Getting Started

Getting Started

Installation

The DataFrames package is available through the Julia package system and can be installed using the following commands:

using Pkg
Pkg.add("DataFrames")

Throughout the rest of this tutorial, we will assume that you have installed the DataFrames package and have already typed using DataFrames to bring all of the relevant variables into your current namespace.

Note

By default Jupyter Notebook will limit the number of rows and columns when displaying a data frame to roughly fit the screen size (like in the REPL).

You can override this behavior by setting the ENV["COLUMNS"] or ENV["LINES"] variables to hold the maximum width and height of output in characters respectively before using the notebook function.

Alternatively, you may want to set the maximum number of data frame rows to print to 100 and the maximum output width in characters to 1000 for every Julia session using some Jupyter kernel file (numbers 100 and 1000 are only examples and can be adjusted). In such case add a "COLUMNS": "1000", "LINES": "100" entry to the "env" variable in this Jupyter kernel file. See here for information about location and specification of Jupyter kernels.

The DataFrame Type

Objects of the DataFrame type represent a data table as a series of vectors, each corresponding to a column or variable. The simplest way of constructing a DataFrame is to pass column vectors using keyword arguments or pairs:

julia> using DataFrames

julia> df = DataFrame(A = 1:4, B = ["M", "F", "F", "M"])
4×2 DataFrame
│ Row │ A     │ B      │
│     │ Int64 │ String │
├─────┼───────┼────────┤
│ 1   │ 1     │ M      │
│ 2   │ 2     │ F      │
│ 3   │ 3     │ F      │
│ 4   │ 4     │ M      │

Columns can be directly (i.e. without copying) accessed via df.col or df[!, :col]. The latter syntax is more flexible as it allows passing a variable holding the name of the column, and not only a literal name. Note that column names are symbols (:col or Symbol("col")) rather than strings ("col"). Columns can also be accessed using an integer index specifying their position.

Since df[!, :col] does not make a copy, changing the elements of the column vector returned by this syntax will affect the values stored in the original df. To get a copy of the column use df[:, :col]: changing the vector returned by this syntax does not change df.

julia> df.A
4-element Array{Int64,1}:
 1
 2
 3
 4

julia> df.A === df[!, :A]
true

julia> df.A === df[:, :A]
false

julia> df.A == df[:, :A]
true

julia> df.A === df[!, 1]
true

julia> df.A === df[:, 1]
false

julia> df.A == df[:, 1]
true

julia> firstcolumn = :A
:A

julia> df[!, firstcolumn] === df.A
true

julia> df[:, firstcolumn] === df.A
false

julia> df[:, firstcolumn] == df.A
true

Column names can be obtained using the names function:

julia> names(df)
2-element Array{Symbol,1}:
 :A
 :B

Constructing Column by Column

It is also possible to start with an empty DataFrame and add columns to it one by one:

julia> df = DataFrame()
0×0 DataFrame


julia> df.A = 1:8
1:8

julia> df.B = ["M", "F", "F", "M", "F", "M", "M", "F"]
8-element Array{String,1}:
 "M"
 "F"
 "F"
 "M"
 "F"
 "M"
 "M"
 "F"

julia> df
8×2 DataFrame
│ Row │ A     │ B      │
│     │ Int64 │ String │
├─────┼───────┼────────┤
│ 1   │ 1     │ M      │
│ 2   │ 2     │ F      │
│ 3   │ 3     │ F      │
│ 4   │ 4     │ M      │
│ 5   │ 5     │ F      │
│ 6   │ 6     │ M      │
│ 7   │ 7     │ M      │
│ 8   │ 8     │ F      │

The DataFrame we build in this way has 8 rows and 2 columns. This can be checked using the size function:

julia> size(df, 1)
8

julia> size(df, 2)
2

julia> size(df)
(8, 2)

Constructing Row by Row

It is also possible to fill a DataFrame row by row. Let us construct an empty data frame with two columns (note that the first column can only contain integers and the second one can only contain strings):

julia> df = DataFrame(A = Int[], B = String[])
0×2 DataFrame

Rows can then be added as tuples or vectors, where the order of elements matches that of columns:

julia> push!(df, (1, "M"))
1×2 DataFrame
│ Row │ A     │ B      │
│     │ Int64 │ String │
├─────┼───────┼────────┤
│ 1   │ 1     │ M      │

julia> push!(df, [2, "N"])
2×2 DataFrame
│ Row │ A     │ B      │
│     │ Int64 │ String │
├─────┼───────┼────────┤
│ 1   │ 1     │ M      │
│ 2   │ 2     │ N      │

Rows can also be added as Dicts, where the dictionary keys match the column names:

julia> push!(df, Dict(:B => "F", :A => 3))
3×2 DataFrame
│ Row │ A     │ B      │
│     │ Int64 │ String │
├─────┼───────┼────────┤
│ 1   │ 1     │ M      │
│ 2   │ 2     │ N      │
│ 3   │ 3     │ F      │

Note that constructing a DataFrame row by row is significantly less performant than constructing it all at once, or column by column. For many use-cases this will not matter, but for very large DataFrames this may be a consideration.

Constructing from another table type

DataFrames supports the Tables.jl interface for interacting with tabular data. This means that a DataFrame can be used as a "source" to any package that expects a Tables.jl interface input, (file format packages, data manipulation packages, etc.). A DataFrame can also be a sink for any Tables.jl interface input. Some example uses are:

df = DataFrame(a=[1, 2, 3], b=[:a, :b, :c])

# write DataFrame out to CSV file
CSV.write("dataframe.csv", df)

# store DataFrame in an SQLite database table
SQLite.load!(df, db, "dataframe_table")

# transform a DataFrame through Query.jl package
df = df |> @map({a=_.a + 1, _.b}) |> DataFrame

Working with Data Frames

Examining the Data

The default printing of DataFrame objects only includes a sample of rows and columns that fits on screen:

julia> df = DataFrame(A = 1:2:1000, B = repeat(1:10, inner=50), C = 1:500)
500×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 1     │ 1     │
│ 2   │ 3     │ 1     │ 2     │
│ 3   │ 5     │ 1     │ 3     │
│ 4   │ 7     │ 1     │ 4     │
⋮
│ 496 │ 991   │ 10    │ 496   │
│ 497 │ 993   │ 10    │ 497   │
│ 498 │ 995   │ 10    │ 498   │
│ 499 │ 997   │ 10    │ 499   │
│ 500 │ 999   │ 10    │ 500   │

Printing options can be adjusted by calling the show function manually: show(df, allrows=true) prints all rows even if they do not fit on screen and show(df, allcols=true) does the same for columns.

The first and last functions can be used to look at the first and last rows of a data frame (respectively):

julia> first(df, 6)
6×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 1     │ 1     │
│ 2   │ 3     │ 1     │ 2     │
│ 3   │ 5     │ 1     │ 3     │
│ 4   │ 7     │ 1     │ 4     │
│ 5   │ 9     │ 1     │ 5     │
│ 6   │ 11    │ 1     │ 6     │

julia> last(df, 6)
6×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 989   │ 10    │ 495   │
│ 2   │ 991   │ 10    │ 496   │
│ 3   │ 993   │ 10    │ 497   │
│ 4   │ 995   │ 10    │ 498   │
│ 5   │ 997   │ 10    │ 499   │
│ 6   │ 999   │ 10    │ 500   │

Also notice that when DataFrame is printed to the console or rendered in HTML (e.g. in Jupyter Notebook) you get an information about type of elements held in its columns. For example in this case:

julia> DataFrame(a = 1:2, b = [1.0, missing],
                 c = categorical('a':'b'), d = [1//2, missing])
2×4 DataFrame
│ Row │ a     │ b        │ c            │ d         │
│     │ Int64 │ Float64⍰ │ Categorical… │ Rationa…⍰ │
├─────┼───────┼──────────┼──────────────┼───────────┤
│ 1   │ 1     │ 1.0      │ 'a'          │ 1//2      │
│ 2   │ 2     │ missing  │ 'b'          │ missing   │

we can observe that:

Taking a Subset

Indexing syntax

Specific subsets of a data frame can be extracted using the indexing syntax, similar to matrices. The colon : indicates that all items (rows or columns depending on its position) should be retained:

julia> df[1:3, :]
3×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 1     │ 1     │
│ 2   │ 3     │ 1     │ 2     │
│ 3   │ 5     │ 1     │ 3     │

julia> df[[1, 5, 10], :]
3×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 1     │ 1     │
│ 2   │ 9     │ 1     │ 5     │
│ 3   │ 19    │ 1     │ 10    │

julia> df[:, [:A, :B]]
500×2 DataFrame
│ Row │ A     │ B     │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 1     │ 1     │
│ 2   │ 3     │ 1     │
│ 3   │ 5     │ 1     │
│ 4   │ 7     │ 1     │
⋮
│ 496 │ 991   │ 10    │
│ 497 │ 993   │ 10    │
│ 498 │ 995   │ 10    │
│ 499 │ 997   │ 10    │
│ 500 │ 999   │ 10    │

julia> df[1:3, [:B, :A]]
3×2 DataFrame
│ Row │ B     │ A     │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 1     │ 1     │
│ 2   │ 1     │ 3     │
│ 3   │ 1     │ 5     │

julia> df[[3, 1], [:C]]
2×1 DataFrame
│ Row │ C     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 3     │
│ 2   │ 1     │

Do note that df[!, [:A]] and df[:, [:A]] return a DataFrame object, while df[!, :A] and df[:, :A] return a vector:

julia> df[!, [:A]]
500×1 DataFrame
│ Row │ A     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │
│ 2   │ 3     │
│ 3   │ 5     │
│ 4   │ 7     │
⋮
│ 496 │ 991   │
│ 497 │ 993   │
│ 498 │ 995   │
│ 499 │ 997   │
│ 500 │ 999   │

julia> df[!, [:A]] == df[:, [:A]]
true

julia> df[!, :A]
500-element Array{Int64,1}:
   1
   3
   5
   7
   9
  11
   ⋮
 991
 993
 995
 997
 999

julia> df[!, :A] == df[:, :A]
true

In the first cases, [:A] is a vector, indicating that the resulting object should be a DataFrame, since a vector can contain one or more column names. On the other hand, :A is a single symbol, indicating that a single column vector should be extracted. Note that in the first case a vector is required to be passed (not just any iterable), so e.g. df[:, (:x1, :x2)] is not allowed, but df[:, [:x1, :x2]] is valid.

It is also possible to use a regular expression as a selector of columns matching it:

julia> df = DataFrame(x1=1, x2=2, y=3)
1×3 DataFrame
│ Row │ x1    │ x2    │ y     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 2     │ 3     │

julia> df[!, r"x"]
1×2 DataFrame
│ Row │ x1    │ x2    │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 1     │ 2     │

A Not selector (from the InvertedIndices package) can be used to select all columns excluding a specific subset:

julia> df[!, Not(:x1)]
1×2 DataFrame
│ Row │ x2    │ y     │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 2     │ 3     │

Finally, you can use Not and All selectors in more complex column selection scenarios. The following examples move all columns whose names match r"x" regular expression respectively to the front and to the end of a data frame:

julia> df = DataFrame(r=1, x1=2, x2=3, y=4)
1×4 DataFrame
│ Row │ r     │ x1    │ x2    │ y     │
│     │ Int64 │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 1     │ 2     │ 3     │ 4     │

julia> df[:, All(r"x", :)]
1×4 DataFrame
│ Row │ x1    │ x2    │ r     │ y     │
│     │ Int64 │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 2     │ 3     │ 1     │ 4     │

julia> df[:, All(Not(r"x"), :)]
1×4 DataFrame
│ Row │ r     │ y     │ x1    │ x2    │
│     │ Int64 │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┼───────┤
│ 1   │ 1     │ 4     │ 2     │ 3     │

The indexing syntax can also be used to select rows based on conditions on variables:

julia> df[df.A .> 500, :]
250×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 501   │ 6     │ 251   │
│ 2   │ 503   │ 6     │ 252   │
│ 3   │ 505   │ 6     │ 253   │
│ 4   │ 507   │ 6     │ 254   │
⋮
│ 246 │ 991   │ 10    │ 496   │
│ 247 │ 993   │ 10    │ 497   │
│ 248 │ 995   │ 10    │ 498   │
│ 249 │ 997   │ 10    │ 499   │
│ 250 │ 999   │ 10    │ 500   │

julia> df[(df.A .> 500) .& (300 .< df.C .< 400), :]
99×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 601   │ 7     │ 301   │
│ 2   │ 603   │ 7     │ 302   │
│ 3   │ 605   │ 7     │ 303   │
│ 4   │ 607   │ 7     │ 304   │
⋮
│ 95  │ 789   │ 8     │ 395   │
│ 96  │ 791   │ 8     │ 396   │
│ 97  │ 793   │ 8     │ 397   │
│ 98  │ 795   │ 8     │ 398   │
│ 99  │ 797   │ 8     │ 399   │

Where a specific subset of values needs to be matched, the in() function can be applied:

julia> df[in.(df.A, Ref([1, 5, 601])), :]
3×3 DataFrame
│ Row │ A     │ B     │ C     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 1     │ 1     │
│ 2   │ 5     │ 1     │ 3     │
│ 3   │ 601   │ 7     │ 301   │

Equivalently, the in function can be called with a single argument to create a function object that tests whether each value belongs to the subset (partial application of in): df[in([1, 5, 601]).(df.A), :].

Column selection using select and select!

You can also use the select and select! functions to select columns in a data frame.

The select function creates a new data frame:

julia> df = DataFrame(x1=1, x2=2, y=3)
1×3 DataFrame
│ Row │ x1    │ x2    │ y     │
│     │ Int64 │ Int64 │ Int64 │
├─────┼───────┼───────┼───────┤
│ 1   │ 1     │ 2     │ 3     │

julia> select(df, Not(:x1)) # drop column :x1 in a new data frame
1×2 DataFrame
│ Row │ x2    │ y     │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 2     │ 3     │

julia> select(df, r"x") # select columns containing 'x' character
1×2 DataFrame
│ Row │ x1    │ x2    │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 1     │ 2     │

It is important to note that select always returns a data frame, even if a single column is selected (as opposed to indexing syntax).

julia> select(df, :x1)
1×1 DataFrame
│ Row │ x1    │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │

julia> df[:, :x1]
1-element Array{Int64,1}:
 1

By default select copies columns of a passed source data frame. In order to avoid copying, pass copycols=false:

julia> df2 = select(df, :x1)
1×1 DataFrame
│ Row │ x1    │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │

julia> df2.x1 === df.x1
false

julia> df2 = select(df, :x1, copycols=false)
1×1 DataFrame
│ Row │ x1    │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │

julia> df2.x1 === df.x1
true

To perform the selection operation in-place use select!:

julia> select!(df, Not(:x1));

julia> df
1×2 DataFrame
│ Row │ x2    │ y     │
│     │ Int64 │ Int64 │
├─────┼───────┼───────┤
│ 1   │ 2     │ 3     │

While the DataFrames package provides basic data manipulation capabilities, users are encouraged to use querying frameworks for more convenient and powerful operations:

See the Data manipulation frameworks section for more information.

Summarizing Data

The describe function returns a data frame summarizing the elementary statistics and information about each column:

julia> df = DataFrame(A = 1:4, B = ["M", "F", "F", "M"])

julia> describe(df)
2×8 DataFrame
│ Row │ variable │ mean   │ min │ median │ max │ nunique │ nmissing │ eltype   │
│     │ Symbol   │ Union… │ Any │ Union… │ Any │ Union…  │ Nothing  │ DataType │
├─────┼──────────┼────────┼─────┼────────┼─────┼─────────┼──────────┼──────────┤
│ 1   │ A        │ 2.5    │ 1   │ 2.5    │ 4   │         │          │ Int64    │
│ 2   │ B        │        │ F   │        │ M   │ 2       │          │ String   │

If you are interested in describing only a subset of columns then the easiest way to do it is to pass a subset of an original data frame to describe like this:

julia> describe(df[!, [:A]))
1×8 DataFrame
│ Row │ variable │ mean    │ min   │ median  │ max   │ nunique │ nmissing │ eltype   │
│     │ Symbol   │ Float64 │ Int64 │ Float64 │ Int64 │ Nothing │ Nothing  │ DataType │
├─────┼──────────┼─────────┼───────┼─────────┼───────┼─────────┼──────────┼──────────┤
│ 1   │ A        │ 2.5     │ 1     │ 2.5     │ 4     │         │          │ Int64    │

Of course, one can also compute descriptive statistics directly on individual columns:

julia> using Statistics

julia> mean(df.A)
2.5

Column-Wise Operations

We can also apply a function to each column of a DataFrame with the aggregate function. For example:

julia> df = DataFrame(A = 1:4, B = 4.0:-1.0:1.0)
4×2 DataFrame
│ Row │ A     │ B       │
│     │ Int64 │ Float64 │
├─────┼───────┼─────────┤
│ 1   │ 1     │ 4.0     │
│ 2   │ 2     │ 3.0     │
│ 3   │ 3     │ 2.0     │
│ 4   │ 4     │ 1.0     │

julia> aggregate(df, sum)
1×2 DataFrame
│ Row │ A_sum │ B_sum   │
│     │ Int64 │ Float64 │
├─────┼───────┼─────────┤
│ 1   │ 10    │ 10.0    │

julia> aggregate(df, [sum, prod])
1×4 DataFrame
│ Row │ A_sum │ B_sum   │ A_prod │ B_prod  │
│     │ Int64 │ Float64 │ Int64  │ Float64 │
├─────┼───────┼─────────┼────────┼─────────┤
│ 1   │ 10    │ 10.0    │ 24     │ 24.0    │

Handling of Columns Stored in a DataFrame

Functions that transform a DataFrame to produce a new DataFrame always perform a copy of the columns by default, for example:

julia> df = DataFrame(A = 1:4, B = 4.0:-1.0:1.0)
4×2 DataFrame
│ Row │ A     │ B       │
│     │ Int64 │ Float64 │
├─────┼───────┼─────────┤
│ 1   │ 1     │ 4.0     │
│ 2   │ 2     │ 3.0     │
│ 3   │ 3     │ 2.0     │
│ 4   │ 4     │ 1.0     │

julia> df2 = copy(df);

julia> df2.A === df.A
false

On the other hand, in-place functions, whose names end with !, may mutate the column vectors of the DataFrame they take as an argument, for example:

julia> x = [3, 1, 2];

julia> df = DataFrame(x=x)
3×1 DataFrame
│ Row │ x     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 3     │
│ 2   │ 1     │
│ 3   │ 2     │

julia> sort!(df)
3×1 DataFrame
│ Row │ x     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 1     │
│ 2   │ 2     │
│ 3   │ 3     │

julia> x
3-element Array{Int64,1}:
 1
 2
 3

julia> df.x[1] = 100
100

julia> df
3×1 DataFrame
│ Row │ x     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 100   │
│ 2   │ 2     │
│ 3   │ 3     │

julia> x
3-element Array{Int64,1}:
 100
   2
   3

In-place functions are safe to call, except when a view of the DataFrame (created via a view, @view or groupby) or when a DataFrame created with copycols=false (or with the DataFrame! function) are in use.

It is possible to have a direct access to a column col of a DataFrame df using the syntaxes df.col, df[!, :col], via the eachcol function, by accessing a parent of a view of a column of a DataFrame, or simply by storing the reference to the column vector before the DataFrame was created with copycols=false (or with the DataFrame! function).

julia> x = [3, 1, 2];

julia> df = DataFrame(x=x)
3×1 DataFrame
│ Row │ x     │
│     │ Int64 │
├─────┼───────┤
│ 1   │ 3     │
│ 2   │ 1     │
│ 3   │ 2     │

julia> df.x == x
true

julia> df[1] !== x
true

julia> eachcol(df, false)[1] === df.x
true

Note that a column obtained from a DataFrame using one of these methods should not be mutated without caution.

The exact rules of handling columns of a DataFrame are explained in The design of handling of columns of a DataFrame section of the manual.

Importing and Exporting Data (I/O)

For reading and writing tabular data from CSV and other delimited text files, use the CSV.jl package.

If you have not used the CSV.jl package before then you may need to install it first:

using Pkg
Pkg.add("CSV")

The CSV.jl functions are not loaded automatically and must be imported into the session.

using CSV

A dataset can now be read from a CSV file at path input using

DataFrame(CSV.File(input))

A DataFrame can be written to a CSV file at path output using

df = DataFrame(x = 1, y = 2)
CSV.write(output, df)

The behavior of CSV functions can be adapted via keyword arguments. For more information, see ?CSV.File, ?CSV.read and ?CSV.write, or checkout the online CSV.jl documentation.