## use 'devtools' and 'knitr' from the user library
packrat::set_opts(external.packages = c("devtools", "knitr"))
## set local repository
packrat::set_opts(local.repos = c("~/projects/R"))
## get the set of 'external packages'
packrat::opts$external.packages()
## set the external packages
packrat::opts$external.packages(c("devtools", "knitr"))Run the code above in your browser using DataLab