Sum of returned list: Error: invalid 'type' (list) of argument

Question

For my script I'm returning a bunch of variables in R, and after splitting up my script in more functions I need to return some lists, along with other data.

I know I can return multiple values by c(value1, value2). But how can I do this when one of the items returned is actually a list? I'm returning listOne, however it looks like the data type gets changed when returning. How can I get return the list without changing its type?

Here's an example:

  B <- function(){
   listOne <- c(1,2,3,4,5,6)
   testString <- "Test"
   return(list(listOne, testString))
 }
  returnlist <- B()

Assigning the variables according to the returned list:

  copy.listOne <- returnlist# [1]
  copy.testString <- returnlist[2]

Expected output:

  listOne <- c(1,2,3,4,5,6)
  print(sum(listOne))
# [1] 21

Actual output:

  print(sum(copy.listOne))
Error in print(sum(copy.listOne)) : 
  error in evaluating the argument 'x' in selecting a method for function 'print': Error in sum(copy.listOne) : invalid 'type' (list) of argument

Solution

When you are working with lists you need to use [[ in order to subset them. In your case when you create the function as follows

  B <- function(){
   listOne <- c(1,2,3,4,5,6)
   testString <- "Test"
   return(list(listOne, testString))
 }
  returnlist <- B()

you then need to use [[ in order to access the elements of that list.

As you can see below all work now properly:

> returnlist[[1]]
[1] 1 2 3 4 5 6
> returnlist[[2]]
[1] "Test"
> sum(returnlist[[1]])
[1] 21