Sorting only odd elements of an associative array

Question

If I have an associative array like

• 3 => 50

• 4 => 12

• 5 => 45

• 6 => 89

• 7 => 5

• 8 => 1

Now I want to sort only the values of odd keys in ascending order. The output should be:

• 7 => 5

• 4 => 12

• 5 => 45

• 6 => 89

• 3 => 50

• 8 => 1

Answer 1

To maintain original keys, you have to first separate even and odd elements:

$odd = $even = array();
array_walk( $array, function( $val, $key ) use ( &$odd, &$even ) { ( $key % 2 ) ? $odd[$key] = $val : $even[$key] = $val; });

Then, sort $odd array:

asort( $odd );

At the end, you reconstruct the array:

$array = array();
while( current( $odd ) || current( $even ) )
{
    if( current( $odd ) )  $array[key($odd)]  = current( $odd );
    if( current( $even ) ) $array[key($even)] = current( $even );
    next( $odd );
    next( $even );
}
print_r( $array );

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Note that your question is a bit ambiguous: it's not totally clear if you base odd/even on key value or key position: this solution consider key values and — trough while and if checks — guarantee that all values are preserved, even if you have more even than odd keys (or vice-versa).