Define a function that installs an R package if it is not available

ensurePkg.R

ensurePkg <- function(x, lib.loc=NULL) {
    if (is.null(lib.loc)) {
        home <- Sys.getenv("HOME")
        if (nchar(home)) {
            lib.loc <- file.path(home, ".OA.Rlibs")
        } else {
            lib.loc <- "./.OA.Rlibs"
        }
        if (!file.exists(lib.loc))
            dir.create(lib.loc)
    }
    if (!require(x, character.only=TRUE, lib.loc=lib.loc)) {
        install.packages(x, lib=lib.loc,
                         repo="http://cran.stat.auckland.ac.nz")
        require(x, character.only=TRUE, lib.loc=lib.loc)
    }
    lib.loc
}

ensurePkgGitHub <- function(x, ..., lib.loc=NULL) {
    ensurePkg("devtools", lib.loc)
    if (is.null(lib.loc)) {
        home <- Sys.getenv("HOME")
        if (nchar(home)) {
            lib.loc <- file.path(home, ".OA.Rlibs")
        } else {
            lib.loc <- "./.OA.Rlibs"
        }
        if (!file.exists(lib.loc))
            dir.create(lib.loc)
    }
    if (!require(x, character.only=TRUE, lib.loc=lib.loc)) {
        .libPaths(lib.loc)
        local({
            r <- getOption("repos")
            r["CRAN"] <- "http://cran.stat.auckland.ac.nz"
            options(repos = r)
        })
        install_github(x, ...)
        require(x, character.only=TRUE, lib.loc=lib.loc)
    }
    lib.loc
}