How to get a random value from 1~N but excluding several specific values in PHP?

Question

rand(1,N) but excluding array(a,b,c,..),

is there already a built-in function that I don\'t know or do I have to implement it myself(how?)

Answer 1

What you need to do is calculate an array of skipped locations so you can pick a random position in a continuous array of length M = N - #of exceptions and easily map it back to the original array with holes. This will require time and space equal to the skipped array. I don't know php from a hole in the ground so forgive the textual semi-psudo code example.

1. Make a new array Offset[] the same length as the Exceptions array.
2. in Offset[i] store the first index in the imagined non-holey array that would have skipped i elements in the original array.
3. Now to pick a random element. Select a random number, r, in 0..M the number of remaining elements.
4. Find i such that Offset[i] <= r < Offest[i+i] this is easy with a binary search
5. Return r + i

Now, that is just a sketch you will need to deal with the ends of the arrays and if things are indexed form 0 or 1 and all that jazz. If you are clever you can actually compute the Offset array on the fly from the original, it is a bit less clear that way though.

Answer 2

I don't think there's such a function built-in ; you'll probably have to code it yourself.

To code this, you have two solutions :

• Use a loop, to call rand() or mt_rand() until it returns a correct value
  • which means calling rand() several times, in the worst case
  • but this should work OK if N is big, and you don't have many forbidden values.
• Build an array that contains only legal values
  • And use array_rand to pick one value from it
  • which will work fine if N is small

Answer 3

The simplest way...

<?php

function rand_except($min, $max, $excepting = array()) {

    $num = mt_rand($min, $max);

    return in_array($num, $excepting) ? rand_except($min, $max, $excepting) : $num;
}
?>

Answer 4

No built-in function, but you could do this:

function randWithout($from, $to, array $exceptions) {
    sort($exceptions); // lets us use break; in the foreach reliably
    $number = rand($from, $to - count($exceptions)); // or mt_rand()
    foreach ($exceptions as $exception) {
        if ($number >= $exception) {
            $number++; // make up for the gap
        } else /*if ($number < $exception)*/ {
            break;
        }
    }
    return $number;
}

That's off the top of my head, so it could use polishing - but at least you can't end up in an infinite-loop scenario, even hypothetically.

Note: The function breaks if $exceptions exhausts your range - e.g. calling randWithout(1, 2, array(1,2)) or randWithout(1, 2, array(0,1,2,3)) will not yield anything sensible (obviously), but in that case, the returned number will be outside the $from-$to range, so it's easy to catch.

If $exceptions is guaranteed to be sorted already, sort($exceptions); can be removed.

Eye-candy: Somewhat minimalistic visualisation of the algorithm.

Answer 5

This is the fastest & best performance way to do it :

$all =  range($Min,$Max);
$diff = array_diff($all,$Exclude);
shuffle($diff );
$data = array_slice($diff,0,$quantity);

Answer 6

Maybe its too late for answer, but I found this piece of code somewhere in my mind when trying to get random data from Database based on random ID excluding some number.

$excludedData = array(); // This is your excluded number
$maxVal = $this->db->count_all_results("game_pertanyaan"); // Get the maximum number based on my database

$randomNum = rand(1, $maxVal); // Make first initiation, I think you can put this directly in the while > in_array paramater, seems working as well, it's up to you
while (in_array($randomNum, $excludedData)) {
  $randomNum = rand(1, $maxVal);
}

$randomNum; //Your random number excluding some number you choose