Efficiently pick n random elements from PHP array (without shuffle)

Question

I have the following code to pick $n elements from an array $array in PHP:

shuffle($array);
$result = array_splice($array, 0, $n);
         


        

Answer 1

The trick is to use a variation of shuffle or in other words a partial shuffle.

performance is not the only criterion, statistical efficiency, i.e unbiased sampling is as important (as the original shuffle solution is)

function random_pick( $a, $n ) 
{
  $N = count($a);
  $n = min($n, $N);
  $picked = array_fill(0, $n, 0); $backup = array_fill(0, $n, 0);
  // partially shuffle the array, and generate unbiased selection simultaneously
  // this is a variation on fisher-yates-knuth shuffle
  for ($i=0; $i<$n; $i++) // O(n) times
  { 
    $selected = mt_rand( 0, --$N ); // unbiased sampling N * N-1 * N-2 * .. * N-n+1
    $value = $a[ $selected ];
    $a[ $selected ] = $a[ $N ];
    $a[ $N ] = $value;
    $backup[ $i ] = $selected;
    $picked[ $i ] = $value;
  }
  // restore partially shuffled input array from backup
  // optional step, if needed it can be ignored, e.g $a is passed by value, hence copied
  for ($i=$n-1; $i>=0; $i--) // O(n) times
  { 
    $selected = $backup[ $i ];
    $value = $a[ $N ];
    $a[ $N ] = $a[ $selected ];
    $a[ $selected ] = $value;
    $N++;
  }
  return $picked;
}

NOTE the algorithm is strictly O(n) in both time and space, produces unbiased selections (it is a partial unbiased shuffling) and produces output which is proper array with consecutive keys (not needing extra array_values etc..)

Use example:

$randomly_picked = random_pick($my_array, 5);
// or if an associative array is used
$randomly_picked_keys = random_pick(array_keys($my_array), 5);
$randomly_picked = array_intersect_key($my_array, array_flip($randomly_picked_keys));

For further variations and extensions of shuffling for PHP:

1. PHP - shuffle only part of an array
2. PHP shuffle with seed
3. How can I take n elements at random from a Perl array?