All the ways to do one-hot encoding

General Usage
2 
julia> using StatsBase

julia> x = ["a", "b", "c", "a", "b", "d"]
6-element Vector{String}:
 "a"
 "b"
 "c"
 "a"
 "b"
 "d"

julia> indicatormat(x)
4×6 Matrix{Bool}:
 1  0  0  1  0  0
 0  1  0  0  1  0
 0  0  1  0  0  0
 0  0  0  0  0  1

but normally I do your option 1 :smiley: :

julia> df = DataFrame(x=x)
6×1 DataFrame
 Row │ x
     │ String
─────┼────────
   1 │ a
   2 │ b
   3 │ c
   4 │ a
   5 │ b
   6 │ d

julia> select(df, [:x => ByRow(isequal(v))=> Symbol(v) for v in unique(df.x)])
6×4 DataFrame
 Row │ a      b      c      d
     │ Bool   Bool   Bool   Bool
─────┼────────────────────────────
   1 │  true  false  false  false
   2 │ false   true  false  false
   3 │ false  false   true  false
   4 │  true  false  false  false
   5 │ false   true  false  false
   6 │ false  false  false   true

(DataConvenience.jl is nice :+1:)

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