Pivoting using tidyr
Last updated on 2024-07-02 | Edit this page
Overview
Questions
- How am I storing data in my data sets?
- How does R tend to expect me to store my data?
- How can I convert (or pivot) my data from one form to another?
Objectives
- Distinguish between “long” and “wide” formats (and “in-between” formats) for storing data in a table.
- Recognize and explain the benefits and drawbacks of storing data in each format.
- Use the
pivotfunctions in thetidyrpackage to convert data from one format to another.
Introduction
Important: There is no one universal, “right” way to store data in a table! There are many advantages and disadvantages to each way and, in some ways, so long as your organizational system follows good data science storage conventions, is consistent, and works for you and your team, then you’re probably doing it “right.”
That said, from a computing standpoint, there are two broad ways to think about how data are stored in a table–in “wide” format or in “long” format:
In “wide” format, each row is usually a grouping, site, individual, etc. for which multiple observations are recorded, and each column is everything you know about that entity, perhaps at several different time points.
In “long” format, each row is a single observation (even if you have several observations per entity), and the columns serve to clarify which entity each observations belongs to.
Notice that, regardless, the exact same information is being stored in the table–it’s just arrayed differently!
This is actually a spectrum–data can be organized in a super long format, a super wide format, or somewhere in between (if the data set has enough complexity to allow it)!
Also, when I say “long” and “wide,” I don’t mean in the physical sense of how much data you’re storing. Obviously, if you have a lot of data, you’re going to have a lot of rows and/or columns potentially, so your data set might look “long” even though it’s organized in a “wide” format.
Challenge
Important: One consequence of data organization is that it influences the ease (or difficulty!) of using a programming language like R to manipulate your data.
Consider: How would you use dplyr’s
mutate() function to calculate a “points per rebound” value
for each team using the “wide” format data set shown above? How about
for the “long” format data set?
Then, consider: How would you use dplyr’s
summarize() function to get an average number of assists
across all teams using the “wide” format data set shown above? How about
the “long” format data set?
With the data in “wide” format, using mutate() to
calculate “points per rebound” for each team would be easy because the
two values for each team (Points and Rebounds)
are in the same row. So, we’d do something like this:
R
dataset %>%
mutate(pts_per_rebound = Points/Rebounds)
However, with the data in “long” format, it’s not at all obvious how
(or even if) one could use mutate() to get what we want.
The numbers we’re trying to slam together are in different rows now.
This demonstrates that, in some ways, mutate() is a
“rowwise” tool–its designed for operations that leverage data stored
more “widely.”
Similarly, with the data in “wide” format, using
summarize() to calculate an average number of
Assists across all teams would be easy because all the
numbers we’re trying to smash together are in the same column. So, we’d
do something like this:
R
dataset %>%
summarize(mean_assists = mean(Assists))
Yes, you can use summarize() without using
group_by() if you have no groups to group by!
However, once again, the “long” format version presents
difficulties–there are numbers in the Value column we’d
need to avoid here to calculate our average. So, it’d require us to use
filter() first to remove these:
R
dataset %>%
filter(Variable == Assists) %>% #Notice I have to filter by the Variable column...
summarize(mean_assists = mean(Value)) #...But take the mean of the Value column.
So, summarize() seems like a tool also designed
for “wide” format data, but that’s only because our wide-format data
have already been grouped by team. In fact,
group_by() and summarize() are best thought of
as tools designed to work best with “long” format data because we need
to be able to easily sub-divide our data into groups to then be able to
summarize effectively and long formats have more columns for “grouping
variables.”
This whole exercise is designed to show that while, to some extent, data organization is “personal preference,” it also has implications for how we manipulate our data (and how easy or hard it will be to do so).
While this is perhaps over-generalizing, I would say that humans tend to prefer reading and recording data in “wider” formats. Notice that, when we report data in tables or record data on data sheets, we tend to do so across rows rather than down columns. Recording data in long format, in particular, tends to feel tedious because it requires us to fill out “grouping variable” columns many times with the same information.
However, computers tend to be designed to “prefer”
manipulating data in “longer” formats (regardless of what the
previous example may have led you to believe!). Computers don’t “see”
natural groupings in data like humans can, so they count on having
columns that clarify these groups (like continent and
country do in the gapminder dataset), and those types of
columns only exist in “longer” formats. In particular, many
dplyr operations will actually be easier if your data are
in “long” format, and ggplot2 especially expects
“long” format to render plots with various groups correctly.
This may seem like a dilemma–we’re torn between how we’d
prefer the data to look and how R would prefer them to look.
But, remember, it’s all the same data, just arranged differently. So, it
seems like it’d be possible to “reshape” our data to suit both needs.
That’s, in part, what the tidyr package’s functions
pivot_longer() and pivot_wider() are for.
Preparation and setup
Note: This lesson uses the gapminder data set. This data set can be downloaded here. Make sure to load the data set into your global environment before continuing.
R
gap = read.csv("data/gapminder_data.csv", header = TRUE)
Also, this lesson will revolve around use of the optional “add-on”
package tidyr. tidyr is a package in the
“tidyverse”–an array of useful tools that are designed to look similar
and work well together and that make R much more powerful to use but
also a lot different than “Base R.” Everything we do in this lesson can
be done in “Base R” instead, but it won’t (typically) be as efficient,
easy, or clear!
tidyr, like many add-on packages, is
not installed with R. So, we must install it before we
can use it:
R
install.packages("tidyr") #Only run once!
You only need to install a package once, so there’s no need to run
the above command more than once. However, packages are updated
occasionally. When updates are available, you can re-install new
versions using the same install.packages() function.
Once a package is installed, it still isn’t “turned on” by default.
So, to turn on tidyr so that we can access its unique
features, we use the library() function:
R
library(tidyr)
The above command must be run every time you start up a new session
of R and want access to tidyr’s features!
We’ll also need the dplyr package here so that we can
use pipes. See the dplyr lesson for details!
Minding the Gapminder
Challenge
Let’s look at the gapminder data set once more to remind ourselves of its structure and contents:
R
head(gap)
OUTPUT
country year pop continent lifeExp gdpPercap
1 Afghanistan 1952 8425333 Asia 28.801 779.4453
2 Afghanistan 1957 9240934 Asia 30.332 820.8530
3 Afghanistan 1962 10267083 Asia 31.997 853.1007
4 Afghanistan 1967 11537966 Asia 34.020 836.1971
5 Afghanistan 1972 13079460 Asia 36.088 739.9811
6 Afghanistan 1977 14880372 Asia 38.438 786.1134Consider: Is the gapminder data set in “long” format or “wide” format? Why do you say that?
Sorry, this is sort of a trick question because the right answer is “both” (or “neither”).
On the one hand, the gapminder data set is not as long as it
could be. For example, right now, there are three columns
containing numeric data (pop, lifeExp, and
gdpPercap). We could instead have a single
Value column to hold these data, as we had with our fake
sports data earlier, and a second column that lists what kind of
Value each row represents.
On the other hand, the gapminder data set is also not as
wide as it could be either. For example, right now, there are
three “grouping variables” (country, year, and
continent). We could instead have a single row per
country and have separate columns for the data from each
year (i.e., pop1952, pop1957,
pop1962, etc.).
So, “data orientation” is a spectrum, and the gapminder data set exists sort of in the middle of it! Hold onto this idea–in the rest of this lesson, we’re going to see how to make those longer and wider versions we just imagined!
PIVOT_LONGER
We’ll begin by seeing how to make our data even longer than it is now
by combining all the values in the pop,
lifeExp, and gdpPercap columns into a single
column called Value. We’ll then add a second column
(officially called a “key” column) that clarifies which
Statistic is being reported in the Value
column in that row.
The tidyr verb that corresponds with this goal is
pivot_longer(). As with most dplyr verbs (and
tidyverse verbs in general!), the first input to
pivot_longer() is always the data frame
you are trying to reshape (or “pivot”). In this case, that will be our
gapminder data set.
After providing our data set, we will provide three more inputs in this particular case:
The columns we are eliminating and collapsing into a single column. Here, that will be
pop,lifeExp, andgdpPercap. We’ll provide these inputs to thecolsparameter.The name of the new column that will hold all the values that used to be held by the columns we’re eliminating. So, our population, life expectancy, and GDP data in this case. We’ll call this new column
Value, and we’ll provide this input to thevalues_toparameter.The name of the new “key” column that will clarify which statistic is being held in the
Valuecolumn in a given row. We’ll call this new columnStatistic, and we’ll provide this input to thenames_toparameter.
Let’s put all this together and see what it looks like!
R
gap_longer = gap %>% #Review the dplyr lesson if you forget how to use pipes!
pivot_longer(cols = c(pop, lifeExp, gdpPercap), #We have to use c() here to bundle the column names together because these three columns are not consecutive.
values_to = "Value", #Notice the quotation marks--these are essential here!
names_to = "Statistic")
head(gap_longer)
OUTPUT
# A tibble: 6 × 5
country year continent Statistic Value
<chr> <int> <chr> <chr> <dbl>
1 Afghanistan 1952 Asia pop 8425333
2 Afghanistan 1952 Asia lifeExp 28.8
3 Afghanistan 1952 Asia gdpPercap 779.
4 Afghanistan 1957 Asia pop 9240934
5 Afghanistan 1957 Asia lifeExp 30.3
6 Afghanistan 1957 Asia gdpPercap 821. The first thing I want you to notice (by looking over in the
‘Environment Pane’) is that our new data set, gap_longer is
indeed much longer than before: it’s now up to 5112 rows!
It also has fewer columns: 5 instead of the 6 we started with.
Challenge
The other important thing to notice here is the contents of the
Statistic column. Where did R get this column’s contents
from?
It used the names of the old columns, the ones we got rid of! It figures if those names were good enough to serve as column names in the original data, they must be good enough to be “keys” in this new organization too!
PIVOT_WIDER
Now, we’ll go the other way–we’ll make our gapminder data set wider
than it already is. We’ll make a data set that has a single row for each
country (countries will be our groups) and we’ll have a
different column for every year x “Statistic” combo we have
for each country.
The tidyr verb that corresponds with this goal is
pivot_wider(), and it works very similarly to
pivot_longer(). Besides our data frame (first input), we’ll
provide pivot_wider() two more inputs:
- The column(s) that we’re going to eliminate by spreading their
contents out over several new columns. In our new, wider data set, we’re
going to have a column for each
yearx “Statistic” combination, so we’re going to eliminate the currentpop,lifeExp, andgdpPercapcolumns as we know them (though their names will get used to make the names for the new columns we’re creating). We’ll provide them to thevalues_fromparameter. - The column that we’re going to eliminate by instead inserting it
into the names of the new columns we’re creating. Here, that’s going to
be the
yearcolumn (I promise this’ll make more sense when you see it!). We’ll provide this to thenames_fromparameter.
Let’s see what this looks like!
R
gap_wider = gap %>%
pivot_wider(values_from = c(pop, lifeExp, gdpPercap),
names_from = year)
head(gap_wider)
OUTPUT
# A tibble: 6 × 38
country continent pop_1952 pop_1957 pop_1962 pop_1967 pop_1972 pop_1977
<chr> <chr> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
1 Afghanistan Asia 8425333 9240934 10267083 11537966 13079460 14880372
2 Albania Europe 1282697 1476505 1728137 1984060 2263554 2509048
3 Algeria Africa 9279525 10270856 11000948 12760499 14760787 17152804
4 Angola Africa 4232095 4561361 4826015 5247469 5894858 6162675
5 Argentina Americas 17876956 19610538 21283783 22934225 24779799 26983828
6 Australia Oceania 8691212 9712569 10794968 11872264 13177000 14074100
# ℹ 30 more variables: pop_1982 <dbl>, pop_1987 <dbl>, pop_1992 <dbl>,
# pop_1997 <dbl>, pop_2002 <dbl>, pop_2007 <dbl>, lifeExp_1952 <dbl>,
# lifeExp_1957 <dbl>, lifeExp_1962 <dbl>, lifeExp_1967 <dbl>,
# lifeExp_1972 <dbl>, lifeExp_1977 <dbl>, lifeExp_1982 <dbl>,
# lifeExp_1987 <dbl>, lifeExp_1992 <dbl>, lifeExp_1997 <dbl>,
# lifeExp_2002 <dbl>, lifeExp_2007 <dbl>, gdpPercap_1952 <dbl>,
# gdpPercap_1957 <dbl>, gdpPercap_1962 <dbl>, gdpPercap_1967 <dbl>, …As the name correctly suggests, gap_wider is indeed much
wider than our original gapminder data set: It has 38 columns instead of
the original 6. We also have fewer rows: Just 142 (1 per
country) compared to the original 1704.
Importantly, our new columns have intuitive, predictable names:
gdpPercap_1972, pop_1992, and
lifeExp1987, for example. Hopefully this is making more
sense now that you’ve seen it in action!
Challenge
You’ve now seen pivot_longer() and
pivot_wider(). Maybe you’ve noticed that they seem like
“opposites?” They are! They’re designed to “undo” the other’s
work, in fact!
So, use pivot_wider() to “rewind”
gap_longer back to the organization of our original data
set.
This task is thankfully relatively easy. We tell R that it
should pull the names for the new columns in our wider table from the
Statistic column, then pull the values for those new
columns from the old Value column:
R
gap_returned1 = gap_longer %>%
pivot_wider(names_from = Statistic,
values_from = Value)
head(gap_returned1)
OUTPUT
# A tibble: 6 × 6
country year continent pop lifeExp gdpPercap
<chr> <int> <chr> <dbl> <dbl> <dbl>
1 Afghanistan 1952 Asia 8425333 28.8 779.
2 Afghanistan 1957 Asia 9240934 30.3 821.
3 Afghanistan 1962 Asia 10267083 32.0 853.
4 Afghanistan 1967 Asia 11537966 34.0 836.
5 Afghanistan 1972 Asia 13079460 36.1 740.
6 Afghanistan 1977 Asia 14880372 38.4 786.Challenge
Now, use pivot_longer() to “rewind”
gap_wider back to the organization of the original data
set. This transformation will be a little more complicated,
though; you’ll need to specify slightly different inputs:
- For the
names_toparameter, specify exactlyc(".value", "year").".value"is a special input value here that has a particular meaning–see if you can guess what it is! You can use?pivot_longer()to research the answer, if you’d like. - You’ll also need to specify
"_"as an input for thenames_sepparameter. See if you can guess why. - Lastly, you won’t be needing to specify anything for the
values_toparameter this time–".value"is taking care of the need to put anything there.
Here’s how we’d use pivot_longer() to “rewind” to our
original data set:
R
gap_returned2 = gap_wider %>%
pivot_longer(cols = pop_1952:gdpPercap_2007,
names_to = c(".value", "year"),
names_sep = "_")
head(gap_returned2)
OUTPUT
# A tibble: 6 × 6
country continent year pop lifeExp gdpPercap
<chr> <chr> <chr> <dbl> <dbl> <dbl>
1 Afghanistan Asia 1952 8425333 28.8 779.
2 Afghanistan Asia 1957 9240934 30.3 821.
3 Afghanistan Asia 1962 10267083 32.0 853.
4 Afghanistan Asia 1967 11537966 34.0 836.
5 Afghanistan Asia 1972 13079460 36.1 740.
6 Afghanistan Asia 1977 14880372 38.4 786.Our inputs for names_to were first telling R “the names
of the new columns should come from the first parts of the
names of the columns we’re getting rid of.” That was the
".values" bit!
Then, they were telling R “the other column you should make should
simply be called year.”
Lastly, by saying names_sep = "_", we were indicating
that R should hack apart the old column names at the underscores (aka
separate the old names at the _) to find the
proper bits to use in the new column names.
So, R pulled apart the old column names, creating pop,
lifeExp, and gdpPercap columns from the front
halves and then years to fill the new year column from the
back halves of those old column names. Pretty incredible, huh??
Try to rephrase the above explanation in your own words.
Key Points
- Use the
tidyrpackage to reshape the organization of your data. - Use
pivot_longer()to go towards a “longer” layout. - Use
pivot_wider()to go towards a “wider” layout.