Get a random number from a broken range of numbers

I would like to draw a random number from the interval 1,49 but I would like to add a number as an exception ( let's say 44 ) , I cannot use round(rand(1,49)) .So I decided to make an array of 49 numbers (1-49), unset($aray[44]) and apply array_rand().

Now I want to draw a number from the interval [$left,49], how can I do that using the same array that I used before? The array now misses value 44.

Solution

The function pick takes an array as an argument with all the numbers you have already picked. It will then pick a number between the start and the end that IS NOT in that array. It will add this number into that array and return the number.

function pick(&$picked, $start, $end) {

    sort($picked);

    if($start > $end - count($picked)) {
        return false;
    }

    $pick = rand($start, $end - count($picked));
    foreach($picked as $p) {
        if($pick >= $p) {
            $pick += 1;
        } else {
            break;
        }
    }

    $picked[] = $pick;

    return $pick;
}

This function will efficiently get a random number that is not in the array AND WILL NEVER INFINITELY RECURSE!

To use this like you want:

$array = array(44); // you have picked 44 for example
$num = pick($array, 1, 49); // get a random number between 1 and 49 that is not in $array  

// $num will be a number between 1 and 49 that is not in $arrays

How the function works

Say you are getting a number between 1 and 10. And you have picked two numbers (e.g. 2 and 6). This will pick a number between 1 and (10 minus 2) using rand: rand(1, 8).

It will then go through each number that has been picked and check if the number is bigger.

For Example:

If rand(1, 8) returns 2. 
  It looks at 2 (it is >= then 2 so it increments and becomes 3)
  It looks at 6 (it is not >= then 6 so it exits the loop)
  The result is: 3

If rand(1, 8) returns 3
  It looks at 2 (it is >= then 2 so it increments and becomes 4)
  It looks at 6 (it is not >= then 6 so it exits the loop)
  The result is 4

If rand(1, 8) returns 6
  It looks at 2 (it is >= then 2 so it increments and becomes 7)
  It looks at 6 (it is >= then 6 so it increments and becomes 8)
  The result is: 8

If rand(1, 8) returns 8
  It looks at 2 (it is >= then 2 so it increments and becomes 9)
  It looks at 6 (it is >= then 6 so it increments and becomes 10)
  The result is: 10

Therefore a random number between 1 and 10 is returned and it will not be 2 or 6.

I implemented this a long time ago to randomly place mines in a 2-dimensional array (because I wanted random mines, but I wanted to guarantee the number of mines on the field to be a certain number)