Sort a flat array with the shortest strings alternating the outermost elements and the longest string in the middle

Question

As an academic/contrived task, I'd like to sort an indexed, 1-dimensional array so that the longest string is in the middle and its neighbouring elements should be the 2nd and 3rd longest and so on.

How can I alternate the position of the shortest strings working from the front and back of the array, and finish with the longest string(s) in the middle?

Sample input:

$array = ["Hello", "World", "this", "is", "a", "test"];

Desired result:

["a", "test", "Hello", "World", "this", "is"]

Explanation:

• a is shortest and must be positioned first
• is is 2nd shortest, so it takes last position
• test and this are tied for 3rd shortest, alphabetize to break the tie which determines that test comes after a and this comes before is
• Hello and World are tied for the longest strings, so alphabetize to break the tie and position Hello after test and World before this

A secondary example with an odd number of elements:

$array = ['hundreds', 'human', 'AI', 'replace', 'may', 'developers', 'soon'];

Desired result:

['AI', 'soon', 'replace', 'developers', 'hundreds', 'human', 'may']

Answer 1

Here is one way to do it (there's most likely a better solution):

function sort_length($str1, $str2)
{
    if(strlen($str1) == strlen($str2))
    {
        return 0;
    }
    return strlen($str1) > strlen($str2) ? -1 : 1;
}
$words = array("Hello", "World", "this", "is", "a", "test");
usort($words, 'sort_length');

$new_list = array();
$boolean = true;
foreach($words as $word)
{
    if($boolean)
    {
        array_push($new_list, $word);
    }
    else
    {
        array_unshift($new_list, $word);
    }
    $boolean = !$boolean;
}

//print_r($new_list);

Answer 2

I would do the following:

1. Sort the original array by length of the strings (with uasort())
2. Split the array in half by putting every element in one of two arrays (in a foreach loop)
3. Reverse the second array (with array_reverse())
4. Merge the two array together (with array_merge())

Answer 3

A first attempt could be to first sort them as normal. Then iterate through this array, copying to a new one, with the copy destination alternating between the start and end of the new array, where the start index is incremented and the end index is decremented.

Answer 4

Here’s a solution that preserves the array keys:

// function to compare two strings by their length
function cmp_strlen($a, $b) {
    return strlen($a) - strlen($b);
}

// sort array by length while preserving the keys
uasort($arr, 'cmp_strlen');

$ordered = array();
$keys = array_keys($arr);

// fill $ordered with odd keys in order
for ($i=0, $n=count($keys); $i<$n; $i+=2) {
    $ordered[$keys[$i]] = $arr[$keys[$i]];
}
// fill $ordered with even keys in reverse order
for ($i=((count($keys)>>1)<<1)-1; $i>0; $i-=2) {
    $ordered[$keys[$i]] = $arr[$keys[$i]];
}