usePackages.R

# This script will issue the command require() for all the packages we want to use and, 
# if the require() commands return FALSE, then install the missing packages.
# We can run this script at the beginning of every R session to make sure 
# that we have all the packages we need. 
# Call loadPkg() for a single package, and loadPkgs() for a list of packages.

loadPkg <- function(pkgname){
  # Test to see if package pkgname is installed. 
  # character.only=TRUE means pkgname is a character string with the name of the package we want to use. 
  if(require(pkgname,character.only = TRUE)){
    # paste0() concatenates strings without any separator
    print(paste0("'",pkgname,"' is loaded correctly"))
  } else {
    # The require() function returned FALSE so we will try to install the package from the CRAN site
    print(paste0("Trying to install '",pkgname,"'"))
    install.packages(pkgname,character.only = TRUE,repos="http://cran.us.r-project.org")
    if(require(pkgname,character.only = TRUE)){
      print(paste0("'",pkgname,"' is installed and loaded."))
    } else {
      print(paste0("Could not install '",pkgname,"'"))
    }
  }
}

# If we provide a vector of package names, we can load them all as follows:
loadPkgs <- function(pkgnames){
  for (pkgname in pkgnames)loadPkg(pkgname)
}