R using temporary options settings inside a function

Question

I have a function which requires that I set an option, namely stringsAsFactors=FALSE. My all-too common approach was to store the existing value at the onset of the function

op <- option("stringsAsFactors")

and before calling return(x) make sure to call

options(op)

While that works, it's a lengthy and complex function that can return from several places, so I need to either make sure every return(x) is preceded by options(op), or create a tiny function (itself inside my function) that does only that:

returnfunc <- function(x) { options(op) return(x) }

So my question is: what would be a better way to do that? Specefically, one that would prevent that the function exits with an error and leave the parameter (unduly) modified?

Answer 1

There's a built in on.exit function that does exactly this. For example

f<-function() {
    op <- options(stringsAsFactors=FALSE)
    on.exit(options(op))

    read.table(...)
}

You can change what ever options you want in the options() call and it returns all the values before the changes as list so it's easy to reset. You can pass whatever expression you want to on.exit, here we just set the options back to what they were. This will run when ever the function exits. See ?on.exit for more info.

Answer 2

You could also add a default argument that allows the user to change it if they wish, and then pass the argument into the function that uses it.

For example,

f <- function(x, stringsAsFactors = FALSE) {

        read.csv(..., stringsAsFactors = stringsAsFactors)

}

This way, you can change it to TRUE if you want, and no options() are changed. You really don't want to be messing around with options() inside of functions.