PHP array of probability with fractions

Question

i have the following array:

public $percentage = array(
   0 => 20.30,
   1=> 19.96,
   2=> 14.15,
   3=> 45.59
);

// it sums in 100%

I need a random function to return the key by the percentage of the value,

for example: the possibility to get 0 is 20.30% and the possibility to get 2 is 14.15%, the first user got 0, the second one got 2.

Please let me know what is the function you suggest me to use.

you want a function which has probability of 20.3% to return 0, 14.15% to return 2 etc? — Marko D
– Marko D, Commented Mar 24, 2013 at 10:16

Antony · Accepted Answer · 2013-03-24 10:29:54Z

7

Convert the percentages to an accumulated probability, then compare it with a random number.

If the random number falls into a category, outputs the result. If not, move onto the next one until one is found. This allows you to output a number based on the percentage probability stated in the array.

$percentage = array(
   0 => 20.30,
   1=> 19.96,
   2=> 14.15,
   3=> 45.59
);
$random = mt_rand(0,10000)/100;
foreach ($percentage as $key => $value) {
    $accumulate += $value;
    if ($random <= $accumulate) {
        echo $key;
        break;
    }
}

edited Mar 24, 2013 at 10:29

answered Mar 24, 2013 at 10:20

Antony

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Comments

saada · Accepted Answer · 2013-03-24 10:28:54Z

2

$random_n = mt_rand(0,10000)/100;
while(true){
  if($random_n <= $percentage[0])
    echo 0; break;
  else if($random_n <= $percentage[1])
    echo 1; break;
  else if($random_n <= $percentage[2])
    echo 2; break;
  else if($random_n <= $percentage[3])
    echo 3; break;
  else
    $random_n = mt_rand(0,10000)/100; //generate a new random #
}

edited Mar 24, 2013 at 10:28

answered Mar 24, 2013 at 10:16

saada

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1 Comment

Antony Over a year ago

No. The problem is, you have a random number generated, let's say it is 10, then according to the array, 0 is given as the result. If the random number is 25, 3 is given as the result. And if the random number is 50, nothing will be shown.

Alexey · Accepted Answer · 2013-03-24 10:19:57Z

1

<?php
$percentage = $tmp = array(
   0 => 20.30,
   1=> 19.96,
   2=> 14.15,
   3=> 45.59
);

sort($tmp);

$rand = mt_rand(0,100);
foreach($tmp as $percent) {
    if($percent >= $rand) {
        echo array_search($percent,$percentage);
        die();
    }
}

echo (count($percentage) - 1);

answered Mar 24, 2013 at 10:19

Alexey

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Comments

jbr · Accepted Answer · 2013-03-24 10:19:18Z

0

I'd do something like:

Take a random number between 1 and 10000 (because you have four significant digits)
If the number is between 1 and 2030 then it's a zero, if it's between 2031 and (2031+1996) then it's a 1 etc.
...
Profit

answered Mar 24, 2013 at 10:19

jbr

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