PHP的array_filter对面？

Question

I'm familiar with array_filter, and I'm trying to think more functional, but I was wondering if there is a way to keep the discarded values? For example, if I had an array like:

<?php  
  $arr = array( 1, 2, 3, 4 );
  $arrGreaterThanTwo = array_filter($arr, function($item){
   return $item > 2;
 });

The results would be: array( 3, 4 ).

1. Is there a way to keep the discarded values array( 1, 2 )?
2. Or do I need to reuse array_filter again on the original array and return item <= to 2?
3. If I use array_filter again to grab array( 1, 2 ), wouldn't that be inefficient by looping over the original array twice?

At the end, I'm just trying to loop over once with array_filter and keep the discarded values too into a separate array.

Thanks in advance!

Answer 1

If you want to get the other array values apart from the ones filtered, use array_diff()

array_diff — Computes the difference of arrays

$arrRemaining = array_diff($arr, $arrGreaterThanTwo);

Output:

Array
(
  [0] => 1
  [1] => 2
)

Answer 2

function array_partition(array $array, callable $fn){
    $result = [[],[]];
    foreach ( $array as $value ){
        $result[$fn($value)?0:1][]=$value;
    }
    return $result;
}
// example usage:
$res = array_partition([1,2,3,4,5,6,7,8,9,10], function($i) { return $i&1==1;});

Although this is not functional implementation it has a functional interface just like array_filter .

Answer 3

Updated Response

function test($arrOne, $fn){
  $restArr = [];

  $filterResults = array_filter($arrOne, function($item) use (&$restArr, $fn) {
      if( $fn($item) ) return $item;
      $restArr[] = $item;
  });

  return array( $filterResults, $restArr );
}

print_r( test( array(1,2,3,4), function($item){
  return $item > 2;
} ) );

Does anyone have a problem with this?

Answer 4

I might recommend you write your own function for it. This is array_bifilter – instead of returning one array, it returns two. The first is the truthy values, the second is the falsey values. And yes, it only iterates thru the input array once.

function array_bifilter(array $xs, callable $f) {
  return array_reduce($xs, function ($acc, $x) use ($f) {
    if (call_user_func($f, $x))
      array_push($acc[0], $x);
    else
      array_push($acc[1], $x);
    return $acc;
  }, [[], []]);
}

list($t, $f) = array_bifilter([1,2,3,4], function($x) { return $x > 2; });

echo json_encode(['true' => $t, 'false' => $f], JSON_PRETTY_PRINT), PHP_EOL;

Output (in JSON format for easy reading)

{
    "true": [
        3,
        4
    ],
    "false": [
        1,
        2
    ]
}