Sorting only odd elements of an associative array - php

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).

Answer 2

Try this:
<?php
$array = array(
3 => 50,
4 => 12,
5 => 45,
6 => 89,
7 => 5,
8 => 1
);
$oddElems = array();
$evenElems = array();
$i = 0;
foreach($array as $index => $value)
{
if($i % 2 == 0)
$oddElems[$index] = $value;
else
$evenElems[$index] = $value;
$i++;
}
//sort odd elements
asort($oddElems);
$result = array();
while(!empty($oddElems) || !empty($evenElems))
{
$lastEvenElemKey = array_keys($evenElems, end($evenElems))[0];
$evenElem = array_pop($evenElems);
if($evenElem !== null)
$result[$lastEvenElemKey] = $evenElem;
$lastOddElemKey = array_keys($oddElems, end($oddElems))[0];
$oddElem = array_pop($oddElems);
if($oddElem !== null)
$result[$lastOddElemKey] = $oddElem;
}
echo '<pre>';
$result = array_reverse($result, true);
print_r($result);
Result is:
Array
(
[7] => 5
[4] => 12
[5] => 45
[6] => 89
[3] => 50
[8] => 1
)

Answer 3

This is how you could do this in PHP, is this what you are looking for ?
$array(3 => 50, 4 => 12, 5 => 45, 6 => 89, 7 => 5, 8 => 1);
function sort_it($array) {
$final_array[];
$array_only_odd_keys[];
$array_only_odd_stuff[];
$array_only_even_keys[];
$array_only_even_stuff[];
foreach($array as $key => $value) {
if($key & 1) {
//Key is odd
array_push($array_only_odd_stuff, $key => $value);
array_push($array_only_odd_keys, $key);
}
else {
// Key is even
array_push($array_only_even_stuff, $key => $value);
array_push($array_only_even_keys, $key);
}
}
$array_only_odd_keys = asort($array_only_odd_keys);
for ($x = 0; $x <= count($array)/2; $x++) {
if ($x & 1) {
array_push($final_array, $array_only_odd_keys[$x], $array_only_odd_stuff[$array_only_odd_keys[$x]]);
}
else {
array_push($final_array, $array_only_even_keys[$x], $array_only_odd_stuff[$array_only_even_keys[$x]]);
}
}
return $final_array;
This is how i'd do it in PHP though only if you want the final array to switch between even and odd keys like in your example.

Answer 4

The easiest way is probably to separate the array into even and odd arrays, sort the odd array, and then glue them back together:
$odd = $even = [];
foreach (array_chunk($myArray, 2, true) as $pair) {
$odd += array_slice($pair, 0, 1, true);
$even += array_slice($pair, 1, 1, true);
}
asort($odd);
$result = [];
for ($i = 0; $i < count($odd); $i++) {
$result += array_slice($odd, $i, 1, true);
$result += array_slice($even, $i, 1, true);
}
This rather naïve implementation assumes there is an equal number of odd and even numbers, they're always alternating and the array starts with an odd number.