Categorical Data

Often, we have to deal with factors that take on a small number of levels:

julia> v = ["Group A", "Group A", "Group A", "Group B", "Group B", "Group B"]
6-element Array{String,1}:
 "Group A"
 "Group A"
 "Group A"
 "Group B"
 "Group B"
 "Group B"

The naive encoding used in an Array represents every entry of this vector as a full string. In contrast, we can represent the data more efficiently by replacing the strings with indices into a small pool of levels. This is what the CategoricalArray type does:

julia> using CategoricalArrays

julia> cv = CategoricalArray(v)
6-element CategoricalArrays.CategoricalArray{String,1,UInt32}:
 "Group A"
 "Group A"
 "Group A"
 "Group B"
 "Group B"
 "Group B"

CategoricalArrays support missing values via the Missings package.

julia> using Missings

julia> cv = CategoricalArray(["Group A", missing, "Group A",
                              "Group B", "Group B", missing])
6-element CategoricalArrays.CategoricalArray{Union{Missing, String},1,UInt32}:
 "Group A"
 missing
 "Group A"
 "Group B"
 "Group B"
 missing

In addition to representing repeated data efficiently, the CategoricalArray type allows us to determine efficiently the allowed levels of the variable at any time using the levels function (note that levels may or may not be actually used in the data):

julia> levels(cv)
2-element Array{String,1}:
 "Group A"
 "Group B"

The levels! function also allows changing the order of appearance of the levels, which can be useful for display purposes or when working with ordered variables.

julia> levels!(cv, ["Group B", "Group A"]);

julia> levels(cv)
2-element Array{String,1}:
 "Group B"
 "Group A"

julia> sort(cv)
6-element CategoricalArrays.CategoricalArray{Union{Missing, String},1,UInt32}:
 "Group B"
 "Group B"
 "Group A"
 "Group A"
 missing
 missing

By default, a CategoricalArray is able to represent 2³²differents levels. You can use less memory by calling the compress function:

julia> cv = compress(cv)
6-element CategoricalArrays.CategoricalArray{Union{Missing, String},1,UInt8}:
 "Group A"
 missing
 "Group A"
 "Group B"
 "Group B"
 missing

Often, you will have factors encoded inside a DataFrame with Array columns instead of CategoricalArray columns. You can convert one or more columns of the DataFrame using the categorical! function, which modifies the input DataFrame in-place.

julia> using DataFrames

julia> df = DataFrame(A = ["A", "B", "C", "D", "D", "A"],
                      B = ["X", "X", "X", "Y", "Y", "Y"])
6×2 DataFrame
│ Row │ A      │ B      │
│     │ String │ String │
├─────┼────────┼────────┤
│ 1   │ A      │ X      │
│ 2   │ B      │ X      │
│ 3   │ C      │ X      │
│ 4   │ D      │ Y      │
│ 5   │ D      │ Y      │
│ 6   │ A      │ Y      │

julia> eltypes(df)
2-element Array{Type,1}:
 String
 String

julia> categorical!(df, :A) # change the column `:A` to be categorical
6×2 DataFrame
│ Row │ A            │ B      │
│     │ Categorical… │ String │
├─────┼──────────────┼────────┤
│ 1   │ A            │ X      │
│ 2   │ B            │ X      │
│ 3   │ C            │ X      │
│ 4   │ D            │ Y      │
│ 5   │ D            │ Y      │
│ 6   │ A            │ Y      │

julia> eltypes(df)
2-element Array{Type,1}:
 CategoricalArrays.CategoricalString{UInt32}
 String

julia> categorical!(df) # change all columns to be categorical
6×2 DataFrame
│ Row │ A            │ B            │
│     │ Categorical… │ Categorical… │
├─────┼──────────────┼──────────────┤
│ 1   │ A            │ X            │
│ 2   │ B            │ X            │
│ 3   │ C            │ X            │
│ 4   │ D            │ Y            │
│ 5   │ D            │ Y            │
│ 6   │ A            │ Y            │

julia> eltypes(df)
2-element Array{Type,1}:
 CategoricalArrays.CategoricalString{UInt32}
 CategoricalArrays.CategoricalString{UInt32}

Using categorical arrays is important for working with the GLM package. When fitting regression models, CategoricalArray columns in the input are translated into 0/1 indicator columns in the ModelMatrix with one column for each of the levels of the CategoricalArray. This allows one to analyze categorical data efficiently.

See the CategoricalArrays package for more information regarding categorical arrays.