Install a R package if not already installed, then load it

loadOrInstall.R

loadOrInstall <- function(pkgname, CRANmirror.index=1) {
    chooseCRANmirror(ind=CRANmirror.index)
    if (! pkgname %in% installed.packages()[,1]) {
        write("installing", stderr())
        install.packages(pkgname, dep=T)
    } else { write("already installed", stderr()) }
    library(pkgname, character.only = TRUE)
    write("loaded",stderr())
}