Sorting PHP array so that duplicate values are next to each other

Question

Using PHP I have this array:

[0]: John
[1]: Brian
[2]: Julia
[3]: Adam
[4]: Brian
[5]: Jonathan
[6]: Amanda
[7]: Julia
[8]: Nathan

I want to sort the array so that the order is as close to the original as possible, but with duplicates stacked, creating the following array:

[0]: John
[1]: Brian
[2]: Brian (duplicate value moved from index 4)
[3]: Julia
[4]: Julia (duplicate value moved from index 7)
[5]: Adam
[6]: Jonathan
[7]: Amanda
[8]: Nathan

I'm assuming that it's a matter of nested foreach loops, but I'm unsure how to apply unset() in a nested foreach loop.

EDIT: Since i probably didn't explain it very well I want to keep the duplicates in the array. I do not want to remove the duplicates.

Duplicate it. I don't want to delete the duplicated values, just stack them together. — user3296337
– user3296337, Commented May 24, 2016 at 20:04
So really, you are not even sorting. What you are doing is moving the second and later duplicates to right after the first found value. — Jonathan Kuhn
– Jonathan Kuhn, Commented May 24, 2016 at 20:11

RomanPerekhrest · Accepted Answer · 2016-05-24 22:28:25Z

1

The solution using array_shift, array_intersect and in_array functions:

$arr = [0=> 'John',1=> 'Brian',2=> 'Julia',3=> 'Adam',4=> 'Brian',5=> 'Jonathan',6=> 'Amanda',7=> 'Julia',8=> 'Nathan'];

$size = count($arr);
$i = 0;
$result = [];
while ($i < $size) {
    $el = array_shift($arr);  // current value
    if (!in_array($el, $result)) $result[] = $el;
    $dups = array_intersect($arr, [end($result)]);  // finding duplicates
    if (count($dups)) $result = $result + $dups;    // adding duplicates to "stack" if exist
    $i++;
}

print_r($result);

The output:

Array
(
    [0] => John
    [1] => Brian
    [2] => Brian
    [3] => Julia
    [4] => Julia
    [5] => Adam
    [6] => Jonathan
    [7] => Amanda
    [8] => Nathan
)

edited May 24, 2016 at 22:28

answered May 24, 2016 at 20:35

RomanPerekhrest

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michaJlS · Accepted Answer · 2016-05-24 20:15:11Z

1

Not the most efficient solution, but works:

function dupsort(array $input)
{
    $output = array();
    $moved = array();
    foreach ($input as $key => $val)
    {
        if (isset($moved[$key])) {
            continue;
        }

        $moved[$key] = true;
        $output[] = $val;

        foreach ($input as $dupKey => $dupVal) {

            if ($dupVal!==$val) {
                continue;
            }

            if (isset($moved[$dupKey])) {
                continue;
            }

            $moved[$dupKey] = true;
            $output[] = $dupVal;
        }
    }

    return $output;
}

answered May 24, 2016 at 20:15

michaJlS

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gbalduzzi · Accepted Answer · 2016-05-24 20:28:33Z

1

It works, just tested.

for ($i = 0; $i < count($array); $i++) {
    for ($j = $i; $j < count($array); $j++) {

        if ($i < $j && $array[$i] == $array[$j]) {
            $insert = array($array[$j]);
            unset($array[$j]);
            array_splice($array,$i,0,$insert);
        }
    }
}

answered May 24, 2016 at 20:28

gbalduzzi

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Comments

Pevara · Accepted Answer · 2016-05-24 20:32:25Z

1

Allow me to offer a slightly more resource friendly solution:

  function dupsort($data) {
    $result = [];

    foreach($data as $word) {

        // remove all matches of the current word from the source
        $before = count($data);
        $data = array_filter($data, function($x) use ($word) {
            return $x !== $word;
        });

        // add the word to the output as many times as it got removed from source
        $newCount = count($result) + $before - count($data);
        $result = array_pad($result, $newCount, $word);
    }

    return $result;
  }

answered May 24, 2016 at 20:32

Pevara

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Frederico Schardong · Accepted Answer · 2016-05-24 20:07:15Z

0

Just use the sort function.

$a = array("a", "b", "c", "a");

sort($a);
var_dump($a);

Result is:

array(4) { [0]=> string(1) "a" [1]=> string(1) "a" [2]=> string(1) "b" [3]=> string(1) "c" }

answered May 24, 2016 at 20:07

Frederico Schardong

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3 Comments

Dale Over a year ago

I thought this at first but it's not quite what OP is looking for

Frederico Schardong Over a year ago

Are you saying OP wants those "(duplicate value moved from index 4)" as part of the result?

user3296337 Over a year ago

@FredericoSchardong Yes! That is exactly what I want as part of the result

mickmackusa · Accepted Answer · 2022-12-23 05:34:43Z

0

Use just one loop to create a reduced lookup map of the first occurrences of each unique value as well as an ordering array which will hold the earliest index of each value.

Then call array_multisort() to sort the original array by the ordering array.

No iterated function calls are written.

Code: (Demo)

$order = [];
foreach ($array as $i => $v) {
    $lookup[$v] ??= $i;
    $order[] = $lookup[$v];
}
array_multisort($order, $array);
var_export($array);

edited Dec 23, 2022 at 5:34

answered Dec 23, 2022 at 4:19

mickmackusa♦

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