How should I sort this array by key with usort?

Question

I think I might have read every usort article on StackOverflow, but I can't work out this one. It might be that usort is not the tool I need? Here's a bit of the array that I'm working with (I have it assigned to $allPages):

Array
(
    [0] => Page Object
        (
            [id] => 4
            [slug] => articles
            [created_on] => 2009-08-06 07:16:00
        )

    [1] => Page Object
        (
            [id] => 99
            [slug] => a-brief-history
            [created_on] => 2011-04-25 12:07:26
        )

    [2] => Page Object
        (
            [id] => 98
            [slug] => we-arrive
            [created_on] => 2011-04-24 13:52:35
        )

    [3] => Page Object
        (
            [id] => 83
            [slug] => new-year
            [created_on] => 2011-01-02 14:05:12
        )
)

I am trying ultimately to sort on the created_on value, but for the moment, I'd settle on being able to sort on any of them! When I try the normal cmp($a, $b) type callback with usort -- as, for example, in this answer on a usort question -- I just get a blank. Example:

function cmp($a, $b) {
  return strcmp($a["slug"], $b["slug"]);
}
usort($allPages, 'cmp')

And print_r gives me nothing. This is with PHP 5.2.n, not 5.3 btw.

Guidance, please? And thankyou!

Animalist

What do you mean by created_on value. – Monetary

Answer 1

Your items in the array are objects, not associative arrays, so you need to refer to them like this:

function cmp($a, $b) {
  return strcmp($a->slug, $b->slug);
}
usort($allPages, 'cmp')

Jocundity

PHP uses arrows, not periods. But the idea is the same. – Cisco

Haha, sorry, I work with PHP and Ruby daily and sometimes get them mixed up when I switch so quickly. Fixed. – Jocundity

that cracked it! Sort of a "doh!" moment for me. Many thanks! – China

Answer 2

Your dump of the array says that the elements are Page Objects, not arrays. By chance, do you need to say $a->created_on instead of $a['created_on']? Using object notation instead of array notation.

Just guessing...

Cisco

Yes, and thanks for nudging Kelly's code. It was fixed by the time I was able to get back to this. :) – China

Answer 3

As @Tesserex suggests, you need to use object notation instead of array notation.

If you had notices turned on, you'd get error messages about accessing an object as an array.

Another thing to consider, is, your Pages don't all have a 'created_on' attribute, some have a 'published_on' attribute. You'll have to do some error checking / logic inside your usort method to make sure that the key you want to sort by is available, and do something when it's not.

Convex

Thanks for the heads-up on the error messages. In the environment that this code is for, it is a pain to turn on error reporting, but I ought to have taken the trouble. And well caught on the different date attributes: in fact, they all have both. That was just my mistake in trimming down the objects for presenting here. – China

error_reporting(E_ALL); and ini_set('display_errors', 'on'); should work in most situations. not really well known methods \: – Convex