R: Meaning of "extdata"?

Solution 1:

This is a convention, not a formally defined term. (However, it's a convention defined by the package authors and coded in the package structure; it's not something you can change unless you mess around with the package structure yourself.) "extdata" is presumably short for "external data".

However, this doesn't mean that you need to use "extdata" when you are structuring your own code; you only need it when finding the files that are included by the package. cron_rscript("~/my_cron_jobs/foo.R") should work fine (provided you actually have something there, and provided that the ~ == home directory shortcut works across OS, which I think it does).

system.file() takes a package argument, but otherwise strings its arguments together into a file path; i.e. system.file(package = "cronR", "extdata", "helloworld.R") means

• look in the system folder that R has set up for the cronR package (in my case that is /usr/local/lib/R/site-library/cronR, but the precise location will vary by OS and configuration)
• within that folder look in the extdata folder
• within that folder look for helloworld.R

So this command will refer in my case to /usr/local/lib/R/site-library/cronR/extdata/helloworld.R.

Since "/" works as a path separator (at least when used from within R) for all current operating systems, you would get the same results from system.file(package="cronR", "extdata/helloworld.R")