How to filter an associative arrays using array of keys in PHP?

Question

I have an associative arrays and an array of keys.

$A = array('a'=>'book', 'b'=>'pencil', 'c'=>'pen');
$B = array('a', 'b');

How I build an associative array from all element of $A where the key is in $B? For the example above, the answer should be

$C = array('a'=>'book', 'b'=>'pencil');

score 19 · Answer 1 · answered Aug 18 '11 at 13:23

19

$keys = array_flip($B);
$C = array_intersect_key($A,$keys);

answered Aug 18 '11 at 13:23

Jon Page

476
3
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score 4 · Answer 2 · edited May 23 '17 at 12:33

4

array_intersect_key($A,array_combine($B,$B))

or better: array_intersect_key($my_array, array_flip($allowed))

from the question: PHP: How to use array_filter() to filter array keys?

edited May 23 '17 at 12:33

Community

1
1

answered Aug 18 '11 at 13:21

Simon Elliston Ball

4,375
1
21
18

Michael Berkowski · Answer 3 · 2011-08-18T13:18:54.503

2

Here's a simple solution which checks that the key exists in $A before appending it to $C

$A = array('a'=>'book', 'b'=>'pencil', 'c'=>'pen');
$B = array('a', 'b');

$C = array();
foreach ($B as $bval) {
  // If the $B key exists in $A, add it to $C
  if (isset($A[$bval])) $C[$bval] = $A[$bval];
}

var_dump($C);

// Prints:
array(2) {
  ["a"]=>
  string(4) "book"
  ["b"]=>
  string(6) "pencil"
}

edited Aug 18 '11 at 13:18

answered Aug 18 '11 at 13:13

Michael Berkowski

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46
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390

Obviously wrong :/. OP need to filter an array with another, your solution doesn't. And the result in `$C` should be an associative array, in your solution it's an indexed one. – Clement Herreman Aug 18 '11 at 13:15
Edited to make `$C` associative – Michael Berkowski Aug 18 '11 at 13:16
1

Edit: And I didn't mean to be ironic, you solution **is working** and is - I personnaly found - more elegant than mine. – Clement Herreman Aug 18 '11 at 14:05

score 1 · Accepted Answer · answered Aug 18 '11 at 13:13

1

$keys = array_keys($B);
$C = array();
foreach ($A as $key => $value)
{
  if (in_array($key, $keys))
  {
    $C[$key] = $value;
  }
}

answered Aug 18 '11 at 13:13

Clement Herreman

10,274
4
35
57

score 1 · Answer 5 · answered Aug 18 '11 at 13:43

To my immense surprise, the foreach loop method is faster.

The following quick benchmark script gives me results: array_intersect_key: 0.76424908638 foreach loop: 0.6393928527832

$A = array('a'=>'book', 'b'=>'pencil', 'c'=>'pen');
$B = array('a', 'b');

$start = microtime(true);
for ($i = 0 ; $i < 1000000; $i++) {
$c = array_intersect_key($A,array_flip($B));
}

$t1 = microtime(true);

for ($i = 0; $i < 1000000; $i++) {
$C = array();
    foreach ($B as $bval) {
          // If the $B key exists in $A, add it to $C
          if (isset($A[$bval])) $C[$bval] = $A[$bval];
    }
}

$t2 = microtime(true);
echo "array_intersect_key: " . ($t1 - $start), "\n";
echo "foreach loop: " . ($t2 - $t1), "\n";

Awesome \o/ (and some more characters to reach 15). – Clement Herreman Aug 23 '11 at 08:09 — Clement Herreman, Aug 23 '11 at 08:09

How to filter an associative arrays using array of keys in PHP?

5 Answers5