The Art of R Programming

You can check which packages are currently loaded by typing this:

> .path.package()

B.2 Loading a Package from Your Hard Drive.....................................

If you need a package that is in your R installation but not loaded into mem-

ory yet, you can load it using thelibrary()function. For instance, suppose

you wish to generate multivariate normal random vectors. The function

mvrnorm()in the packageMASSdoes this. So, load the package as follows:

> library(MASS)

Themvrnorm()function will now be ready to use. And so will its docu-

mentation (before you loadedMASS, enteringhelp(mvrnorm)would have gener-

ated an error message).

B.3 Downloading a Package from the Web.......................................

The package you want may not be in your R installation. One of the big

advantages of open source software is that people love to share. People all

over the world have written their own special-purpose R packages, placing

them in the CRAN repository and elsewhere.

NOTE User contributions to CRAN go through a vetting process and are generally of high

quality. They are, however, not tested as throughly as R itself.

B.3.1 Installing Packages Automatically.................................

One way to install a package is to use theinstall_packages()function. For

example, suppose you wish to use themvtnormpackage, which computes

multivariate normal cumulative distribution functions and other quantities.

First, choose a directory in which you wish to install the package (and maybe

others in the future), say/a/b/c. Then at the R prompt, type this:

> install.packages("mvtnorm","/a/b/c/")

This will cause R to automatically go to CRAN, download the package,

compile it, and load it into a new directory:/a/b/c/mvtnorm.

You do need to tell R where to find that package once it’s installed,

which you can do via the.libPaths()function:

> .libPaths("/a/b/c/")

This will add that new directory to the ones R was already using. If you

use that directory often enough, you may wish to add that call to.libPaths()

in your.Rprofilestartup file in your home directory.

356 Appendix B