DocumentationHSLReferenceFunctionHH\Lib\Dict\group_by

HH\Lib\Dict\group_by

Requires the Hack Standard Library to be installed.

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This function is available as Dict\group_by() in Facebook's www repository.

Returns a new dict where

keys are the results of the given function called on the given values

namespace HH\Lib\Dict;

function group_by<Tk as arraykey, Tv>(
  Traversable<Tv> $values,
  (function(Tv): ?Tk) $key_func,
): dict<Tk, vec<Tv>>;

values are vecs of original values that all produced the same key.

If a value produces a null key, it's omitted from the result.

Time complexity: O(n * f), where f is the complexity of $key_func Space complexity: O(n)

Parameters

Traversable<Tv> $values
(function(Tv): ?Tk) $key_func

Returns

dict<Tk, vec<Tv>>

Examples

Group numbers by their parity (separate even and odd numbers):

$numbers = vec[1, 1, 2, 3, 5, 8, 14];
$groups = Dict\group_by($numbers, $value ==> $value % 2);
\print_r($groups);

Output

Dict
(
    [1] => Vec
        (
            [0] => 1
            [1] => 1
            [2] => 3
            [3] => 5
        )

    [0] => Vec
        (
            [0] => 2
            [1] => 8
            [2] => 14
        )

)