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up vote 57 down vote favorite
15

The callback function in array_filter() only passes in the array's values, not the keys.

If I have:

$my_array = array("foo" => 1, "hello" => "world");

$allowed = array("foo", "bar");

What's the best way to delete all keys in $my_array that are not in the $allowed array?

Desired output:

$my_array = array("foo" => 1);
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4 Answers

up vote 94 down vote accepted

With array_intersect_key and array_flip:

var_dump(array_intersect_key($my_array, array_flip($allowed)));

array(1) {
  ["foo"]=>
  int(1)
}

array_intersect_key is pretty much self explanatory and array_flip flips the keys with the values.

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1  
That's clever, I never thought to use array_flip – GWW Nov 23 '10 at 19:47
+1 for array_flip, you beat me to it. :) – netcoder Nov 23 '10 at 19:47
I'm curious if this is more efficient than my solution though? It's definitely more elegant :) – GWW Nov 23 '10 at 19:48
2  
gorgeous. thank you, @Vincent Savard – maček Nov 23 '10 at 19:53
3  
This is not a general solution because it would mandate that each value is unique. Edit: sorry.. I misread the solution. Flipping on the allowed keys is a good solution (+1) – Matthew Nov 23 '10 at 19:53
show 7 more comments
up vote 6 down vote

I needed to do same, but with a more complex array_filter on the keys.

Here's how I did it, using a similar method.

// Filter out array elements with keys shorter than 4 characters
$a = array(
  0      => "val 0", 
  "one"  => "val one", 
  "two"  => "val two", 
  "three"=> "val three", 
  "four" => "val four", 
  "five" => "val five", 
  "6"    => "val 6"
); 

$f = array_filter(array_keys($a), function ($k){ return strlen($k)>=4; }); 
$b = array_intersect_key($a, array_flip($f));
print_r($b);

This outputs the result:

Array
(
    [three] => val three
    [four] => val four
    [five] => val five
)
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exactly what i needed :) – nico roy Nov 30 '12 at 20:29
up vote 2 down vote

Here is a more flexible solution using a closure:

$my_array = array("foo" => 1, "hello" => "world");
$allowed = array("foo", "bar");
$result = array_flip(array_filter(array_flip($my_array), function ($key) use ($allowed)
{
    return in_array($key, $allowed);
}));
var_dump($result);

Outputs:

array(1) {
  'foo' =>
  int(1)
}

So in the function, you can do other specific tests.

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I wouldn't exactly call this "more flexible"; it feels a lot less straightforward than the accepted solution, too. – maček Jan 26 at 21:33
I agree. It would be more flexible is the condition was a more complex one. – COil Jan 31 at 11:25
up vote 0 down vote

Here's a less flexible alternative using unset():

$array = array(
    1 => 'one',
    2 => 'two',
    3 => 'three'
);
$disallowed = array(1,3);
foreach($disallowed as $key){
    unset($array[$key]);
}

The result of print_r($array) being:

Array
(
    [2] => two
)

This is not applicable if you want to keep the filtered values for later use but tidier, if you're certain that you don't.

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