how does usort work?

问题:

I have looked at the php documentation, tutorials online and none of them how usort is actually working. I have an example i was playing with below.

$data = array(

    array('msg' => 'some text','month' => 11,'level' => 10),

    array('msg' => 'some text','month' => 5,'level' => 10),

    array('msg' => 'some text','month' => 8,'level' => 10),

    array('msg' => 'some text','month' => 12,'level' => 10),

    array('msg' => 'some text','month' => 2,'level' => 10),

    array('msg' => 'some text','month' => 3,'level' => 10),

    array('msg' => 'some text','month' => 4,'level' => 10),

    array('msg' => 'some text','month' => 7,'level' => 10),

    array('msg' => 'some text','month' => 10,'level' => 10),

    array('msg' => 'some text','month' => 1,'level' => 10),

    array('msg' => 'some text','month' => 6,'level' => 10),

    array('msg' => 'some text','month' => 9,'level' => 10)

);

I wanted to be able to sort the months from 12 to 1 (since their unorganized) through some help this was the solution

function cmp($a, $b)
{
    if ($a["month"] == $b["month"]) 
    {
       return 0;
    }
    return ($a["month"] < $b["month"]) ? -1 : 1;
}

usort($data, "cmp");

but i dont understand how the function cmp sorts the array. i tried printing out each variable $a and $b like this:

function cmp($a, $b)
{
   echo "a: ".$a['month']."<br/>;
   echo " b: ".$b['month']."<br/>;
   echo "<br/><br/>";
}

and the output was

a: 3
b: 5

a: 9
b: 3

a: 3
b: 8

a: 6
b: 3

a: 3
b: 12

a: 1
b: 3

a: 3
b: 2

a: 10
b: 3

a: 3
b: 11

a: 7
b: 3

a: 4
b: 3

a: 12
b: 2

a: 5
b: 12

a: 12
b: 11

a: 8
b: 12

a: 5
b: 8

a: 2
b: 11

a: 6
b: 9

a: 7
b: 6

a: 6
b: 4

a: 10
b: 6

a: 1
b: 6

a: 9
b: 4

a: 7
b: 1

a: 10
b: 7

it makes no sense to how the sort is working and why cmp($a, $b) is used. i have tried to print out all its processes as you can see but have not come to any solution to how it all works..

thanks

回答1:

The function cmp itself doesn't do the sorting. It just tells usort if a value is smaller, equal or greater than another value. E.g. if $a = 5 and $b = 9 it will return 1 to indicate that the value in $b is greater than the one in $a.

Sorting is done by usort.

回答2:

The callback provided to the sorting functions in PHP have three return values:

0:  both elements are the same
-1 (<0): the first element is smaller than the second
1 (>0):  the first element is greater

Now, usort probably uses some kind of quicksort or mergesort internally. For each comparison it calls your callback with two elements and then decides if it needs to swap them or not.

回答3:

usort() uses an implementation of Quicksort to sort the array, it calls your cmp function as many times as it needs to to fully sort the array using that algorithm.

回答4:

As the others mentioned, usort uses the Quicksort algorithm. On a side note, you do not need to explicitly do the comparision between two strings. You can use PHP's string compare methods.

The function that you created,

function cmp($a, $b)
{
    if ($a["month"] == $b["month"]) 
    {
       return 0;
    }
    return ($a["month"] < $b["month"]) ? -1 : 1;
}

could simply be written as follows

function compareMyStrings($a, $b){
    return strnatcmp($a["month"], $b["month"]);
}

Hope this helps!

回答5:

This is another solution I found

<?php 
$users = array( array( "peter", "male", "46"), 
                array( "hans", "male", "19"), 
                array( "john", "male", "30"), 
                array( "linda", "female", "54"), 
                array( "erika", "female", "79")); 
usort($users, "whatevername"); 
function whatevername($whatever1, $whatever2) 
{ 
    // $whatever1 and $whatever2 are items from the $user array. 
    // index [2] is the age. 
    // Check if $whatever1 is older than $whatever2. 
    // Return 1 tells usort to swap the positions. 
    return $whatever1[2] > $whatever2[2]; 
} 

echo("<pre>"); 
print_r($users); 
echo("</pre>"); 
?>