
        $odd[]=$val;
    }
    else
    {
        $even[]=$val;
    }
    $count++;
}

Answer 2

    <?php

$array1 = array(0,1,2,3,4,5,6,7,8,9);
$oddarray = array();
$evenarray = array();

$count = 1;

echo "Original: ";
foreach ($array1 as $value)
 {
    echo "$value";
}

echo "<br> Even: ";

foreach ($array1 as $print) 
{
    if ($count%2==1) 
    {
        $evenarray = $print;
        echo "$print";
    }
    $count++;
}

echo "<br> Odd: ";

foreach ($array1 as $print2) {
    if ($count%2!=1) 
    {
        $oddarray[] = $print2;
        echo "$print2";
    }
    $count++;
}

?>

Output:

Original: 0123456789
Even: 02468
Odd: 13579

Answer 3

One solution, using anonymous functions and array_walk:

$odd = array();
$even = array();
$both = array(&$even, &$odd);
array_walk($array, function($v, $k) use ($both) { $both[$k % 2][] = $v; });

This separates the items in just one pass over the array, but it's a bit on the "cleverish" side. It's not really any better than the classic, more verbose

$odd = array();
$even = array();
foreach ($array as $k => $v) {
    if ($k % 2 == 0) {
        $even[] = $v;
    }
    else {
        $odd[] = $v;
    }
}

Answer 4

Based on @Jon's second variant, I made this following for use with PHP Smarty v3 template engine. This is for displaying news/blog with both one or two columns template model.

After the MySql query I'll do the following code :

if(sizeof($results) > 0) {
    $data = array();
    foreach($results as $k => $res) {
        if($k % 2 == 0) {
            $res["class"] = "even";
            $data["all"][] = $data["even"][] = $res;
        }
        else {
            $res["class"] = "odd";
            $data["all"][] = $data["odd"][] = $res;
        }
    }
}

I obtain an array of 3 sub-arrays (including odd/even class) with Smarty syntax of use :

1. all items {foreach $data.all as $article}...{/foreach}
2. odd items only {foreach $data.odd as $article}...{/foreach}
3. even items only {foreach $data.even as $article}...{/foreach}

Hope it helps some people...

Answer 5

Use array_filter (PHP >= 5.6):

$odd = array_filter($array, function ($input) {return $input & 1;}, ARRAY_FILTER_USE_KEY);
$even = array_filter($array, function ($input) {return !($input & 1);}, ARRAY_FILTER_USE_KEY);

Answer 6

As an almost-one-liner, I think this will be my favourite:

$even = $odd = array();
foreach( $array as $k => $v )  $k % 2  ?  $odd[] = $v  :  $even[] = $v;

Or for a tiny little more? speed:

$even = $odd = array();
foreach( $array as $k => $v )  ( $k & 1 ) === 0  ?  $even[] = $v  :  $odd[] = $v;

A bit more verbose variant:

$both = array( array(), array() );
// or, if $array has at least two elements:
$both = array();

foreach( $array as $k => $v )  $both[ $k % 2 ][] = $v;
list( $even, $odd ) = $both;

With array_chunk:

$even = $odd = array();
foreach( array_chunk( $array, 2 ) as $chunk ){
  list( $even[], $odd[] ) = isset( $chunk[1]) ? $chunk : $chunk + array( null, null );
  // or, to force even and odd arrays to have the same count:
  list( $even[], $odd[] ) = $chunk + array( null, null );
}

If $array is guaranteed to have even number of elements:

$even = $odd = array();
foreach( array_chunk( $array, 2 ) as $chunk )
  list( $even[], $odd[] ) = $chunk;

PHP 5.5.0+ with array_column:

$chunks = array_chunk( $array, 2 );
$even = array_column( $chunks, 0 );
$odd  = array_column( $chunks, 1 );

Something similar for older PHP versions. The keys will be 0,2,4,… and 1,3,5,…. If you don't like this, apply an array_values too:

$even = array_intersect_key( $array, array_flip( range( 0, count( $array ), 2 )));
$odd  = array_intersect_key( $array, array_flip( range( 1, count( $array ), 2 )));

or

$even = array_intersect_key( $array, array_fill_keys( range( 0, count( $array ), 2 ), null ));
$odd  = array_intersect_key( $array, array_fill_keys( range( 1, count( $array ), 2 ), null ));