How can I tell if a certain package was already installed?

Question

When I install the yaml package, an annoying error message pops up in RStudio if it had been previously installed. How can I tell if the package was already installed so I can d

Answer 1

This will load yaml, installing it first if its not already installed:

if (!require(yaml)) {
  install.packages("yaml")
  library(yaml)
}

or if you want to parameterize it:

pkg <- "yaml"
if (!require(pkg, character.only = TRUE)) {
  install.packages(pkg)
  if (!require(pkg, character.only = TRUE)) stop("load failure: ", pkg)
}

UPDATE. Parametrization.