21 Introduction to dplyr
dplyr is a package for data manipulation, developed by Hadley Wickham and Romain Francois. It is built to be fast, highly expressive, and open-minded about how your data is stored. It is installed as part of the tidyverse meta-package and, as a core package, it is among those loaded via library(tidyverse).
dplyr’s roots are in an earlier package called plyr, which implements the “split-apply-combine” strategy for data analysis (Hadley Wickham 2011b). Where plyr covers a diverse set of inputs and outputs (e.g., arrays, data frames, lists), dplyr has a laser-like focus on data frames or, in the tidyverse, “tibbles”. dplyr is a package-level treatment of the ddply() function from plyr, because “data frame in, data frame out” proved to be so incredibly important.
Have no idea what I’m talking about? Not sure if you care? If you use these base R functions: subset(), apply(), [sl]apply(), tapply(), aggregate(), split(), do.call(), with(), within(), then you should keep reading. Also, if you use for() loops a lot, you might enjoy learning other ways to iterate over rows or groups of rows or variables in a data frame.
21.0.1 Load dplyr and gapminder
I choose to load the tidyverse, which will load dplyr, among other packages we’ll use incidentally below.
Also load gapminder.
21.0.2 Say hello to the gapminder tibble
The gapminder data frame is a special kind of data frame: a tibble.
gapminder
#> # A tibble: 1,704 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Afghanistan Asia 1952 28.8 8425333 779.
#> 2 Afghanistan Asia 1957 30.3 9240934 821.
#> 3 Afghanistan Asia 1962 32.0 10267083 853.
#> 4 Afghanistan Asia 1967 34.0 11537966 836.
#> 5 Afghanistan Asia 1972 36.1 13079460 740.
#> 6 Afghanistan Asia 1977 38.4 14880372 786.
#> 7 Afghanistan Asia 1982 39.9 12881816 978.
#> 8 Afghanistan Asia 1987 40.8 13867957 852.
#> 9 Afghanistan Asia 1992 41.7 16317921 649.
#> 10 Afghanistan Asia 1997 41.8 22227415 635.
#> # ℹ 1,694 more rowsIt’s tibble-ness is why we get nice compact printing. For a reminder of the problems with base data frame printing, go type iris in the R Console or, better yet, print a data frame to screen that has lots of columns.
Note how gapminder’s class() includes tbl_df; the “tibble” terminology is a nod to this.
Some functions, like print(), know about tibbles and do something special. However, other functions do not, like summary(). In those cases, the tibble will be treated the same as a regular data frame because every tibble is also a regular data frame.
To turn any data frame into a tibble, use as_tibble():
as_tibble(iris)
#> # A tibble: 150 × 5
#> Sepal.Length Sepal.Width Petal.Length Petal.Width Species
#> <dbl> <dbl> <dbl> <dbl> <fct>
#> 1 5.1 3.5 1.4 0.2 setosa
#> 2 4.9 3 1.4 0.2 setosa
#> 3 4.7 3.2 1.3 0.2 setosa
#> 4 4.6 3.1 1.5 0.2 setosa
#> 5 5 3.6 1.4 0.2 setosa
#> 6 5.4 3.9 1.7 0.4 setosa
#> 7 4.6 3.4 1.4 0.3 setosa
#> 8 5 3.4 1.5 0.2 setosa
#> 9 4.4 2.9 1.4 0.2 setosa
#> 10 4.9 3.1 1.5 0.1 setosa
#> # ℹ 140 more rows21.1 Think before you create excerpts of your data
If you feel the urge to store a little snippet of your data:
(canada <- gapminder[241:252, ])
#> # A tibble: 12 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Canada Americas 1952 68.8 14785584 11367.
#> 2 Canada Americas 1957 70.0 17010154 12490.
#> 3 Canada Americas 1962 71.3 18985849 13462.
#> 4 Canada Americas 1967 72.1 20819767 16077.
#> 5 Canada Americas 1972 72.9 22284500 18971.
#> 6 Canada Americas 1977 74.2 23796400 22091.
#> 7 Canada Americas 1982 75.8 25201900 22899.
#> 8 Canada Americas 1987 76.9 26549700 26627.
#> 9 Canada Americas 1992 78.0 28523502 26343.
#> 10 Canada Americas 1997 78.6 30305843 28955.
#> 11 Canada Americas 2002 79.8 31902268 33329.
#> 12 Canada Americas 2007 80.7 33390141 36319.Stop and ask yourself …
Do I want to create mini datasets for each level of some factor (or unique combination of several factors) … in order to compute or graph something?
If YES, use proper data aggregation techniques or faceting in ggplot2 – don’t subset the data. Or, more realistic, only subset the data as a temporary measure while you develop your elegant code for computing on or visualizing these data subsets.
If NO, then maybe you really do need to store a copy of a subset of the data. But seriously consider whether you can achieve your goals by simply using the subset = argument of, e.g., the lm() function, to limit computation to your excerpt of choice. Lots of functions offer a subset = argument!
Copies and excerpts of your data clutter your workspace, invite mistakes, and sow general confusion. Avoid whenever possible.
Reality can also lie somewhere in between. You will find the workflows presented below can help you accomplish your goals with minimal creation of temporary, intermediate objects.
21.2 Use filter() to subset data row-wise
filter() takes logical expressions and returns the rows for which all are TRUE.
filter(gapminder, lifeExp < 29)
#> # A tibble: 2 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Afghanistan Asia 1952 28.8 8425333 779.
#> 2 Rwanda Africa 1992 23.6 7290203 737.filter(gapminder, country == "Rwanda", year > 1979)
#> # A tibble: 6 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Rwanda Africa 1982 46.2 5507565 882.
#> 2 Rwanda Africa 1987 44.0 6349365 848.
#> 3 Rwanda Africa 1992 23.6 7290203 737.
#> 4 Rwanda Africa 1997 36.1 7212583 590.
#> 5 Rwanda Africa 2002 43.4 7852401 786.
#> 6 Rwanda Africa 2007 46.2 8860588 863.filter(gapminder, country %in% c("Rwanda", "Afghanistan"))
#> # A tibble: 24 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Afghanistan Asia 1952 28.8 8425333 779.
#> 2 Afghanistan Asia 1957 30.3 9240934 821.
#> 3 Afghanistan Asia 1962 32.0 10267083 853.
#> 4 Afghanistan Asia 1967 34.0 11537966 836.
#> 5 Afghanistan Asia 1972 36.1 13079460 740.
#> 6 Afghanistan Asia 1977 38.4 14880372 786.
#> 7 Afghanistan Asia 1982 39.9 12881816 978.
#> 8 Afghanistan Asia 1987 40.8 13867957 852.
#> 9 Afghanistan Asia 1992 41.7 16317921 649.
#> 10 Afghanistan Asia 1997 41.8 22227415 635.
#> # ℹ 14 more rowsCompare with some base R code to accomplish the same things:
gapminder[gapminder$lifeExp < 29, ] ## repeat `gapminder`, [i, j] indexing is distracting
subset(gapminder, country == "Rwanda") ## almost same as filter; quite nice actuallyUnder no circumstances should you subset your data the way I did at first:
Why is this approach a terrible idea?
- It is not self-documenting. What is so special about rows 241 through 252?
- It is fragile. This line of code will produce different results if someone changes the row order of
gapminder, e.g. sorts the data earlier in the script.
This call explains itself and is fairly robust.
21.3 Meet the new pipe operator
Before we go any further, we should exploit the new pipe operator that the tidyverse imports from the magrittr package by Stefan Bache. This is going to change your data analytical life. You no longer need to enact multi-operation commands by nesting them inside each other, like so many Russian nesting dolls. This new syntax leads to code that is much easier to write and to read.
Here’s what it looks like: %>%. The RStudio keyboard shortcut: Ctrl+Shift+M (Windows), Cmd+Shift+M (Mac).
Let’s demo, then I’ll explain.
gapminder %>% head()
#> # A tibble: 6 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Afghanistan Asia 1952 28.8 8425333 779.
#> 2 Afghanistan Asia 1957 30.3 9240934 821.
#> 3 Afghanistan Asia 1962 32.0 10267083 853.
#> 4 Afghanistan Asia 1967 34.0 11537966 836.
#> 5 Afghanistan Asia 1972 36.1 13079460 740.
#> 6 Afghanistan Asia 1977 38.4 14880372 786.This code is equivalent to head(gapminder). The pipe operator takes the thing on the left-hand-side and pipes it into the function call on the right-hand-side – literally, drops it in as the first argument.
Never fear, you can still specify other arguments to this function! To see the first 3 rows of gapminder, we could say head(gapminder, 3) or this:
gapminder %>% head(3)
#> # A tibble: 3 × 6
#> country continent year lifeExp pop gdpPercap
#> <fct> <fct> <int> <dbl> <int> <dbl>
#> 1 Afghanistan Asia 1952 28.8 8425333 779.
#> 2 Afghanistan Asia 1957 30.3 9240934 821.
#> 3 Afghanistan Asia 1962 32.0 10267083 853.I’ve advised you to think “gets” whenever you see the assignment operator, <-. Similarly, you should think “then” whenever you see the pipe operator, %>%.
You are probably not impressed yet, but the magic will soon happen.
21.4 Use select() to subset the data on variables or columns
Back to dplyr….
Use select() to subset the data on variables or columns. Here’s a conventional call:
select(gapminder, year, lifeExp)
#> # A tibble: 1,704 × 2
#> year lifeExp
#> <int> <dbl>
#> 1 1952 28.8
#> 2 1957 30.3
#> 3 1962 32.0
#> 4 1967 34.0
#> 5 1972 36.1
#> 6 1977 38.4
#> 7 1982 39.9
#> 8 1987 40.8
#> 9 1992 41.7
#> 10 1997 41.8
#> # ℹ 1,694 more rowsAnd here’s the same operation, but written with the pipe operator and piped through head():
gapminder %>%
select(year, lifeExp) %>%
head(4)
#> # A tibble: 4 × 2
#> year lifeExp
#> <int> <dbl>
#> 1 1952 28.8
#> 2 1957 30.3
#> 3 1962 32.0
#> 4 1967 34.0Think: “Take gapminder, then select the variables year and lifeExp, then show the first 4 rows.”
21.5 Revel in the convenience
Here’s the data for Cambodia, but only certain variables:
gapminder %>%
filter(country == "Cambodia") %>%
select(year, lifeExp)
#> # A tibble: 12 × 2
#> year lifeExp
#> <int> <dbl>
#> 1 1952 39.4
#> 2 1957 41.4
#> 3 1962 43.4
#> 4 1967 45.4
#> 5 1972 40.3
#> 6 1977 31.2
#> 7 1982 51.0
#> 8 1987 53.9
#> 9 1992 55.8
#> 10 1997 56.5
#> 11 2002 56.8
#> 12 2007 59.7and what a typical base R call would look like:
gapminder[gapminder$country == "Cambodia", c("year", "lifeExp")]
#> # A tibble: 12 × 2
#> year lifeExp
#> <int> <dbl>
#> 1 1952 39.4
#> 2 1957 41.4
#> 3 1962 43.4
#> 4 1967 45.4
#> 5 1972 40.3
#> 6 1977 31.2
#> 7 1982 51.0
#> 8 1987 53.9
#> 9 1992 55.8
#> 10 1997 56.5
#> 11 2002 56.8
#> 12 2007 59.721.6 Pure, predictable, pipeable
We’ve barely scratched the surface of dplyr but I want to point out key principles you may start to appreciate. If you’re new to R or “programming with data”, feel free skip this section and move on.
dplyr’s verbs, such as filter() and select(), are what’s called pure functions. To quote from Wickham’s Advanced R Programming book (2015):
The functions that are the easiest to understand and reason about are pure functions: functions that always map the same input to the same output and have no other impact on the workspace. In other words, pure functions have no side effects: they don’t affect the state of the world in any way apart from the value they return.
In fact, these verbs are a special case of pure functions: they take the same flavor of object as input and output. Namely, a data frame or one of the other data receptacles dplyr supports.
And finally, the data is always the very first argument of the verb functions.
These design choices are deliberate. When combined with the new pipe operator, the result is a highly effective, low friction domain-specific language for data analysis.
Furthermore, cheatsheets are really great resources to learn functions. Click the link to download it!
Go to the next section, for more dplyr!