Generating random results by weight in PHP?

Question

I know how to generate a random number in PHP but lets say I want a random number between 1-10 but I want more 3,4,5\'s then 8,9,10\'s. How is this possible? I would post wh

Answer 1

Many of the answers on this page seem to use array bloating, excessive iteration, a library, or a hard-to-read process. Of course, everyone thinks their own baby is the cutest, but I honestly think my approach is lean, simple and easy to read/modify...

Per the OP, I will create an array of values (declared as keys) from 1 to 10, with 3, 4, and 5 having double the weight of the other values (declared as values).

$values_and_weights=array(
    1=>1,
    2=>1,
    3=>2,
    4=>2,
    5=>2,
    6=>1,
    7=>1,
    8=>1,
    9=>1,
    10=>1
);

If you are only going to make one random selection and/or your array is relatively small* (do your own benchmarking to be sure), this is probably your best bet:

$pick=mt_rand(1,array_sum($values_and_weights));
$x=0;
foreach($values_and_weights as $val=>$wgt){
    if(($x+=$wgt)>=$pick){
        echo "$val";
        break;
    }
}

This approach involves no array modification and probably won't need to iterate the entire array (but may).

---

On the other hand, if you are going to make more than one random selection on the array and/or your array is sufficiently large* (do your own benchmarking to be sure), restructuring the array may be better.

The cost in memory for generating a new array will be increasingly justified as:

1. array size increases and
2. number of random selections increases.

The new array requires the replacement of "weight" with a "limit" for each value by adding the previous element's weight to the current element's weight.

Then flip the array so that the limits are the array keys and the values are the array values. The logic is: the selected value will have the lowest limit that is >= $pick.

// Declare new array using array_walk one-liner:
array_walk($values_and_weights,function($v,$k)use(&$limits_and_values,&$x){$limits_and_values[$x+=$v]=$k;});

//Alternative declaration method - 4-liner, foreach() loop:
/*$x=0;
foreach($values_and_weights as $val=>$wgt){
    $limits_and_values[$x+=$wgt]=$val;
}*/
var_export($limits_and_values);

Creates this array:

array (
  1 => 1,
  2 => 2,
  4 => 3,
  6 => 4,
  8 => 5,
  9 => 6,
  10 => 7,
  11 => 8,
  12 => 9,
  13 => 10,
)

Now to generate the random $pick and select the value:

// $x (from walk/loop) is the same as writing: end($limits_and_values); $x=key($limits_and_values);
$pick=mt_rand(1,$x);  // pull random integer between 1 and highest limit/key
while(!isset($limits_and_values[$pick])){++$pick;}  // smallest possible loop to find key
echo $limits_and_values[$pick];  // this is your random (weighted) value

This approach is brilliant because isset() is very fast and the maximum number of isset() calls in the while loop can only be as many as the largest weight (not to be confused with limit) in the array. For this case, maximum iterations = 2!

THIS APPROACH NEVER NEEDS TO ITERATE THE ENTIRE ARRAY