Define global variable using function argument in R

Question

I'm trying to write a function in R that drops columns from a data frame and returns the new data with a name specified as an argument of the function:

drop <- function(my.data,col,new.data) {
new.data <<- my.data[,-col] 
return(new.data)
}

So in the above example, I want a new data frame to exist after the function is called that is named whatever the user inputs as the third argument.

When I call the function the correct data frame is returned, but then if I then try to use the new data frame in the global environment I get object not found. I thought by using the <<- operator I was defining new.data globally.

Can someone help me understand what's going on and if there is a way to accomplish this?

I found this and this that seemed related, but neither quite answered my question.

Solution

Use the assign() function.

  assign("new.data", my.data[,-col], envir = .GlobalEnv) 

The first argument should be a string. In this case, the resultant global variable will be named "new.data". If new.data is the name itself, drop the quotes from the function call.

<<- does not always assign to the global environment.

In general, however, it is better to return things from a function than set global variables from inside a function. The latter is a lot harder to debug.

Comments

[cerpintaxt]

Thanks this is helpful. I'll live with slightly more typing and just use return.

[Christopher Louden]

@JakeBurkhead: I expanded to explain when to quote and when not to.

[algae]

Is there a systematic way to do this for a pile of variables?