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In R, the traditional way to load packages can sometimes lead to situations where several lines of code need to be written just to load packages. These lines can cause errors if the packages are not installed, and can also be hard to maintain, particularly during deployment.

Fortunately, there is a way to create a function in R that will automatically load our packages for us. In this post, I will walk you through conceiving and creating such a function.

In order to write a function that checks if a package is installed, loads it if it is, and installs it if it isn’t, we first need a way to check if a package is installed. Thankfully, this function does the job:
is_installed <- function(mypkg) is.element(mypkg, installed.packages()[,1])

The above function comes from a post on the R mailing list, although I do not know if it is the original source. This function will test if a given function name is in the list of installed packages. We can access the list directly by using installed.packages()[,1] , and we can use the function by trying is_installed("foreach") .

Now that we know how to test if a package is installed or not, we can move on to writing the function. At this moment, we have two hurdles. The first is how to load a package from a character vector of names. The second is how to install a package programmatically. Typically, loading a package will look like this:
library(MASS)

Fortunately for us, there is a character.only option in the library function that allows us to specify the package name as a string.
library("MASS",character.only=TRUE)

The above gives us the functionality that we need to pass the name of a package to a function as a string and have it loaded. Now, we need to find how to install packages, which can be done with the install.packages function:
install.packages("MASS",repos="http://lib.stat.cmu.edu/R/CRAN")

Explicitly setting the repo will avoid having R ask us for it when the function is executed for the first time. I chose statlib for convenience, but feel free to use any repo you like.

Now, we have a way to test if a package is installed, a way to install the package, and a way to load the package. All we need to do is wrap it up with an if statement.
if(!is_installed(package_name))
{
install.packages(package_name,repos="http://lib.stat.cmu.edu/R/CRAN")
}
library(package_name,character.only=TRUE,quietly=TRUE,verbose=FALSE)

The above if statement will test to see if a package is installed, and then install it if it isn't. It will then load the package.

This gets us most of the way to what we want, but if we want to pass a character vector to the function and have it load multiple packages at once, we need to wrap everything in a for loop.
for(package_name in package_names)
{
if(!is_installed(package_name))
{
install.packages(package_name,repos="http://lib.stat.cmu.edu/R/CRAN")
}
library(package_name,character.only=TRUE,quietly=TRUE,verbose=FALSE)
}

This for loop will perform the operations that we need on the character vector package_names. Now, we can just wrap everything into a neat function that is passed a character vector called package_names.
load_or_install<-function(package_names)
{
for(package_name in package_names)
{
if(!is_installed(package_name))
{
install.packages(package_name,repos="http://lib.stat.cmu.edu/R/CRAN")
}
library(package_name,character.only=TRUE,quietly=TRUE,verbose=FALSE)
}
}

We can call the function with the following syntax (substitute the function names with your own):
load_or_install(c("foreach","MASS","doParallel"))

And with that, we are done, and now have a function that can load or install packages as needed from a character vector. This function will terminate with an error if the install.packages function or the library function cannot find the specified package name, but you can fix that by using a try statement, which is an exercise that I will leave to you for the moment.