Hi all,

The answer to this question might be really simple but my thinking process is so clogged up that i can't think of the answer.

Let's say there are 5 students in an exam and their results are stored in array like this:

$student[] = array("John", "59");
$student[] = array("Mary", "42");
$student[] = array("Steve", "72");
$student[] = array("Harry", "31");
$student[] = array("Sally", "59");

How can this same array be re-ordered according to the exam result in descending order?

All of your help is greatly appreciated.

Why store it like that?

The best way would be storing by unique names, since their names are all unique in your example:

PHP Code:

$students = array('John' => 59, 'Mary' => 42, 'Steve' => 72, 'Harry' => 31, 'Sally' => 59); 


this is the same as doing:

PHP Code:

$students['John'] = 59;
$students['Mary'] = 42;
... 


Then to sort, do

PHP Code:

arsort($students); 


And that's it.

If you do want to keep it as a multidimensional array, then (rename your variable to $students[] it makes more sense this way)

PHP Code:

foreach($students as $key => $row)
{
  $student[$key] = $row[0];
  $results[$key] = $row[1];
}
array_multisort($results, SORT_DESC, $student, SORT_ASC, $students); 


Basically the way we look at the information is important for working with multidimensional arrays, we have our main array, which holds all the students, it is built up of smaller arrays which contains the student and results hence my variable naming.

We first need to go through our array and store the information inside it in their own array, so storing the same information in it's own array, this way when sorting, we are still working with the same information (3 times as much), but that's what you asked for and this is the only way I know how we can do it.

It is quite confusing, but knowing that $row[0] is where our students are and $row[1] is where the results are (could have called it $col which may make it clearer for you, I just worked on what PHP output in print_r), we want to sort the results first, then students (if they have the same results, then we'd like the person with the lower alpha character to have higher precedance), then we tell it the main array so that it can update it to reflect the changes for the keys.

Before:

Array
(
    [0] => Array
        (
            [0] => John
            [1] => 59
        )

    [1] => Array
        (
            [0] => Mary
            [1] => 42
        )

    [2] => Array
        (
            [0] => Steve
            [1] => 72
        )

    [3] => Array
        (
            [0] => Harry
            [1] => 31
        )

    [4] => Array
        (
            [0] => Sally
            [1] => 59
        )

)

After:

Array
(
    [0] => Array
        (
            [0] => Steve
            [1] => 72
        )

    [1] => Array
        (
            [0] => John
            [1] => 59
        )

    [2] => Array
        (
            [0] => Sally
            [1] => 59
        )

    [3] => Array
        (
            [0] => Mary
            [1] => 42
        )

    [4] => Array
        (
            [0] => Harry
            [1] => 31
        )

)

Hopefully what I suggest helps you.

Cheers,

MC

MC,

That has to be the best answer i've ever received on any forum.

Thanks!

One question though, to better-help me understand this. Would

PHP Code:

foreach($students as $key => $row)
{
  $student[$key] = $row[0];
  $results[$key] = $row[1];
}
array_multisort($results, SORT_DESC, $student, SORT_ASC, $students); 


Be the same as:

PHP Code:

foreach($students as $key => $row)
{
  $student[] = $row[0];
  $results[] = $row[1];
}
array_multisort($results, SORT_DESC, $student, SORT_ASC, $students); 


I.e. Does [$key] need to be there or can it be replaced with []?

Yes - It appears so because the array keys are numaric starting at 0. If you wanted to perserve the keys, like if they were numeric or string representing a specific value, then that would be best. Looking at mastercomputers array sample above, that is not the case, the keys are numeric starting from 0.

So, to make it even more simple, you could bypass the keys...

foreach($students as $row) { . . .

Read manual about usort(), it will allow you to sort any arrays of any types of data

Thanks for the help guys.

Hey fambi,

You are correct that student[] is the same in this example, however you must realise that this actually means it will add to the end of an array, if it's just first time creating an array (like this example), then it will always start at zero. You can think if $key as a counter in this, starting from 0 and incrementing in each loop.

e.g. if I did this when creating an array

PHP Code:

$student[5] = 'John';
$student[] = 'Peter'; 


In this example, $student[6] == 'Peter'; Even if we've skipped 0-4. So you can have mixed results if you are specifically wanting to overwrite the array but if wanting to append to it, then use your method, it's quite safe doing it this way too, if you don't want results overwritten as you can always sort it afterwards and remove double ups, etc.

Code what you're comfortable with is what I recommend, but you can pick up bad habits if you're not careful. I would have recommended that numbers not be quoted as computers handle numbers better than strings, but that's just a performance thing really as PHP's type handling will probably treat it as a number and string so no need to worry about it's type, in C/C++ however, it's a string.

Cheers,

MC