R comes with a number of default packages, a collection of previously programmed functions for specific tasks, and with datasets. This is usually known as a library, but the R community refers to it as a package. There are two types of R packages:
Default packages that come with the R executable
Add-on packages that do not come during installation; we need to install them manually on downloading
When we open the R console, it automatically loads its default packages with the associated functions, and we do not need to load those packages manually. A list of installed packages can be obtained by typing library() in the R console. However, some of the packages need to load to execute functions. To load a specific package, the corresponding R command is library(package), where package is the name of any library such as plyr, provided that the package has already been installed.
In some situations, we may require a special type of data processing and analysis. If the corresponding packages are not available in the default list, we need to install them. For example, the plyr package is not in the default list, so we need to install it separately.
There are two different ways to install a package:
To download a package from CRAN and install it, follow these steps:
Go to http://www.r-project.org/.
Select any one of the regional servers from the list; for example, select the server from Austria at http://cran.at.r-project.org/.
Click on Contributed extension packages under the Source Code for all Platforms section.
Select Table of available packages, sorted by date of publication or Table of available packages, sorted by name and then download the desired package from the list.
While downloading, users need to choose the file that matches with the platform; for example, a Windows user will download the binary zip file.
Once the download is completed, open R.
Go to the Packages menu and select Install packages from local zip files.
Tip
One potential problem with manual downloads is that, sometimes, particular packages are dependent on other packages that are not included in the manual process of installation. To avoid this problem, we can install the desired package(s) from the R shell, as installing package(s) from the R shell resolves dependencies.
To install a package from within the R console, we can use the install.packages() command; this command will prompt us to select the appropriate server CRAN. Note that to install packages using this approach, the computer must have active Internet connection.
For example, to install the plyr package, we can use the following command:
install.packages("plyr")The previous command will prompt us to select a regional server and, after selecting the server from the available list, the package will be installed on the local computer.