Category to integer mapping: bikeshedding session about dataarrays.jl HOT 9 OPEN

I believe this is a very basic device that is useful out of this package. For example, it may also be useful in constructing contingency tables (see JuliaStats/StatsKit.jl#32).

What about we implement this in Stats.jl and thus provide such support to other packages that may also want it?

from dataarrays.jl.

That works for me. What interface will we be using? Something like `levelsmap(["a", "b", "A", "A"]) -> ["a" => 1, "b" => 2, "A" => 3]?

from dataarrays.jl.

FWIW, there's already indexmap in Stats.jl. (see https://github.com/JuliaStats/Stats.jl#miscelleneous-functions)

from dataarrays.jl.

We don't want to use that as our canonical numbering, right? That could produce numbers that are spaced very unevenly.

from dataarrays.jl.

Oh, yes ... we can then add a levelmap method for this?

from dataarrays.jl.

That seems right to me. Should I submit a PR to discuss implementation details?

from dataarrays.jl.

Sure.

from dataarrays.jl.

Think about it more. Probably, we may want a data structure that maintain cross-reference between levels & indexes.

Something along this line?

immutable LevelMap{T}
    levels::Vector{T}   # index -> level
    indmap::Dict{T, Int}   # level -> index
end

with some functions to make doing the translation convenient.

from dataarrays.jl.

Not sure. For involved indexing operations, it seems like you'd want to maintain all of the indices for each level since that would make it much easier to repeat the levels calculation on subsets of the data.

from dataarrays.jl.