# Introduction

As it is end-of-year I thought of writing a longer post that would be a useful reference to DataFrames.jl users.

In DataFrames.jl we have five functions that can be used to perform transformations of columns of a data frame:

• combine: create a new DataFrame populated with columns that are results of transformations applied to the source data frame columns;
• select: create a new DataFrame that has the same number of rows as the source data frame populated with columns that are results of transformations applied to the source data frame columns; (the exception to the above number of rows invariant is select(df) which produces an empty data frame);
• select!: the same as select but updates the passed data frame in place;
• transform: the same as select but keeps the columns that were already present in the data frame (note though that these columns can be potentially modified by the transformations passed to transform);
• transform!: the same as transform but updates the passed data frame in place.

The same functions also work with GroupedDataFrame with the difference that the transformations are applied to groups and then combined. Here, an important distinction is that combine again allows transformations to produce any number of rows and they are combined in the order of groups in the GroupedDataFrame. On the other hand select, select!, transform and transform! require transformations to produce the same number of rows for each group as the source group and produce a result that has the same row order as the parent data frame of GroupedDataFrame passed. This rule has two important implications:

• it is not allowed to perform select, select!, transform and transform! operations on a GroupedDataFrame whose groups do not cover all rows of the parent data frame;
• select, select!, transform and transform!, contrary to combine, ignore the order of groups in the GroupedDataFrame.

In this post I want to explain what I mean when I write transformations. These transformations follow a so-called DataFrames.jl minilanguage that is largely based on using => operator. You can find a specification of this minilanguage here. However, because the rules have to be precise they are relatively hard to read. Therefore in this post I will introduce them by example. I will give all the examples with data frames as a source, but as noted above they naturally extend to GroupedDataFrame case so I will add the examples for GroupedDataFrame only in cases when it is especially relevant (in order to read these parts of the post please earlier check out how groupby function works in DataFrames.jl).

This post is written under Julia 1.5.3 and DataFrames.jl 0.22.2 (it was also checked to work correctly on Julia 1.6.1 and DataFrames.jl 1.2.0).

The data frame that we are going to use in our examples is defined follows (I sampled some random data to populate it, with the exception of one name to celebrate this recent masterpiece):

julia> using DataFrames

julia> df = DataFrame(id = 1:6,
name = ["Aaron Aardvark", "Belen Barboza",
"春 陈", "Даниил Дубов",
"Elżbieta Elbląg", "Felipe Fittipaldi"],
age = [50, 45, 40, 35, 30, 25],
eye = ["blue", "brown", "hazel", "blue", "green", "brown"],
grade_1 = [95, 90, 85, 90, 95, 90],
grade_2 = [75, 80, 65, 90, 75, 95],
grade_3 = [85, 85, 90, 85, 80, 85])
6×7 DataFrame
│ Int64  String             Int64  String  Int64    Int64    Int64
─────┼────────────────────────────────────────────────────────────────────
1 │     1  Aaron Aardvark        50  blue         95       75       85
2 │     2  Belen Barboza         45  brown        90       80       85
3 │     3  春 陈                 40  hazel        85       65       90
4 │     4  Даниил Дубов          35  blue         90       90       85
5 │     5  Elżbieta Elbląg       30  green        95       75       80
6 │     6  Felipe Fittipaldi     25  brown        90       95       85



# Column selection

These are the most simple operations that the discussed functions can perform. In this case the result of combine and select are the same (using transform is not very useful here, as it retains all the columns anyway).

The allowed column selectors are: column number, column name, a regular expression, Not, Between, Cols, All(), and :. In order to select a column or a set of columns you just pass them as arguments. Here is an example of a simple column selection:

julia> select(df, r"grade", :)
6×7 DataFrame
│ Int64    Int64    Int64    Int64  String             Int64  String
─────┼────────────────────────────────────────────────────────────────────
1 │      95       75       85      1  Aaron Aardvark        50  blue
2 │      90       80       85      2  Belen Barboza         45  brown
3 │      85       65       90      3  春 陈                 40  hazel
4 │      90       90       85      4  Даниил Дубов          35  blue
5 │      95       75       80      5  Elżbieta Elbląg       30  green
6 │      90       95       85      6  Felipe Fittipaldi     25  brown


Above, we use a common pattern showing how one can move columns to the front of the data frame. The r"grade" regular expression picks all columns that contain "grade" string in their name, then : picks all the remaining columns.

An important rule here is that if we pass several column selectors that pick multiple columns, as you can see above, it is allowed that they select the same columns and they are included in the target data frame using first-in-first-out policy. However, it is not allowed to select the same column twice using single column selectors, so this works:

julia> select(df, "name", [2])
6×1 DataFrame
Row │ name
│ String
─────┼───────────────────
1 │ Aaron Aardvark
2 │ Belen Barboza
3 │ 春 陈
4 │ Даниил Дубов
5 │ Elżbieta Elbląg
6 │ Felipe Fittipaldi


but this fails:

julia> select(df, "name", 2)
ERROR: ArgumentError: duplicate output column name: :name


The same rule applies also to more advanced transformations that I cover below (i.e. duplicates are allowed only in plain column selectors picking multiple columns).

# Basic transformations

The simplest way to specify a transformation is

source_column => transformation => target_column_name


In this scenario the source_column is passed as an argument to transformation and stored in target_column_name column.

Here is an example:

julia> using Statistics

julia> select(df, :age => mean => :meanage)
6×1 DataFrame
Row │ meanage
│ Float64
─────┼─────────
1 │    37.5
2 │    37.5
3 │    37.5
4 │    37.5
5 │    37.5
6 │    37.5

julia> combine(df, :age => mean => :meanage)
1×1 DataFrame
Row │ meanage
│ Float64
─────┼─────────
1 │    37.5


Observe that because select produces as many rows in the produced data frame as there are rows in the source data frame, a single value is repeated accordingly. This is not the case for combine. However, if other columns in combine would produce multiple rows the repetition also happens:

julia> combine(df, :age => mean => :meanage, :eye => unique => :eye)
4×2 DataFrame
Row │ meanage  eye
│ Float64  String
─────┼─────────────────
1 │    37.5  blue
2 │    37.5  brown
3 │    37.5  hazel
4 │    37.5  green


Note, however, that it is not allowed to return vectors of different lengths in different transformations:

julia> combine(df, :age, :eye => unique => :eye)
ERROR: ArgumentError: New columns must have the same length as old columns


Let us discuss one more example, this case using GroupedDataFrame. As you can see below vectors get expanded into multiple rows by default, e.g.:

julia> combine(groupby(df, :eye),
:name => (x -> identity(x)) => :name,
:age => mean => :age)
6×3 DataFrame
Row │ eye     name               age
│ String  String             Float64
─────┼────────────────────────────────────
1 │ blue    Aaron Aardvark        42.5
2 │ blue    Даниил Дубов          42.5
3 │ brown   Belen Barboza         35.0
4 │ brown   Felipe Fittipaldi     35.0
5 │ hazel   春 陈                 40.0
6 │ green   Elżbieta Elbląg       30.0


(as a side note observe that I used (x -> identity(x)) transformation although in this case it would be enough just to write :name; the reason is that I wanted to highlight that if you use an anonymous function in the transformation definition you have to wrap it in ( and ) as shown above; otherwise the expression is parsed by Julia in an unexpected way due to the => operator precedence level)

However, in some cases it would be more natural not to expand the :name column into multiple rows. You can easily achieve it by protecting the value returned by the transformation using Ref (just as in broadcasting):

julia> combine(groupby(df, :eye),
:name => (x -> Ref(identity(x))) => :name,
:age => mean => :age)
4×3 DataFrame
Row │ eye     name                               age
│ String  SubArray…                          Float64
─────┼────────────────────────────────────────────────────
1 │ blue    ["Aaron Aardvark", "Даниил Дубов…     42.5
2 │ brown   ["Belen Barboza", "Felipe Fittip…     35.0
3 │ hazel   ["春 陈"]                             40.0
4 │ green   ["Elżbieta Elbląg"]                   30.0


In this case the vectors produced by our transformation are kept in a single row of the produced data frame. Note that in this case we also could have just written:

julia> combine(groupby(df, :eye), :name => Ref => :name, :age => mean => :age)
4×3 DataFrame
Row │ eye     name                               age
│ String  SubArray…                          Float64
─────┼────────────────────────────────────────────────────
1 │ blue    ["Aaron Aardvark", "Даниил Дубов…     42.5
2 │ brown   ["Belen Barboza", "Felipe Fittip…     35.0
3 │ hazel   ["春 陈"]                             40.0
4 │ green   ["Elżbieta Elbląg"]                   30.0


as we were applying identity transformation to our data, which can be skipped.

Often you want to apply some function not to the whole column of a data frame, but rather to its individual elements. Normally one would achieve this using broadcasting like this:

julia> select(df, :name => (x -> uppercase.(x)) => :NAME)
6×1 DataFrame
Row │ NAME
│ String
─────┼───────────────────
1 │ AARON AARDVARK
2 │ BELEN BARBOZA
3 │ 春 陈
4 │ ДАНИИЛ ДУБОВ
5 │ ELŻBIETA ELBLĄG
6 │ FELIPE FITTIPALDI


This pattern is encountered very often in practice, therefore there is a ByRow convenience wrapper for a function that creates its broadcasted variant:

julia> select(df, :name => ByRow(uppercase) => :NAME)
6×1 DataFrame
Row │ NAME
│ String
─────┼───────────────────
1 │ AARON AARDVARK
2 │ BELEN BARBOZA
3 │ 春 陈
4 │ ДАНИИЛ ДУБОВ
5 │ ELŻBIETA ELBLĄG
6 │ FELIPE FITTIPALDI


One can skip specifying a target column name, in which case it is generated automatically by suffixing source column name by function name that is applied to it, e.g.:

julia> select(df, :name => ByRow(uppercase))
6×1 DataFrame
Row │ name_uppercase
│ String
─────┼───────────────────
1 │ AARON AARDVARK
2 │ BELEN BARBOZA
3 │ 春 陈
4 │ ДАНИИЛ ДУБОВ
5 │ ELŻBIETA ELBLĄG
6 │ FELIPE FITTIPALDI


If you want to avoid this then pass renemecols=false keyword argument (this is mostly useful if you perform transformations):

julia> select(df,
:grade_1 => ByRow(x -> x / 100),
:grade_2 => ByRow(x -> x / 100),
:grade_3 => ByRow(x -> x / 100),
renamecols=false)
6×3 DataFrame
│ Float64  Float64  Float64
─────┼───────────────────────────
1 │    0.95     0.75     0.85
2 │    0.9      0.8      0.85
3 │    0.85     0.65     0.9
4 │    0.9      0.9      0.85
5 │    0.95     0.75     0.8
6 │    0.9      0.95     0.85


Similarly to skipping target column name you can skip the transformation part of the pattern we discussed, in which case it is assumed to be the identity function. In effect we get a way to rename columns in transformations:

julia> select(df, r"grade", :grade_1 => :g1, :grade_2 => :g2, :grade_3 => :g3)
6×6 DataFrame
│ Int64    Int64    Int64    Int64  Int64  Int64
─────┼────────────────────────────────────────────────
1 │      95       75       85     95     75     85
2 │      90       80       85     90     80     85
3 │      85       65       90     85     65     90
4 │      90       90       85     90     90     85
5 │      95       75       80     95     75     80
6 │      90       95       85     90     95     85


If you want to perform multiple column transformations that have a similar structure (as in the example above) you can pass a vector of transformations:

julia> select(df,
renamecols=false)
6×3 DataFrame
│ Float64  Float64  Float64
─────┼───────────────────────────
1 │    0.95     0.75     0.85
2 │    0.9      0.8      0.85
3 │    0.85     0.65     0.9
4 │    0.9      0.9      0.85
5 │    0.95     0.75     0.8
6 │    0.9      0.95     0.85


or even shorter using broadcasting and taking advantage of names function:

julia> select(df, names(df, r"grade") .=> ByRow(x -> x / 100), renamecols=false)
6×3 DataFrame
│ Float64  Float64  Float64
─────┼───────────────────────────
1 │    0.95     0.75     0.85
2 │    0.9      0.8      0.85
3 │    0.85     0.65     0.9
4 │    0.9      0.9      0.85
5 │    0.95     0.75     0.8
6 │    0.9      0.95     0.85


You now know almost all about single column selection. The only thing left to learn is that nrow function has a special treatment. It does not require passing source column name (but allows specifying of target column name; the default name is :nrow) and produces number of rows in the passed data frame. Here is an example of both:

julia> combine(groupby(df, :eye), nrow, nrow => :count)
4×3 DataFrame
Row │ eye     nrow   count
│ String  Int64  Int64
─────┼──────────────────────
1 │ blue        2      2
2 │ brown       2      2
3 │ hazel       1      1
4 │ green       1      1


That was a lot of information. Probably this is a good moment to take a short break.

Next we move on to the cases when there are multiple source columns or multiple columns are produced as a result of the transformation.

# Multiple source columns

The simplest way to pass multiple columns to a transformation is to pass their list in a vector. In this case these columns are passed as consecutive positional arguments to the function. Here is an example (also observe how automatic column naming is done in this case):

julia> combine(df,
1×3 DataFrame
│ Float64          Float64          Float64
─────┼───────────────────────────────────────────────────
1 │       0.0710072        -0.536745         0.338062


Alternatively you can pass the columns as a single argument being a NamedTuple, in order to achieve this wrap the list of passed columns in AsTable. Here is a simple example:

julia> combine(df, :grade_1, :grade_2,
6×3 DataFrame
│ Int64    Int64    Bool
─────┼────────────────────────────────────────────
1 │      95       75                      true
2 │      90       80                      true
3 │      85       65                      true
4 │      90       90                     false
5 │      95       75                      true
6 │      90       95                     false


Let us see these two rules at play in a common task of finding a sum of several columns:

julia> select(df, names(df, r"grade") => +, AsTable(names(df, r"grade")) => sum)
6×2 DataFrame
│ Int64                      Int64
─────┼────────────────────────────────────────────────────────
1 │                       255                          255
2 │                       255                          255
3 │                       240                          240
4 │                       265                          265
5 │                       250                          250
6 │                       270                          270


# Multiple target columns

Normally all the transformation functions assume that a single column is returned from a transformation function. Here is a more advanced example:

julia> select(df,
:name => ByRow(x -> (; ([:firstname, :lastname] .=> split(x))...)))
6×1 DataFrame
Row │ name_function
│ NamedTuple…
─────┼───────────────────────────────────
1 │ (firstname = "Aaron", lastname = …
2 │ (firstname = "Belen", lastname = …
3 │ (firstname = "春", lastname = "陈…
4 │ (firstname = "Даниил", lastname =…
5 │ (firstname = "Elżbieta", lastname…
6 │ (firstname = "Felipe", lastname =…


In the above code we have used the following pattern that is a convenient way of programmatic creation of NamedTuples:

julia> (; :a => 1, :b => 2)
(a = 1, b = 2)

julia> (; [:a => 1, :b => 2]...)
(a = 1, b = 2)


However, it is natural to ask if it is possible to produce the separate :firstname and :lastname columns in the data frame. You can do it by adding AsTable as target column names specifier:

julia> select(df, :name =>
ByRow(x -> (; ([:firstname, :lastname] .=> split(x))...)) =>
AsTable)
6×2 DataFrame
Row │ firstname   lastname
│ SubStrin…   SubStrin…
─────┼────────────────────────
1 │ Aaron       Aardvark
2 │ Belen       Barboza
3 │ 春          陈
4 │ Даниил      Дубов
5 │ Elżbieta    Elbląg
6 │ Felipe      Fittipaldi


In general it is not required that one has to produce a vector of NamedTuples to get this result. It is enough to have any standard iterable (e.g. a vector or a Tuple):

julia> select(df, :name => ByRow(split) => AsTable)
6×2 DataFrame
Row │ x1         x2
│ SubStrin…  SubStrin…
─────┼───────────────────────
1 │ Aaron      Aardvark
2 │ Belen      Barboza
3 │ 春         陈
4 │ Даниил     Дубов
5 │ Elżbieta   Elbląg
6 │ Felipe     Fittipaldi


Note that as split produces a vector (without column names) the names are generated automatically as :x1 and :x2. You can override this behavior by specifying the target column names explicitly:

julia> select(df, :name => ByRow(split) => [:firstname, :lastname])
6×2 DataFrame
Row │ firsntame   lastname
│ SubStrin…   SubStrin…
─────┼────────────────────────
1 │ Aaron       Aardvark
2 │ Belen       Barboza
3 │ 春          陈
4 │ Даниил      Дубов
5 │ Elżbieta    Elbląg
6 │ Felipe      Fittipaldi


Several values that can be returned by a transformation are treated to produce multiple columns by default. Therefore they are not allowed to be returned from a function unless AsTable or multiple target column names are specified.

Here is an example:

julia> combine(df, :age => x -> (age=x, age2 = x.^2))
ERROR: ArgumentError: Table returned but a single output column was expected

julia> combine(df, :age => (x -> (age=x, age2 = x.^2)) => AsTable)
6×2 DataFrame
Row │ age    age2
│ Int64  Int64
─────┼──────────────
1 │    50   2500
2 │    45   2025
3 │    40   1600
4 │    35   1225
5 │    30    900
6 │    25    625


The list of return value types treated in this way consists of four elements:

• AbstractDataFrame,
• DataFrameRow,
• AbstractMatrix,
• NamedTuple.

In the example above we returned a NamedTuple. Note, however, that returning a NamedTuple is not the same as returning a vector of NamedTuples (as we did in the topmost example in this section) as this is allowed.

For the purposes of detection of column names conflicts multiple columns returned are treated as a series of single returned columns (so duplicates are not allowed):

julia> select(df, :name => ByRow(split) => [:name, :lastname], :name)
ERROR: ArgumentError: duplicate output column name: :name


Just for completeness of discussion note that this works:

julia> select(df, :name => ByRow(split) => [:name, :lastname], [:name])
6×2 DataFrame
Row │ name       lastname
│ SubStrin…  SubStrin…
─────┼───────────────────────
1 │ Aaron      Aardvark
2 │ Belen      Barboza
3 │ 春         陈
4 │ Даниил     Дубов
5 │ Elżbieta   Elbląg
6 │ Felipe     Fittipaldi



as [:names] is considered to be a multiple column selector (thus it is not producing a duplicate and is just ignored in this case).

The traditional transformation style in the combine function was to pass a transformation as a function (without using the => minilanguage). This style is also allowed currently in all transformation functions. Additionally if you want to use this approach you can pass the function as a first argument of the function (which is often convenient in combination with do block style).

In this traditional style we are not allowed to specify source columns. Therefore such a function always takes a data frame (it is most useful with GroupedDataFrame where the data frame is representing a given group of the source data frame)

Similarly traditional style does not allow specifying target column names. Therefore the following defaults are taken:

• if AbstractDataFrame, DataFrameRow, AbstractMatrix, or NamedTuple is returned then it is assumed to produce multiple columns (in the case of NamedTuple it must contain only vectors or only single values, as if they are mixed an error is thrown).
• all other return values are treated as a single column (where returning a vector produces multiple rows and returning any other value produces one row).

Here are examples of this style:

julia> combine(groupby(df, :eye)) do sdf
return mean(sdf.age)
end
4×2 DataFrame
Row │ eye     x1
│ String  Float64
─────┼─────────────────
1 │ blue       42.5
2 │ brown      35.0
3 │ hazel      40.0
4 │ green      30.0

julia> combine(groupby(df, :eye), sdf -> mean(sdf.age))
4×2 DataFrame
Row │ eye     x1
│ String  Float64
─────┼─────────────────
1 │ blue       42.5
2 │ brown      35.0
3 │ hazel      40.0
4 │ green      30.0

julia> combine(groupby(df, :eye),
sdf -> (count = nrow(sdf),
mean_age = mean(sdf.age),
std_age = std(sdf.age)))
4×4 DataFrame
Row │ eye     count  mean_age  std_age
│ String  Int64  Float64   Float64
─────┼───────────────────────────────────
1 │ blue        2      42.5   10.6066
2 │ brown       2      35.0   14.1421
3 │ hazel       1      40.0  NaN
4 │ green       1      30.0  NaN


# Special treatment of grouping columns

As a special rule for processing of GroupedDataFrame object is that it is not allow to change the values stored in the grouping columns as they are kept by default.

Here is a simple example:

julia> combine(groupby(df, :eye), :eye => ByRow(uppercase) => :eye)
ERROR: ArgumentError: column :eye in returned data frame is not equal to grouping key :eye


However, you are allowed to do this if you drop the grouping columns by passing keepkeys=false keyword argument:

julia> transform(groupby(df, :eye),
:eye => ByRow(uppercase) => :eye,
keepkeys=false)
6×7 DataFrame
│ Int64  String             Int64  String  Int64    Int64    Int64
─────┼────────────────────────────────────────────────────────────────────
1 │     1  Aaron Aardvark        50  BLUE         95       75       85
2 │     2  Belen Barboza         45  BROWN        90       80       85
3 │     3  春 陈                 40  HAZEL        85       65       90
4 │     4  Даниил Дубов          35  BLUE         90       90       85
5 │     5  Elżbieta Elbląg       30  GREEN        95       75       80
6 │     6  Felipe Fittipaldi     25  BROWN        90       95       85


(note that for transform the key columns would be retained in the produced data frame even with keepkeys=false; in this case this keyword argument only influences the fact if we check that key columns have not changed in this case)

# Conclusions

This was long. As a conclusion let me comment that all the above-mentioned styles can be freely combined in a single transformation call:

julia> combine(groupby(df, :eye),
sdf -> nrow(sdf), nrow, :age => :AGE)
6×8 DataFrame