Random value from array by weight in php

Question

I understand that this question has likely been asked, but I don't understand the prior questions enough to know if they do what I want.

$fruits = array('20' => 'apple', '40' => 'orange', '40' => 'pear');

The keys are percentages of the chance of value getting picked. I would like to mt_rand() a number between 0 and 99 and return a value from $fruits based on those percentages.

It's very possible I'm so confused because I don't know how to explain what I'm looking for.

Thank you in advance for the help.

Edit: want a random value from $fruits, based on these chances:

I want a 40% chance of getting an orange, a 40% chance of getting a pear, and an 80% chance of getting an apple.

Edit: To further clarify, since either a lot of the answers got it wrong, (or I just don't understand their code), I needed a result regardless of what number I pick, not just 20, 40, or 40.

Answer 1

First of all, your array only has two items (apple and pear) because pear overwrites orange due to having the same key. So use the opposite order. Also, don't put quotes around the integers:

$fruits = array('apple' => 20, 'orange' => 40, 'pear' => 40);

You should choose a random number, then compare each weight of the array plus the sum of the previous weights until you get a match:

$rand = rand(1,100);
$sum = 0;
$chosenFruit;

foreach ($fruits as $f=>$v) {
    $sum += $v;
    if ( $sum >= $rand ) {
        $chosenFruit = $f;
        break;
    }
}

echo "We have chosen the {$chosenFruit}!";

You could even make the procedure more resilient by replacing the value of 100 in the rand() function with a calculated sum of the $fruits array values.

Answer 2

I think something like this will do what you want:

sample

(click the submit button multiple times on the sample to get the code to re-execute)

$fruits = array('apple' => '20', 'orange' => '40', 'pear' => '40');

$newFruits = array();
foreach ($fruits as $fruit=>$value)
{
    $newFruits = array_merge($newFruits, array_fill(0, $value, $fruit));
}

$myFruit = $newFruits[array_rand($newFruits)];

This creates an array ($newFruits), which is a numerically-indexed array with 100 elements. 20 of those elements are 'apple', 40 are 'orange', and 40 are 'pear'. Then we select a random index from that array. 20 times out of 100 you will get 'apple', 40 times out of 100 you will get 'orange', and 40 times out of 100 you will get 'pear'.

Answer 3

There are multiple roads to go down with this one and they pretty much depend on your data. Using percentages, 0 - 100, you could have 100 items in your array that each map to a percentage, or you can have a distribution with assigned values (much like the array in your sample).

Assuming the former, you can use:

$key = mt_rand(0, 100);
$fruit = isset($fruits[$key]) ? $fruits[$key] : false;

The above could also work with a distribution, to check if a fruit "exists", but normally you would want "if the percentage is <= the value, use this fruit". Something like that can be done with:

$key = mt_rand(0, 100);
$fruit = false;
foreach ($fruits as $percent => $value) {
    if ($percent <= $key) {
        $fruit = $value;
        break;
    }
}

Now, the fun part. What if you wanted any of the fruits that have a percentage above the value? You can modify the above to store each fruit that matches into an array and then pick a random value from that array:

$key = mt_rand(0, 100);
$matches = array();
foreach ($fruits as $percent => $value) {
    if ($percent <= $key) {
        $matches[] = $value;
    }
}
$select = mt_rand(0, count($matches) - 1);
$fruit = $matches[$select];

Answer 4

  $key = rand(1,98); // rand() is inclusive of the range, hence 1-98.
  foreach( $fruits as $k => $v){
  if( $k == $key) {
    echo 'Chosen fruit is:' . $v; 
    break;
    } 
  }