Readers here at the Sharp Sight blog will know how much we emphasize “foundational” data science skills.
If you want to be effective as a junior data scientist, you need to master the fundamental skills.
One of those fundamental skills is data manipulation. To be a really effective data scientist, you need to be masterful at performing essential data manipulations. This is because a very large proportion of your work will just involve getting and cleaning data.
Among the simple data manipulation tasks that you need to be able to perform are:
- selecting columns from data
- subsetting rows of data
- aggregating data
- summarising data (calculating summary statistics)
- sorting data
- creating new variables
In this blog post, we’ll talk about the last skill in that list. Using mutate in R to create new variables.
A quick introduction to the dplyr mutate function
Let’s quickly run through the basics of mutate.
Before we do that though, let’s talk about
What is dplyr?
If you’re reading this blog post, you’re probably an R user. And there’s a good chance that you’re trying to figure out how to use the functions from
If you’re not 100% familiar with it,
dplyr is an add-on package for the R programming language. The
dplyr package is a toolkit that is exclusively for data manipulation. More specifically, it is a toolkit for performing the data manipulation tasks that I listed above. It has one function for each of those core data manipulation tasks:
select()selects columns from data
filter()subsets rows of data
summarise()summarises data (calculating summary statistics)
mutate()creates new variables
For the most part,
dplyr only does these tasks. It essentially has one function for each of them. (Note that these
dplyr “functions” are sometimes called “verbs”.)
Part of what makes
dplyr great is that it is “compact.” There are only 5 or 6 major tools and they are simple to use.
Now that we’ve discussed what
dplyr is, let’s focus in on the
mutate() function so you can learn how to use mutate in R.
What is the mutate function?
mutate() function is a function for creating new variables. Essentially, that’s all it does. Like all of the
dplyr functions, it is designed to do one thing.
How to use mutate in R
mutate() is very straightforward. In fact, using any of the
dplyr functions is very straightforward, because they are quite well designed.
When you use
mutate(), you need typically to specify 3 things:
- the name of the dataframe you want to modify
- the name of the new variable that you’ll create
- the value you will assign to the new variable
So when you use
mutate(), you’ll call the function by name. Then the first argument is the dataframe that you want to manipulate.
For example, if you had a dataframe named
df, that would be the first item inside of the parenthasis (i.e., the first “argument” to the mutate function):
mutate() – like all of the
dplyr functions – strictly operates on dataframes. It’s not set up to work with lists, matrices, vectors, or other data structures.
Ok, so the first argument is the name of the dataframe.
The second argument is a “name-value” pair. That might sound a little arcane, so let me explain it.
When you use
mutate(), you’re basically creating a variable. The new variable needs a name, but it also needs a value that gets assigned to that name. So when you use mutate, you provide the name and the new value … a name-value pair.
Let’s take a look at our syntax example again:
You can see here in this dummy code example that we’re creating a new variable called
new_variable. The value assigned to
new_variable is the value of
existing_var multiplied by 2. Note that in this example, we’re assuming a dataframe called df that already has a variable called
That’s really it. To use mutate in R, all you need to do is call the function, specify the dataframe, and specify the name-value pair for the new variable you want to create.
Example: how to use mutate in R
The explanation I just gave is pretty straightforward, but to make it more concrete, let’s work with some actual data.
Here, I’ll show you how to use the
mutate() function from
First, let’s load a few packages. We’ll load
dplyr so we have access to the
mutate() function. We’ll also load the
ISLR is a package that contains several datasets. For the record, this package is actually related to the excellent book, an Introduction to Statistical Learning … a book about machine learning. We won’t be doing any machine learning here, but if you’re interested, get that book.
Ok. Here’s the code to load the packages.
#-------------- # LOAD PACKAGES #-------------- library(dplyr) library(ISLR)
We’ll be working with the
Auto dataframe from
Before we actually do anything with the data, let’s just inspect it.
Here we’ll print out the dataframe.
#------------- # INSPECT DATA #------------- print(Auto)
When you print it out, you can see that the data is a little hard to read.
print() will print out every row of data.
Before we move on, let’s fix that.
The reason that the
print() function prints out every row of data is because the
Auto dataframe is an old-fashioned
data.frame object, not a
tibble. Tibbles print better. Keep in mind that tibbles actually are dataframes, but they are modified dataframes. One of the things that is different about tibbles is that they print out with better formatting.
That being the case, I’m going to quickly coerce
Auto to a
As I do this, I’ll also rename it to
auto_specs. The name
Auto is a little non-descript, and it starts with a capital letter, which I don’t like. So very quickly, I’ll rename it while I’m coercing it to a tibble.
#------------------------------- # RENAME DATA & COERCE TO TIBBLE #------------------------------- auto_specs <- as.tibble(Auto) print(auto_specs)
This is much better.
You can see that when we print it out now, the
auto_specs has a slightly more readable structure. This is because we coerced this data to a
Ok. Now we're ready to use
This is very straightforward.
We're going to call the
mutate() function, and the first argument (the first item inside the paranthesis) is the dataframe we're going to modify,
After that (and separated by a comma) we have the name-value pair for our new variable. The name of the new variable is
hp_to_weight and the value is
horsepower divided by
#---------------------------------- # CREATE NEW VARIABLE WITH mutate() #---------------------------------- auto_specs_new <- mutate(auto_specs, hp_to_weight = horsepower / weight) print(auto_specs_new)
That's basically it. Using mutate in R to create a new variable is as simple as that.
A quick warning ... save your data
There's one thing that I want to point out. Notice that to the left hand side of the
mutate() function, I've used the assignment operator,
I did this so that the new output of
mutate() is "saved" to the
All of the
dplyr functions work with dataframes. The inputs to the
dplyr functions are dataframes. The outputs are also dataframes. So, mutate outputs a dataframe.
But by default, the
dplyr functions send the output directly to the console.
What that means is that the
dplyr functions do not automatically change the input dataframe.
Let me repeat that. The
dplyr functions do not automatically change the input dataframe.
What that means is that if you don't use the assignment operator to save the output with a name, the changes will not be saved to the input dataset.
To see this, try running
mutate() without saving the output to a new dataframe. Run the
mutate() function, and then print out the original input dataframe.
mutate(auto_specs, hp_to_weight = horsepower / weight) colnames(auto_specs)
Take a look at the column names.
hp_to_weight is not one of them!
mutate() does not directly modify the input dataframe. It leaves the input dataframe unchanged and then produces an output dataframe which is sent to the console by default (i.e., the console just prints the output).
If you want to save the output, you need to use the assignment operator and and save the output to an object name:
auto_specs_new <- mutate(auto_specs, hp_to_weight = horsepower / weight) print(auto_specs_new)
Notice that when we print out
auto_specs_new, it now has the new variable
Let this be a reminder: if you want to add a new variable to a dataframe with
mutate(), and you want that change to be permanent, you need to save the output by using the assignment operator.
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