a priority or weight problem

[Mgccl]

first, we can see this script

$weight[]=1
$weight[]=2
$weight[]=8
$rand = array_rand($weight, 1)

This will randomly chose on from the array...
But what about if I want to chose them by it's weight? the array with contains "8" when chose by random will have 8 times more chance to get chosed?
so when chose in those thing, the array contains 8 will have 8 out of 11 chance to be chosed....

I haven't thought of any way to do that. because random is only randomly chose one, so we don't know which group that one will fall...
Anyone got suggestions?

[bradgrafelman]

Well, that depends. Where are you pulling the values for $weight from? Just picking random numbers out of the sky?

If I knew more about where these numbers are picked from, I'd have a better example, but you could do something like this:

$options = array(1, 2, 8); // the different weights you have
$weights = array();

foreach($options as $weight_value)
     for($i = 0; $i < $weight_value; $i++)
          $weights[] = $weight_value;

print_r($weights);

The output of that code snippet looks like this:

Array
(
    [0] => 1
    [1] => 2
    [2] => 2
    [3] => 8
    [4] => 8
    [5] => 8
    [6] => 8
    [7] => 8
    [8] => 8
    [9] => 8
    [10] => 8
)

As you can see, the probability of each value as you described is correct now.

[Mgccl]

Great, but now a new problem comes...

$thearray[] = array("url"=>"http://localhost",
                                "weight"=>"8");
$thearray[] = array("url"=>"http://www.php.net",
                                "weight"=>"1");

and now I want randomly chose an array of url, but each url have different weight (defined by the weight array), and have the chance of chose the array which contain http://localhost 8 times larger than the other.
is there any way to do that? because your explaination made there are no way of putting other values in the array because you can't just assign the array numbers to something else...

    [0] => 1
    [1] => 2
    [2] => 2
    [3] => 8
    [4] => 8
    [5] => 8
    [6] => 8
    [7] => 8
    [8] => 8
    [9] => 8
    [10] => 8

like now, how do you know which one could be assigned another value, because you can't get enough info of from this for me to assign other things to some of those arrays, you can't just replace all the 8 into something else because there might be other things that weights 8 when the script get larger...(maybe it's possible.. but I just can't think of a way to do it)

[laserlight]

1. Sort the array by weight.
2. Cumulatively sum the weights of the array elements.
3. Generate a random number within the range of the weight sum.
4. Select the element for which the random number falls within that weight range.

Here is a draft implementation of the algorithm I outlined:

<?php
// fill the array
$array[] = array(
	"url" => "http://localhost",
	"weight" => 8);
$array[] = array(
	"url" => "http://www.php.net",
	"weight" => 1);
$array[] = array(
	"url" => "http://www.example.com",
	"weight" => 4);

// sort the array by weight
usort($array, create_function('$a,$b', 'return $a["weight"]-$b["weight"];'));

// cumulatively sum the weights of the array elements
for ($i = 1; $i < count($array); ++$i) {
	$array[$i]["weight"] += $array[$i-1]["weight"];
}

// generate a random number within the range of the weight sum
$randnum = mt_rand($array[0]["weight"], $array[count($array)-1]["weight"]);

// select the element for which the random number falls within that weight range
$chosen_index = 0;
for ($i = 0; $i < count($array); ++$i) {
	if ($array[$i]["weight"] >= $randnum) {
		$chosen_index = $i;
		break;
	}
}

echo $array[$chosen_index]["url"];
?>