Random weighted distribution

Question

For example I have an array like:

$items = [
    '0' => [
        'name' => 'item1', 
        'percent' => 10
        ],
    '2' => [
        'name' => 'item2', 
        'percent' => 20
        ],
    '3' => [
        'name' => 'item3', 
        'percent' => 30
        ],
    '4' => [
        'name' => 'item4', 
        'percent' => 40
        ],
];

And function:

function echoRandomItem(){
}

If I call this function it should return 'name' of the item depending on the 'percent' value. Basically if I call this function 100 times it should return item1 10 times, item2 20 times, item3 30 times, item4 40 times.

Hello, we're not developers for hire. Try to make this yourself, then if you have bugs in it, we'll be able to help you. — Olivier Grégoire, Feb 14 '17 at 18:48
Hey man, I cannot figure out the algorithm itself and want some help to figure it out. Maybe you know some formula that may help? At first sight the task seems easy, but I cannot grasp it. — niQitos, Feb 14 '17 at 18:51
It's called "random weighted distribution". Look for that on the Internet with the language you're using. By the way you've not even said which language you're using. — Olivier Grégoire, Feb 14 '17 at 18:53
Ok, thanks. This is my first time here so thank you for bearing with me. By the way it's PHP — niQitos, Feb 14 '17 at 18:56

niQitos · Answer 1 · 2017-02-14T20:54:34.877

Here is a solution I've come up with thanks to Oliver's hint. $items is a given array. wrand() function is responsible for random weighted distribution calculation.

$items = [
    '1' => [
        'name' => 'item1', 
        'rand' => 10
        ],
    '2' => [
        'name' => 'item2', 
        'rand' => 20
        ],
    '3' => [
        'name' => 'item3', 
        'rand' => 30
        ],
    '4' => [
        'name' => 'item4', 
        'rand' => 40
        ],
];

function wrand($data) {

    foreach ($data as $value) {
        $itemsRand[] = $value ['rand'];
    }

    $totalw = array_sum($itemsRand);
    $rand   = rand(1, $totalw);

    $curw   = 0;
    foreach ($data as $val) {
        $curw += $val['rand'];
        if ($curw >= $rand) return $val['name'];
    }

    return end($data);
}

Further code simply calls wrand() function 100 times and counts the results.

static $item1 = 0;
static $item2 = 0;
static $item3 = 0;
static $item4 = 0;

for ($i = 0; $i < 100; $i++){
    $k = wrand($items);

    if ($k == 'item1') {
        $item1++;
    } elseif ($k == 'item2'){
        $item2++;
    } elseif ($k == 'item3'){
        $item3++;
    } elseif ($k == 'item4'){
        $item4++;
    }
}

echo "item1 = $item1<br>";
echo "item2 = $item2<br>";
echo "item3 = $item3<br>";
echo "item4 = $item4";

Random weighted distribution

1 Answers1