Visualisation is an important tool for insight generation, but it is rare that you get the data in exactly the right form you need for visualisation. Often you'll need to create some new variables or summaries, or maybe you just want to rename the variables or reorder the observations in order to make the data a little easier to work with. You'll learn how to do all that (and more!) in this chapter which will teach you how to transform your data using the dplyr package.
To explore the basic data manipulation verbs of dplyr, we'll use the `flights` data frame from the nycflights13 package. This data frame contains all `r format(nrow(nycflights13::flights), big.mark = ",")` flights that departed from New York City in 2013. The data comes from the US [Bureau of Transportation Statistics](http://www.transtats.bts.gov/DatabaseInfo.asp?DB_ID=120&Link=0), and is documented in `?nycflights13`.
You might notice that this data frame prints little differently to other data frames you might have used: it only shows the first few rows and all the columns that fit on one screen (To see the whole dataset, you can run `View(flights)` which will open the dataset in the RStudio viewer). It prints differently because it's a __tibble__. Tibbles are data frames, but slightly tweaked to work better in the tidyverse. For now, you don't need to worry about the differences; we'll come back to tibbles in more detail in [wrangle].
You might also have noticed the row of three letter abbreviations under the column names. These describe the type of each variable:
These can all be used in conjunction with `group_by()` which changes the scope of each function from operating on the entire dataset to operating on it group-by-group. These six functions provide the verbs for a language of data manipulation.
`filter()` allows you to subset observations. The first argument is the name of the data frame. The second and subsequent arguments are the expressions that filter the data frame. For example, we can select all flights on January 1st with:
When you run this line of code, dplyr executes the filtering operation and returns a new data frame. dplyr functions never modify their inputs, so if you want to save the results, you'll need to use the assignment operator `<-`:
R provides the standard suite of numeric comparison operators: `>`, `>=`, `<`, `<=`, `!=` (not equal), and `==` (equal).
When you're starting out with R, the easiest mistake to make is to use `=` instead of `==` when testing for equality. When this happens you'll get a somewhat uninformative error:
Note the order isn't like English. The following expression doesn't find on months that equal 11 or 12. Instead it finds all months that equal `11 | 12`, an expression that evaluates to `TRUE`. In a numeric context (like here), `TRUE` becomes one, so this finds all flights in January, not November or December (It is the equivalent of `filter(flights, month == 1)`).
Sometimes you can simplify complicated subsetting by remembering De Morgan's law: `!(x & y)` is the same as `!x | !y`, and `!(x | y)` is the same as `!x & !y`. For example, if you wanted to find flights that weren't delayed (on arrival or departure) by more than two hours, you could use either of the following two filters:
Note that R has both `&` and `|` and `&&` and `||`. `&` and `|` are vectorised: you give them two vectors of logical values and they return a vector of logical values. `&&` and `||` are scalar operators: you give them individual `TRUE`s or `FALSE`s. They're used in `if` statements when programming. You'll learn about that later on in Chapter ?.
Sometimes you want to find all rows after the first `TRUE`, or all rows until the first `FALSE`. The cumulative functions `cumany()` and `cumall()` allow you to find these values:
```{r}
df <- data_frame(
x = c(FALSE, TRUE, FALSE),
y = c(TRUE, FALSE, TRUE)
)
filter(df, cumany(x)) # all rows after first TRUE
filter(df, cumall(y)) # all rows until first FALSE
Whenever you start using multipart expressions in your `filter()`, it's typically a good idea to make the expressions explicit variables with `mutate()` so that you can more easily check your work. You'll learn about `mutate()` in the next section.
One important feature of R that can make comparison tricky is the missing value, `NA`. `NA` represents an unknown value so missing values are "contagious": any operation involving an unknown value will also be unknown.
`filter()` only includes rows where the condition is `TRUE`; it excludes both `FALSE` and `NA` values. If you want to preserve missing values, ask for them explicitly:
`arrange()` works similarly to `filter()` except that instead of filtering or selecting rows, it reorders them. It takes a data frame and a set of column names (or more complicated expressions) to order by. If you provide more than one column name, each additional column will be used to break ties in the values of preceding columns:
It's not uncommon to get datasets with hundreds or even thousands of variables. In this case, the first challenge is often narrowing in on the variables you're actually interested in. `select()` allows you to rapidly zoom in on a useful subset using operations based on the names of the variables:
But because `select()` drops all the variables not explicitly mentioned, it's not that useful. Instead, use `rename()`, which is a variant of `select()` that keeps variables by default:
The `select()` function works similarly to the `select` argument in `base::subset()`. `select()` is its own function in dplyr because the dplyr philosophy is to have small functions that each do one thing well.
Besides selecting sets of existing columns, it's often useful to add new columns that are functions of existing columns. This is the job of `mutate()`.
`mutate()` always adds new columns at the end of your dataset so we'll start by creating a narrower dataset so we can see the new variables. Remember that when you're in RStudio, the easiest way to see all the columns is `View()`
There are many functions for creating new variables that you can use with `mutate()`. The key property is that the function must be vectorised: it needs to return the same number of outputs as inputs. There's no way to list every possible function that you might use, but here's a selection of functions that are frequently useful:
That's not terribly useful unless we pair it with `group_by()`. This changes the unit of analysis from the complete dataset to individual groups. When you use the dplyr verbs on a grouped data frame they'll be automatically applied "by group". For example, if we applied exactly the same code to a data frame grouped by date, we get the average delay per date:
Together `group_by()` and `summarise()` provide one of the tools that you'll use most commonly when working with dplyr: grouped summaries. But before we go any further with this, we need to introduce a powerful new idea: the pipe.
Imagine that we want to explore the relationship between the distance and average delay for each location. Using what you already know about dplyr, you might write code like this:
This code is a little frustrating to write because we have to give each intermediate data frame a name, even though we don't care about it. Naming things well is hard, so this slows us down.
This focuses on the transformations, not what's being transformed, which makes the code easier to read. You can read it as a series of imperative statements: group, then summarise, then filter. As suggested by this reading, a good way to pronounce `%>%` when reading code is "then".
Behind the scenes, `x %>% f(y)` turns into `f(x, y)`, and `x %>% f(y) %>% g(z)` turns into `g(f(x, y), z)` and so on. You can use the pipe to rewrite multiple operations in a way that you can read left-to-right, top-to-bottom. We'll use piping frequently from now on because it considerably improves the readability of code, and we'll come back to it in more detail in Chapter XYZ.
Most of the packages you'll learn through this book have been designed to work with the pipe (tidyr, dplyr, stringr, purrr, ...). The only exception is ggplot2: it was developed considerably before the pipe was discovered. Unfortunately the next iteration of ggplot2, ggvis, which does use the pipe, isn't ready for prime time yet.
We get a lot of missing values! That's because aggregation functions obey the usual rule of missing values: if there's any missing value in the input, the output will be a missing value. `x %>% f(y)` turns into `f(x, y)` you'll learn more about aggregation functions in Section 5.7.4. Fortunately, all aggregation functions have an `na.rm` argument which removes the missing values prior to computation:
Whenever you do any aggregation, it's always a good idea to include either a count (`n()`), or a count of non-missing values (`sum(!is.na(x))`). That way you can check that you're not drawing conclusions based on very small amounts of non-missing data.
Not surprisingly, there is much more variation in the average delay when there are few flights. The shape of this plot is very characteristic: whenever you plot a mean (or many other summaries) vs. number of observations, you'll see that the variation decreases as the sample size increases.
When looking at this sort of plot, it's often useful to filter out the groups with the smallest numbers of observations, so you can see more of the pattern and less of the extreme variation in the smallest groups. This is what the following code does, and also shows you a handy pattern for integrating ggplot2 into dplyr flows. It's a bit painful that you have to switch from `%>%` to `+`, but once you get the hang of it, it's quite convenient.
RStudio tip: useful keyboard shortcut is Cmd + Shift + P. This resends the previously sent chunk from the editor to the console. This is very convenient when you're (e.g.) exploring the value of `n` in the example above. You send the whole block once with Cmd + Enter, then you modify the value of `n` and press Cmd + Shift + P to resend the complete block.
There's another common variation of this type of pattern. Let's look at how the average performance of batters in baseball is related to the number of times they're at bat. Here I use the Lahman package to compute the batting average (number of hits / number of attempts) of every major league baseball player. When I plot the skill of the batter against the number of times batted, you see two patterns:
This also has important implications for ranking. If you naively sort on `desc(ba)`, the people with the best batting averages are clearly lucky, not skilled:
You can find a good explanation of this problem at <http://varianceexplained.org/r/empirical_bayes_baseball/> and <http://www.evanmiller.org/how-not-to-sort-by-average-rating.html>.
Be careful when progressively rolling up summaries: it's OK for sums and counts, but you need to think about weighting for means and variances, and it's not possible to do it exactly for rank-based statistics like the median (i.e. the sum of groupwise sums is the overall sum, but the median of groupwise medians is not the overall median).
A grouped filter is a grouped mutate followed by an ungrouped filter. I generally avoid them except for quick and dirty manipulations: otherwise it's hard to check that you've done the manipulation correctly.
Functions that work most naturally in grouped mutates and filters are known as window functions (vs. summary functions used for summaries). You can learn more about useful window functions in the corresponding vignette: `vignette("window-functions")`.
