r - Update data frame via function doesn't work

Question

I ran into a little problem using R…

In the following data frame

test <- data.frame(v1=c(rep(1,3),rep(2,3)),v2=0) 

I want to change values for v2 in the rows where v1 is 1.

test[test$v1==1,"v2"] <- 10

works just fine.

test
  v1 v2
1  1 10
2  1 10
3  1 10
4  2  0
5  2  0
6  2  0

However, I need to do that in a function.

test <- data.frame(v1=c(rep(1,3),rep(2,3)),v2=0)

test.fun <- function (x) {
    test[test$v1==x,"v2"] <- 10
    print(test)
}

Calling the function seems to work.

test.fun(1)
  v1 v2
1  1 10
2  1 10
3  1 10
4  2  0
5  2  0
6  2  0

However, when I now look at test:

test
  v1 v2
1  1  0
2  1  0
3  1  0
4  2  0
5  2  0
6  2  0

it didn’t work. Is there a command that tells R to really update the data frame in the function? Thanks a lot for any help!

Answer 1

test in your function is a copy of the object from your global environment (I'm assuming that's where it is defined). Assignment happens in the current environment unless specified otherwise, so any changes that happen inside the function apply only to the copy inside the function, not the object in your global environment.

And it's good form to pass all necessary objects as arguments to the function.

Personally, I would return(test) at the end of your function and make the assignment outside of the function, but I'm not sure if you can do this in your actual situation.

test.fun <- function (x, test) {
    test[test$v1==x,"v2"] <- 10
    return(test)
}
test <- data.frame(v1=c(rep(1,3),rep(2,3)),v2=0)
(test <- test.fun(1, test))
#  v1 v2
#1  1 10
#2  1 10
#3  1 10
#4  2  0
#5  2  0
#6  2  0

If it is absolutely necessary to modify an object outside your function directly, so you need to tell R that you want to assign the local copy of test to the test in the .GlobalEnv.

test.fun <- function (x, test) {
    test[test$v1==x,"v2"] <- 10
    assign('test',test,envir=.GlobalEnv)
    #test <<- test  # This also works, but the above is more explicit.
}
(test.fun(1, test))
#  v1 v2
#1  1 10
#2  1 10
#3  1 10
#4  2  0
#5  2  0
#6  2  0

Using assign or <<- in this fashion is fairly uncommon, though, and many experienced R programmers will recommend against it.