php percentage chance

Question

This is really more a question of approach, but I'm presenting it in php.

Suppose we had a list of four percentages that a give event will occur on iteration.

array=('walk the dog'=>.25,'read the paper'=>.25,'drink coffee'=>.0,'listen to music'=>.50)

(Keys are just tests - in practice this is going to be used in a strategy pattern that applies different methods to an object off chance using call_user_func())

In a loop, what would be the best way to choose one of these events by random-adjusted selection taking their respective weights into account (IE, for this particular configuration, there will be a lot of music listened to and not a lot of coffee drank)

Currently I have something like:

for($i=1;$i<=3;$i++){
  $rand = 1/rand(1,10);
  //choose from the list
}

But this would require mapping the percentages to value ranges (50-100% become listening to music, 0-25% walking the dog and 25-50 reading the paper) because the 'value' for that iteration is calculated as a floating point number between 0 and 1. This kind of changes the way the percentages need to be recorded.

Is there any kind of approach that would preserve the identity of the values as the real probability they should be chosen rather than ranges? It would definitely make the code more configurable since I have to make these values adjustable. Please let me know if I've been too vague.

There are 4 items in your list btw, not three. – MrGlass Jan 10 '12 at 15:46 — MrGlass, Jan 10 '12 at 15:46

score 5 · Accepted Answer · answered Jan 10 '12 at 16:03

I'd do something like:

$arr = array (
    'walk the dog'    =>  array('min' =>  0, 'max' =>  25),
    'read the paper'  =>  array('min' => 25, 'max' =>  50),
    'drink coffee'    =>  array('min' =>  0, 'max' =>   0),
    'listen to music' =>  array('min' => 50, 'max' => 100),
);

$rnd = rand(1,100);
foreach($arr as $k =>$v) {
    if ($rnd > $v['min'] && $rnd <= $v['max']) {
        echo $k,"\n";
    }
}

CNeo · Answer 2 · 2012-01-10T16:13:41.667

3

Have you considered using the percentage values as a cumulative number?

$array = array('walk the dog'=>.25,'read the paper'=>.25,'drink coffee'=>.0,'listen to music'=>.50);

$rand = rand();
$cur = 0;
foreach ($key=>$val in $array) {
  $cur += $val;
  if ($rand <= $cur)
    return $key;
}

This assumes the values in the array add up to 1. Ordering doesn't matter either...

edited Jan 10 '12 at 16:13

answered Jan 10 '12 at 15:56

CNeo

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OptimusCrime · Answer 3 · 2012-01-10T15:56:17.783

1

$arr = array('walk the dog'=>25,'read the paper'=>25,'listen to music'=>50);
$rand = rand(1,100);
$sum = 0;
$action = '';

foreach ($arr as $k => $v) {
    $sum .= $k;
    if ($sum <= $k)
        $action = $v;
}

Removed those with a key value of zero, and multiplied the key-value with 100 to make it more readable.

edited Jan 10 '12 at 15:56

answered Jan 10 '12 at 15:45

OptimusCrime

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This is what I was more or less looking to apply, but the problem is choosing the correct one from the list. Since there are four potential events for any one loop pass (3 if we discount coffee, since it is set to zero in this config). – DeaconDesperado Jan 10 '12 at 15:48
Is the sum of these keys always 100 (or 1) – OptimusCrime Jan 10 '12 at 15:50
Yes, new ones added to the list would decrease the probability of the others coming up. Summed together they will always equal 1 (or 100 if calculated on that scale.) – DeaconDesperado Jan 10 '12 at 15:51
I think you might have a typo swapping $k for $v, but yes this does work. – DeaconDesperado Jan 10 '12 at 16:22
4

What's the point of generating the random number if it's not even used? – dukevin Jun 24 '12 at 05:38

crenate · Answer 4 · 2012-01-11T11:42:52.350

0

You can restructure the array in such a way that the events are stored with decreasing probability (for performance) and the value corresponding to an event is the probability of this particular event plus the value of a previous element (which means that the last element will always have the value 1.).

Then you just iterate over the array and return the event for which its value is greater than $rand.

edited Jan 11 '12 at 11:42

answered Jan 10 '12 at 15:57

crenate

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php percentage chance

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