PHP usort tree array

Question

I want to sort a tree hierarchy array with structure as below based on a key (in this example: timestamp).

$topics = array(
  array('name' => 'n1', 'timestamp' => 5000, 'children' => array()),
  array('name' => 'n2', 'timestamp' => 4000, 'children' => array(
    array('name' => 'n3', 'timestamp' => 6000, 'children' => array()),
    array('name' => 'n4', 'timestamp' => 2000, 'children' => array(
      array('name' => 'n5', 'timestamp' => 4000, 'children' => array()),
      array('name' => 'n6', 'timestamp' => 3000, 'children' => array())
    )), 
  )),
  array('name' => 'n7', 'timestamp' => 1000, 'children' => array())
);

My sort function:

function sequenceSort(&$a, &$b) {
  if (!empty($a['children'])) {
    usort($a['children'], 'sequenceSort');
  }
  if ($a['timestamp'] == $b['timestamp']) {
    return 0;
  }
  return $a['timestamp'] < $b['timestamp'] ? -1 : 1;
}

usort($topics, 'sequenceSort');
print_a($topics);

At some levels it produces the correct output, at another it doesn't, e.g:

What's wrong with this?

Note that you should order your input array differently to see if it really sorts. The second level now also fails but you don't see that because you have already sorted it correctly. — jeroen
– jeroen, Commented Aug 9, 2018 at 7:28
@jeroen I've edited the question with more random input order. — Justinus Hermawan
– Justinus Hermawan, Commented Aug 9, 2018 at 7:32

Barmar · Accepted Answer · 2018-08-09 07:51:43Z

1

This doesn't work because usort() doesn't pass the array elements by reference, even though you've declared the parameters with &. I added:

$a['touched'] = true;

to the comparison function, and when I printed the result these keys were nowhere to be found.

However, even if it worked, this seems like a very poor way to do it, as it will sort the children multiple times -- every time the parent is being compared with another parent, it will have to re-sort the children, and grandchildren, etc.

It would be better to write a recursive function that sorts one level, then iterates over the children, and so on.

function sortRecurse(&$array) {
    usort($array, function($a, $b) {
        return $a['timestamp'] - $b['timestamp'];
    });
    foreach ($array as &$subarray) {
        sortRecurse($subarray['children']);
    }
}

DEMO

edited Aug 9, 2018 at 7:51

answered Aug 9, 2018 at 7:24

Barmar

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6 Comments

jeroen Over a year ago

That's what I thought, but this does not sort correctly either.

Barmar Over a year ago

@JustinusHermawan It doesn't work because the elements aren't passed by reference to the comparison function. So Kaspar's comment above is correct.

Justinus Hermawan Over a year ago

@barmar Yes, it works. So, the mistake here is usort() function does not retrieve params by reference, right?

Barmar Over a year ago

It gets its own parameter by reference, but it doesn't pass parameters to the comparison function by reference.

Sakezzz Over a year ago

so bad, i am late sandbox.onlinephpfunctions.com/code/…

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