recursion - Elixir map_reduce 引用最后一个元素

Question

我如何循环(递归地或使用 reduce)Map 的 List，同时与列表中的最后一个元素进行比较？

例如，假设我有一个这样的 map 列表:

datetime = Timex.beginning_of_day(Timex.now)
data = [
  %{a: 0, cluster: 0, time: datetime},
  %{a: 1, cluster: 0, time: Timex.shift(datetime, minutes: 3)},
  %{a: 2, cluster: 0, time: Timex.shift(datetime, minutes: 6)},
  %{a: 3, cluster: 0, time: Timex.shift(datetime, minutes: 9)},
  %{a: 4, cluster: 1, time: Timex.shift(datetime, minutes: 12)},
  %{a: 5, cluster: 1, time: Timex.shift(datetime, minutes: 15)},
  %{a: 6, cluster: 0, time: Timex.shift(datetime, minutes: 18)},
  %{a: 7, cluster: 0, time: Timex.shift(datetime, minutes: 21)},
  %{a: 8, cluster: 0, time: Timex.shift(datetime, minutes: 23)},
  %{a: 9, cluster: 2, time: Timex.shift(datetime, minutes: 26)},
  %{a: 10, cluster: 2, time: Timex.shift(datetime, minutes: 29)},
  %{a: 11, cluster: 2, time: Timex.shift(datetime, minutes: 32)},
  %{a: 12, cluster: 1, time: Timex.shift(datetime, minutes: 35)},
  %{a: 13, cluster: 1, time: Timex.shift(datetime, minutes: 38)},
]

我想修改每个 map 的 cluster 属性，以根据其所在的顺序来表示它所在的组。

如果 cluster id 不重复，则使用 group_by 非常好。

我想在他们改变组时对他们进行分组，结果是这样的:

[
  %{a: 0, cluster: 0, time: datetime},
  %{a: 1, cluster: 0, time: Timex.shift(datetime, minutes: 3)},
  %{a: 2, cluster: 0, time: Timex.shift(datetime, minutes: 6)},
  %{a: 3, cluster: 0, time: Timex.shift(datetime, minutes: 9)},
  %{a: 4, cluster: 1, time: Timex.shift(datetime, minutes: 12)},
  %{a: 5, cluster: 1, time: Timex.shift(datetime, minutes: 15)},
  %{a: 6, cluster: 2, time: Timex.shift(datetime, minutes: 18)},
  %{a: 7, cluster: 2, time: Timex.shift(datetime, minutes: 21)},
  %{a: 8, cluster: 2, time: Timex.shift(datetime, minutes: 23)},
  %{a: 9, cluster: 3, time: Timex.shift(datetime, minutes: 26)},
  %{a: 10, cluster: 3, time: Timex.shift(datetime, minutes: 29)},
  %{a: 11, cluster: 3, time: Timex.shift(datetime, minutes: 32)},
  %{a: 12, cluster: 4, time: Timex.shift(datetime, minutes: 35)},
  %{a: 13, cluster: 4, time: Timex.shift(datetime, minutes: 38)},
]

为此，我需要将列表中的当前项与前一项进行比较。我从这样的东西开始(如下)并停止了，因为我知道它不会引用前一个项目来将当前项目与前一个项目进行比较:

Enum.map_reduce(data, 0, fn(x, acc) -> cluster_grouping(x, acc) end)

def cluster_grouping(x, acc) do
  cond do
    x.cluster == acc -> {Map.put(x, :cluster, acc), acc}
    x.cluster > acc -> {Map.put(x, :cluster, acc), acc + 1}
  end
end

Answer 1

您需要在累加器中保留两个整数:当前生成的簇(每次更改都会递增 1)和簇的最后一个原始值。

datetime = Timex.beginning_of_day(Timex.now)
data = [
  %{a: 0, cluster: 0, time: datetime},
  %{a: 1, cluster: 0, time: Timex.shift(datetime, minutes: 3)},
  %{a: 2, cluster: 0, time: Timex.shift(datetime, minutes: 6)},
  %{a: 3, cluster: 0, time: Timex.shift(datetime, minutes: 9)},
  %{a: 4, cluster: 1, time: Timex.shift(datetime, minutes: 12)},
  %{a: 5, cluster: 1, time: Timex.shift(datetime, minutes: 15)},
  %{a: 6, cluster: 0, time: Timex.shift(datetime, minutes: 18)},
  %{a: 7, cluster: 0, time: Timex.shift(datetime, minutes: 21)},
  %{a: 8, cluster: 0, time: Timex.shift(datetime, minutes: 23)},
  %{a: 9, cluster: 2, time: Timex.shift(datetime, minutes: 26)},
  %{a: 10, cluster: 2, time: Timex.shift(datetime, minutes: 29)},
  %{a: 11, cluster: 2, time: Timex.shift(datetime, minutes: 32)},
  %{a: 12, cluster: 1, time: Timex.shift(datetime, minutes: 35)},
  %{a: 13, cluster: 1, time: Timex.shift(datetime, minutes: 38)},
]

Enum.map_reduce(data, {0, 0}, fn x, {i, last} -> 
  i = if x.cluster == last, do: i, else: i + 1
  {Map.put(x, :cluster, i), {i, x.cluster}}
end)
|> elem(0)
|> IO.inspect

输出:

[
  %{a: 0, cluster: 0, time: #DateTime<2018-02-18 00:00:00Z>},
  %{a: 1, cluster: 0, time: #DateTime<2018-02-18 00:03:00Z>},
  %{a: 2, cluster: 0, time: #DateTime<2018-02-18 00:06:00Z>},
  %{a: 3, cluster: 0, time: #DateTime<2018-02-18 00:09:00Z>},
  %{a: 4, cluster: 1, time: #DateTime<2018-02-18 00:12:00Z>},
  %{a: 5, cluster: 1, time: #DateTime<2018-02-18 00:15:00Z>},
  %{a: 6, cluster: 2, time: #DateTime<2018-02-18 00:18:00Z>},
  %{a: 7, cluster: 2, time: #DateTime<2018-02-18 00:21:00Z>},
  %{a: 8, cluster: 2, time: #DateTime<2018-02-18 00:23:00Z>},
  %{a: 9, cluster: 3, time: #DateTime<2018-02-18 00:26:00Z>},
  %{a: 10, cluster: 3, time: #DateTime<2018-02-18 00:29:00Z>},
  %{a: 11, cluster: 3, time: #DateTime<2018-02-18 00:32:00Z>},
  %{a: 12, cluster: 4, time: #DateTime<2018-02-18 00:35:00Z>},
  %{a: 13, cluster: 4, time: #DateTime<2018-02-18 00:38:00Z>}
]