Loading Packages

Once you’ve installed a new package, you’re nearly ready to start using the functionality provided by that package, but there’s still one final step: you must load the package into your active R session.

The following code produces an error because we’re trying to using the ggplot(), aes(), and geom_boxplot() functions that are provided by the ggplot2 package, but we haven’t yet loaded that package.

ggplot(iris, aes(Species, Petal.Length)) + geom_boxplot()

Error in ggplot(iris, aes(Species, Petal.Length)): could not find function "ggplot"

An R package is really just a small piece of software. Like any other software, you only need to install a given R package once (just like you only need to install RStudio once), but you’ll need to load the package every time you want to use it in a new R session (like you need to open RStudio every time you want to use it).

We load R packages with the library() function.

library(dplyr)

The library() function can only load one package at a time, so you need to call library() multiple times to load multiple packages.

library(dplyr)
library(ggplot2)

Running the following code should now produce a simple plot, because we’re loaded the ggplot2 package before trying to use the functions it provides.