在Elixir中按值过滤地图的有效方法

问题描述

在Elixir中，将是一种通过其值过滤 Map 的有效方法.

In Elixir, what would be an efficient way to filter a Map by its values.

现在我有以下解决方案

%{foo: "bar", biz: nil, baz: 4}
|> Enum.reject(fn {_, v} -> is_nil(v) end)
|> Map.new

这个解决方案对我来说似乎效率很低.当在 Map 上调用时， Enum.reject/2 返回关键字.因为我想要一个 Map ，所以我需要调用 Map.new/1 将那个关键字转换回给我.

This solution seems pretty inefficient to me. When called on a Map, Enum.reject/2 returns a Keywords. Since I want a Map, I need to call Map.new/1 to convert that Keywords back to me.

这似乎效率低下，因为 Enum.reject/2 必须遍历 Map 一次，然后推测 Map.new/1 具有再次遍历关键字.

This seems inefficient because Enum.reject/2 has to iterate over the Map once and then presumably, Map.new/1 has to iterate over the Keywords another time.

什么是更有效的解决方案?

What would be a more efficient solution?

推荐答案

您可以使用:maps.filter/2 ，它过滤地图并且不创建任何中间列表:

You can use :maps.filter/2, which filters a map and does not create any intermediate list:

iex(1)> :maps.filter fn _, v -> v != nil end, %{foo: "bar", biz: nil, baz: 4}
%{baz: 4, foo: "bar"}

一个简单的基准测试确认这比 Enum.filter + Map.new 更快:

A simple benchmark confirms this is faster than Enum.filter + Map.new:

map = for i <- 1..100000, into: %{}, do: {i, Enum.random([nil, 1, 2])}

IO.inspect :timer.tc(fn ->
  map
  |> Enum.reject(fn {_, v} -> is_nil(v) end)
  |> Map.new
end)

IO.inspect :timer.tc(fn ->
  :maps.filter fn _, v -> v != nil end, map
end)

{44728,
 %{48585 => 1, 60829 => 2, 12995 => 1, 462 => 2, 704 => 2, 28954 => 2,
   29635 => 2, 78798 => 1, 92572 => 1, 89750 => 2, 39389 => 2, 62855 => 2,
   79313 => 1, 92062 => 2, 61871 => 1, 92856 => 2, 75920 => 1, 59922 => 1,
   37912 => 2, 30420 => 2, 51211 => 2, 7994 => 2, 78269 => 2, 9765 => 2,
   38352 => 2, 6653 => 1, 82555 => 2, 54031 => 2, 45138 => 1, 41351 => 1,
   40746 => 1, 5961 => 1, 66704 => 2, 33823 => 1, 47603 => 1, 86873 => 1,
   81009 => 2, 96255 => 1, 36219 => 1, 1328 => 2, 33314 => 1, 54477 => 2,
   40189 => 2, 27028 => 1, 31676 => 1, 94037 => 1, 32388 => 1, 4351 => 1,
   46309 => 1, ...}}
{28638,
 %{48585 => 1, 60829 => 2, 12995 => 1, 462 => 2, 704 => 2, 28954 => 2,
   29635 => 2, 78798 => 1, 92572 => 1, 89750 => 2, 39389 => 2, 62855 => 2,
   79313 => 1, 92062 => 2, 61871 => 1, 92856 => 2, 75920 => 1, 59922 => 1,
   37912 => 2, 30420 => 2, 51211 => 2, 7994 => 2, 78269 => 2, 9765 => 2,
   38352 => 2, 6653 => 1, 82555 => 2, 54031 => 2, 45138 => 1, 41351 => 1,
   40746 => 1, 5961 => 1, 66704 => 2, 33823 => 1, 47603 => 1, 86873 => 1,
   81009 => 2, 96255 => 1, 36219 => 1, 1328 => 2, 33314 => 1, 54477 => 2,
   40189 => 2, 27028 => 1, 31676 => 1, 94037 => 1, 32388 => 1, 4351 => 1,
   46309 => 1, ...}}

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