Saving and retrieving temp files in R packages - r

Question

How should I save a file in my R package?
In other words, the user of my package will download a remote text file, and I'd like that file to be available to the user the next time they load the package.
The function I've created currently saves the file using tempfile(). As it's written, the download will happen every time the function is called, and a new temp file will be created. I think this presents a problem - if the user runs the function many times, the /var folder will become bloated with temp files. To fix this, I'd like to check if the file already exists and use the existing version if it does instead of downloading it. Unfortunately, the path returned by tempfile() is lost when the function terminates, so the path is not available to check if the file exists the next time the function is called.
Can I save the file in my packge's /data directory. How do I find this directory from the user's current working directory?
Saving in a tempfile is not necessary. I'm more interested to learn how to persist data in my package.
Thank you.

Answer 1

You can use tempdir() and then a fixed file name:
f <- file.path(tempdir(), "mypackage_thefile.csv")
if (file.exists(f)) {
d <- read.csv(f)
} else {
# download it and save to f
}
These files are removed after each session (when the tempdir is deleted)
You can also provide the data with your package if it is not too large (read it with R and then save it as a compressed 'rds' file with saveRDS). But you can normally not have the user download it to the package directory (often write-protected).
To add data to a package, either put it in the data folder (use save to create a .RData file, for retrieval with data() ), or in the inst/extdata folder, in any appropriate format, for retrieval with
f <- system.file("extdata/filename", package="pkgname")
See the writing R extensions manual