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### DataFrames basics - poke at your data!

Let's explore some of the basic functionalities of DataFrames in Julia. If you've had some experience with R's DataFrames or Python's Pandas then this should be smooth sailing for you. If you have no previous dataframes experience, don't worry this is the most basic intro you can imagine! :)

Why DataFrames? We already know that Julia has built in support for array like objects, so why would we need another rectengular data format? Well, in data science there are many cases when we have a mixture of numerical and string data in the same table. Think of for example an employment table. One column might be whether the individual is employed or not (Boolean) another might contain info on the industry that person works in (string) and another the salary as a float. DataFrames are great for handling such data as it keeps your observations together without penalising you for mixing data types.
In a previous post I've already shown how to read in tabular file formats using CSV.jl, so we'll skip that part and use the RDatasets package to play around with a real classic, the iris dataset. It's not a requirement to know what's in this data, but do a quick Google search if you're curious. In summary this table has info on a bunch of pretty flowers.
Let's read in this file:
using DataFrames
using RDatasets

# we need the iris table from the datasets directory
iris = dataset("datasets", "iris")

As a first step it might be a good idea to actually know what the columns are called in this data. Ie. what it is that we're looking at. DataFrames.jl provides the same syntax as R for this:
names(iris)

5-element Array{Symbol,1}:
:SepalLength
:SepalWidth
:PetalLength
:PetalWidth
:Species

If you look closely, you can see that these names are actually Symbols and not strings. There is a very good reason for this but I won't go into the details here. Just remember that you can create symbols by using : before a name or by using the Symbol constructor:
a_string = "column_name"
a_symbol = :column_name
another_symbol = Symbol("column_name")

This will be important when we come to subsetting DataFrames.
Checking how many rows and columns we have is also a good idea:
size(iris)

(150, 5)

This tells us that we have 150 rows (first index) and 5 columns (2nd index).
You can always peek at your data by looking at the first few rows. This is useful when you're trying to familiarise yourself with the structure of the table and get an idea about the values it holds:
head(iris)

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
15.13.51.40.2setosa
24.93.01.40.2setosa
34.73.21.30.2setosa
44.63.11.50.2setosa
55.03.61.40.2setosa
65.43.91.70.4setosa
You can also check the last few rows using tail:
tail(iris)

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
16.73.35.72.5virginica
26.73.05.22.3virginica
36.32.55.01.9virginica
46.53.05.22.0virginica
56.23.45.42.3virginica
65.93.05.11.8virginica
If you're not satisfyed with the default 5 rows, you can change it by passing a 2nd argument to both head and tail:
head(iris, 3)

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
15.13.51.40.2setosa
24.93.01.40.2setosa
34.73.21.30.2setosa
If you're working with Julia you usually don't need to do this though, as simply saying iris will print the first few and the last few columns of the dataset.
As I mentioned in the beginning, a DataFrame can hold columns with different types. To see the types, you can use the eltypes function:
eltypes(iris)

5-element Array{Type,1}:
Float64
Float64
Float64
Float64
CategoricalArrays.CategoricalString{UInt8}

You can also use showcols() and see the number of missing values too. But note that this prints the values rather than returning an array of DataFrame, so I'd recommend you stick to eltypes.
Notice how Species has a CategoricalString type. You can think of this as a factor variable in R. It allows us to represent low cardinality strings as integers but show us the human readable values (levels) instead. In this case we only have 3 Species in the dataset, so it makes sense to have them as categorical variables.
If you want to gain a quick intuition about your dataset then make friends with describe(). This handy function shows us the main summary stats for each column and also tells us their types!
describe(iris)

SepalLength
Summary Stats:
Mean:           5.843333
Minimum:        4.300000
1st Quartile:   5.100000
Median:         5.800000
3rd Quartile:   6.400000
Maximum:        7.900000
Length:         150
Type:           Float64

SepalWidth
Summary Stats:
Mean:           3.057333
Minimum:        2.000000
1st Quartile:   2.800000
Median:         3.000000
3rd Quartile:   3.300000
Maximum:        4.400000
Length:         150
Type:           Float64

PetalLength
Summary Stats:
Mean:           3.758000
Minimum:        1.000000
1st Quartile:   1.600000
Median:         4.350000
3rd Quartile:   5.100000
Maximum:        6.900000
Length:         150
Type:           Float64

PetalWidth
Summary Stats:
Mean:           1.199333
Minimum:        0.100000
1st Quartile:   0.300000
Median:         1.300000
3rd Quartile:   1.800000
Maximum:        2.500000
Length:         150
Type:           Float64

Species
Summary Stats:
Length:         150
Type:           CategoricalArrays.CategoricalString{UInt8}
Number Unique:  3

Now that we have a rough idea about what we're working with let's start slicing and dicing. It's quite commont that we would like to extract a column or columns from our dataset. We can do this by referring to the columns by their names or indexing using brackets []. Remeber that this DataFrame has symbols as column names!
iris[:SepalLength]

150-element Array{Float64,1}:
5.1
4.9
4.7
4.6
5.0
5.4
4.6
5.0
4.4
4.9
⋮
6.9
5.8
6.8
6.7
6.7
6.3
6.5
6.2
5.9

iris[1]

150-element Array{Float64,1}:
5.1
4.9
4.7
4.6
5.0
5.4
4.6
5.0
4.4
4.9
⋮
6.9
5.8
6.8
6.7
6.7
6.3
6.5
6.2
5.9

Then we can operate on these arrays as usual:
sum(iris[1])

876.5

It's also easy to select rows. The trick here is that by default DataFrames looks for columns when we use integers as indexes. To select rows, we just need to tell the DataFrames which columns we want to pick too.
If we want all available columns, we can just pass a colon : for the second argument:
# this gives us the 1st row and all the columns
iris[1,:]

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
15.13.51.40.2setosa
Slicing is also valid, so we can say: give us all the rows between 1 and 10:
iris[1:10, :]

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
15.13.51.40.2setosa
24.93.01.40.2setosa
34.73.21.30.2setosa
44.63.11.50.2setosa
55.03.61.40.2setosa
65.43.91.70.4setosa
74.63.41.40.3setosa
85.03.41.50.2setosa
94.42.91.40.2setosa
104.93.11.50.1setosa
But there's more! We can also pass in a boolean vector (same length as many rows we have). But why and how do we get such an array? One typical usecase is that we want to select rows based on some logic. Let's say we want to find all flowers where PetalLength is over 5. We know how to do that comparison already:
# the dot tells Julia to compare each element of the array
over_five = iris[:PetalLength] .> 5

This gets a true value for each element where this logic is satisfyed and false values otherwise. Now we can pass this in as a selector, to end up with observations that we actually need:
head(iris[over_five,:])

SepalLengthSepalWidthPetalLengthPetalWidthSpecies
16.02.75.11.6versicolor
26.33.36.02.5virginica
35.82.75.11.9virginica
47.13.05.92.1virginica
56.32.95.61.8virginica
66.53.05.82.2virginica
In order to try some additional indexing, I'm going to indtroduce you to a higher order function, called colwise. Higher order you say? A higher order function is a function that takes a function as an argument. Why would we need this? To make our code pretty and easily readable.
In our case, colwise takes a function and a DataFrame and then applies that function column wise (to each column) to the DataFrame. A simple example would be to get the maximum value for each column:
colwise(maximum, iris)

ERROR: ArgumentError: Unordered CategoricalValue objects cannot be tested for order using <. Use isless instead, or call the ordered! function on the parent array to change this

Oh, but this gives us an error. Fotunately the error is pretty clear. Get into the habit of reading your error messages! It'll save you loads of time later on. Here the error is about comparing CategoricalValue objects. It's clear that to find the maximum of an array we need to be able to compare values to each other, but for categorical values this doesn't make much sense. Who is to say that setosa is better than virginica? To solve the error, we just need to exclude the categorical Species column:
colwise(maximum, iris[:,1:4])

4-element Array{Float64,1}:
7.9
4.4
6.9
2.5

Look how I just selected the first 4 columns... This is not recommended. Let's say you write this piece of code and then your collegaue comes in and jumbles the iris dataset, then the above might give an error as before. To avoid this, we should pick the columns implicitly by their names:
# first we find all column names that are not Species
numerical_cols = filter(x->x!=:Species, names(iris))
numerical_cols

4-element Array{Symbol,1}:
:SepalLength
:SepalWidth
:PetalLength
:PetalWidth

Yes, filter is another higher order function. Can you guess what it does? :)
Now we can safely index using this array of Symbols:
head(iris[:,numerical_cols])

SepalLengthSepalWidthPetalLengthPetalWidth
15.13.51.40.2
24.93.01.40.2
34.73.21.30.2
44.63.11.50.2
55.03.61.40.2
65.43.91.70.4
And to prove that this solves the over-eager co-worker problem I'll reshuffle the datase, by reversing the columns just to prove a point:
new_iris = iris[:,names(iris)[end:-1:1]]

# this will now fail
colwise(maximum, new_iris[:,1:4])

ERROR: ArgumentError: Unordered CategoricalValue objects cannot be tested for order using <. Use isless instead, or call the ordered! function on the parent array to change this

# but this still works
colwise(maximum, new_iris[numerical_cols])

4-element Array{Float64,1}:
7.9
4.4
6.9
2.5

Thanks for reading all the way to the end. Now you should have all the skills required to do some very simple poking at your data using DataFrames.jl. Come back here later for some more advanced tutorials on how to aggregate and visualise your data.
Also, if you have friends who are not too keen on R/Python or just interested in learning something awesome share this blog with them as I focus on using Julia for Data Science.

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