4.5 Types and Categorical Data

As discussed in Section 4.1, CSV.jl will do its best to guess what kind of types your data have as columns. However, this won’t always work perfectly. In this section, we show why suitable types are important and we fix wrong data types. To be more clear about the types, we show the text output for DataFrames instead of a pretty-formatted table. In this section, we work with the following dataset:

function wrong_types()
    id = 1:4
    date = ["28-01-2018", "03-04-2019", "01-08-2018", "22-11-2020"]
    age = ["adolescent", "adult", "infant", "adult"]
    DataFrame(; id, date, age)
end
wrong_types()

4×3 DataFrame
 Row │ id     date        age
     │ Int64  String      String
─────┼───────────────────────────────
   1 │     1  28-01-2018  adolescent
   2 │     2  03-04-2019  adult
   3 │     3  01-08-2018  infant
   4 │     4  22-11-2020  adult

Because the date column has the wrong type, sorting won’t work correctly:

sort(wrong_types(), :date)

4×3 DataFrame
 Row │ id     date        age
     │ Int64  String      String
─────┼───────────────────────────────
   1 │     3  01-08-2018  infant
   2 │     2  03-04-2019  adult
   3 │     4  22-11-2020  adult
   4 │     1  28-01-2018  adolescent

To fix the sorting, we can use the Date module from Julia’s standard library as described in Section 3.5.1:

function fix_date_column(df::DataFrame)
    strings2dates(dates::Vector) = Date.(dates, dateformat"dd-mm-yyyy")
    dates = strings2dates(df[!, :date])
    df[!, :date] = dates
    df
end
fix_date_column(wrong_types())

4×3 DataFrame
 Row │ id     date        age
     │ Int64  Date        String
─────┼───────────────────────────────
   1 │     1  2018-01-28  adolescent
   2 │     2  2019-04-03  adult
   3 │     3  2018-08-01  infant
   4 │     4  2020-11-22  adult

Now, sorting will work as intended:

df = fix_date_column(wrong_types())
sort(df, :date)

4×3 DataFrame
 Row │ id     date        age
     │ Int64  Date        String
─────┼───────────────────────────────
   1 │     1  2018-01-28  adolescent
   2 │     3  2018-08-01  infant
   3 │     2  2019-04-03  adult
   4 │     4  2020-11-22  adult

For the age column, we have a similar problem:

sort(wrong_types(), :age)

4×3 DataFrame
 Row │ id     date        age
     │ Int64  String      String
─────┼───────────────────────────────
   1 │     1  28-01-2018  adolescent
   2 │     2  03-04-2019  adult
   3 │     4  22-11-2020  adult
   4 │     3  01-08-2018  infant

This isn’t right, because an infant is younger than adults and adolescents. The solution for this issue and any sort of categorical data is to use CategoricalArrays.jl:

4.5.1 CategoricalArrays.jl

using CategoricalArrays

With the CategoricalArrays.jl package, we can add levels that represent the ordering of our categorical variable to our data:

function fix_age_column(df)
    levels = ["infant", "adolescent", "adult"]
    ages = categorical(df[!, :age]; levels, ordered=true)
    df[!, :age] = ages
    df
end
fix_age_column(wrong_types())

4×3 DataFrame
 Row │ id     date        age
     │ Int64  String      Cat…
─────┼───────────────────────────────
   1 │     1  28-01-2018  adolescent
   2 │     2  03-04-2019  adult
   3 │     3  01-08-2018  infant
   4 │     4  22-11-2020  adult

NOTE: Also note that we are passing the argument ordered=true which tells CategoricalArrays.jl’s categorical function that our categorical data is “ordered.” Without this any type of sorting or bigger/smaller comparisons would not be possible.

Now, we can sort the data correctly on the age column:

df = fix_age_column(wrong_types())
sort(df, :age)

4×3 DataFrame
 Row │ id     date        age
     │ Int64  String      Cat…
─────┼───────────────────────────────
   1 │     3  01-08-2018  infant
   2 │     1  28-01-2018  adolescent
   3 │     2  03-04-2019  adult
   4 │     4  22-11-2020  adult

Because we have defined convenient functions, we can now define our fixed data by just performing the function calls:

function correct_types()
    df = wrong_types()
    df = fix_date_column(df)
    df = fix_age_column(df)
end
correct_types()

4×3 DataFrame
 Row │ id     date        age
     │ Int64  Date        Cat…
─────┼───────────────────────────────
   1 │     1  2018-01-28  adolescent
   2 │     2  2019-04-03  adult
   3 │     3  2018-08-01  infant
   4 │     4  2020-11-22  adult

Since age in our data is ordinal (ordered=true), we can properly compare categories of age:

df = correct_types()
a = df[1, :age]
b = df[2, :age]
a < b

true

which would give wrong comparisons if the element type were strings:

"infant" < "adult"

false

Support this project
CC BY-NC-SA 4.0 Jose Storopoli, Rik Huijzer, Lazaro Alonso