Select Keys - Hhvm — ContextQMD

:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::

Returns a new dict containing only the keys found in both the input container and the given Traversable

Hack

namespace HH\Lib\Dict;

function select_keys<Tk as arraykey, Tv>(
  KeyedContainer<Tk, Tv> $container,
  Traversable<Tk> $keys,
): dict<Tk, Tv>;

The dict will have the same ordering as the $keys Traversable.

Time complexity: O(k), where k is the size of $keys. Space complexity: O(k), where k is the size of $keys.

Parameters

KeyedContainer<Tk,Tv> $container
Traversable<Tk>$keys

Returns

dict<Tk, Tv>

Examples

basic-usage.hack

$dict_with_all_keys = dict["key1" => "value 1", "key2" => "value 2", "key3" => "value 3"];

$present_required_keys = vec["key1"];
$keys_not_present_in_dict = vec["incorrect_keys"];

$dict_with_present_keys = Dict\select_keys($dict_with_all_keys, $present_required_keys);
echo"Result when keys present in dict: \n";
\print_r($dict_with_present_keys);

$dict_with_keys_not_present = Dict\select_keys($dict_with_all_keys, $keys_not_present_in_dict);
echo"Result when keys not present in dict: \n";
\print_r($dict_with_keys_not_present);