PHP rand() exclude certain numbers

Question

I have this:

<?php  $n = rand(1,1600); echo $n ?>

I want to exclude from random numbers let's say 234, 1578 ,763 , 1274 and other numbers. How would I do that?

Answer 1

As the volume of "blacklisted" integers approaches the volume of the full range of integers, it becomes increasingly compelling to take the advice of @regenschein.

A non-iterative approach might look like this:

$range = range(1, 1600);
$blacklist = [234, 1578, 763, 1274]; // 4 blacklisted versus 1600 full range is NOT compelling
$valids = array_diff($range, $blacklist);
echo array_values($valids)[rand(0, count($valids) - 1)];
// or
echo $valids[array_rand($valids)];
// the two approaches use different randomizers

Or if you'd be just as happy shuffling, you could do:

$blacklist = [234, 1578, 763, 1274];
$range = range(1, 1600);
$valids = array_diff($range, $blacklist);
shuffle($valids);
echo $valids[0];

*Note array_diff() is particularly great if you want to pass multiple blacklist arrays -- just comma-separate them.

For example:

var_export($valids = array_diff(range(1, 100), range(5, 50), range(61, 99), [55]));

Output:

array (
  0 => 1,
  1 => 2,
  2 => 3,
  3 => 4,
  50 => 51,
  51 => 52,
  52 => 53,
  53 => 54,
  55 => 56,
  56 => 57,
  57 => 58,
  58 => 59,
  59 => 60,
  99 => 100,
)