I’d like to query an Explorer DataFrame containing a “select” column such that the output gives, for each row, the field/column that was pointed to by the “select” column. Example:

iex>DF.print(my_df)
+--------------------------------------------+
| Explorer DataFrame: [rows: 3, columns: 3]  |
+--------------+--------------+--------------+
|     col1     |     col2     |    select    |
|   <string>   |   <string>   |   <string>   |
+==============+==============+==============+
| a            | b            | col1         |
+--------------+--------------+--------------+
| c            | d            | col2         |
+--------------+--------------+--------------+
| e            | f            | col2         |
+--------------+--------------+--------------+

The output I want is:

#Explorer.Series<
  Polars[3]
  string ["a", "d", "f"]
>

This pops up while processing categorical data and is related to a use-case involving one-hot encoding that was discussed here.

I was hoping to be able to use syntax that is fairly short, something like

my_df[.., my_df["select"]]

…but I haven’t been able to get that working yet. What I have gotten working is:

defmodule Janky do
  def dynamic_select(dataframe) do
    # use mutate_with combined with Series.select
    DF.mutate_with(dataframe, fn df ->
      [selection: 
          Enum.reduce(
            Enum.reject(dataframe.names, fn x -> x == "select" end),
            "nil",
            fn x, acc ->
              this_column = S.equal(df["select"], x)
              S.select(this_column, df[x], acc)
            end
          )
      ]
    end)
    |> DF.pull("selection")
  end
end

iex> Janky.dynamic_select(my_df)
#Explorer.Series<
  Polars[3]
  string ["a", "d", "f"]
>

This seems like a lot of code to do something fairly straightforward—is there anything more sleek built into Explorer?

In Pandas, there used to be DataFrame.lookup(), which I guess has been replaced by something slightly more verbose. I feel like this type of dynamic selection is supported by dplyr too…

Anything built into Explorer (or on the roadmap) for this?