将列表转换为元组列表

Question

I have a list of lists like this:

[
    ["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42", "1538077790705827"],
     ["5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42","1538078530667847"],
     ["5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42","1538077778390908"],
     ["5b71d7e458c37fa04a7ce768","5bad45b1e990057961313822","1538082492283531"]
]

I want to transform it to a list like:

[ 
  {"5b3f77502dfe0deeb8912b42",
   [{"5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42", "1538077790705827"},
    {"5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42", "1538078530667847"},
    {"5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42" ,"1538077778390908"}
   ]},

{"5bad45b1e990057961313822",
   [{"5b71d7e458c37fa04a7ce768","5bad45b1e990057961313822","1538082492283531"}
   ]}
]

So the key will creating tuple out of the second item in the original list

["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42" , "1538077790705827"],

Answer 1

First, Enum.group_by the second element of the list. Then, map every value of the resulting map's value to List.to_tuple . Enum.map when used with a map automatically converts each key/value pair to a tuple.

list = [
  ["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42", "1538077790705827"],
  ["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42", "1538078530667847"],
  ["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42", "1538077778390908"],
  ["5b71d7e458c37fa04a7ce768", "5bad45b1e990057961313822", "1538082492283531"]
]

list
|> Enum.group_by(&Enum.at(&1, 1))
|> Enum.map(fn {k, v} -> {k, Enum.map(v, &List.to_tuple/1)} end)
|> IO.inspect

Output:

[
  {"5b3f77502dfe0deeb8912b42",
   [
     {"5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42",
      "1538077790705827"},
     {"5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42",
      "1538078530667847"},
     {"5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42",
      "1538077778390908"}
   ]},
  {"5bad45b1e990057961313822",
   [
     {"5b71d7e458c37fa04a7ce768", "5bad45b1e990057961313822",
      "1538082492283531"}
   ]}
]

Answer 2

I would use a map to index my list of lists according to the second element, and then transform them using Enum.map/2 and List.to_tuple/1 .

if

l = [
    ["5b71d7e458c37fa04a7ce768", "5b3f77502dfe0deeb8912b42", "1538077790705827"],
     ["5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42","1538078530667847"],
     ["5b71d7e458c37fa04a7ce768","5b3f77502dfe0deeb8912b42","1538077778390908"],
     ["5b71d7e458c37fa04a7ce768","5bad45b1e990057961313822","1538082492283531"]
]

then

l
    |> Enum.reduce(%{}, fn [a,b,c], acc -> put_in(acc[b], (acc[b] && acc[b] || []) ++ [[a,b,c]]) end)
    |> Enum.map(fn {a,b} -> {a, Enum.map(b, &List.to_tuple/1)} end)

does the work.