function returning more than one value

Question

still trying to get into the R logic... what is the "best" way to unpack the results from a function returning multiple values?

I can't do this apparently:

R> functionReturningTwoValues <- function() { return(c(1, 2)) }
R> functionReturningTwoValues()
[1] 1 2
R> a, b <- functionReturningTwoValues()
Error: unexpected ',' in "a,"
R> c(a, b) <- functionReturningTwoValues()
Error in c(a, b) <- functionReturningTwoValues() : object 'a' not found

must I really do the following?

R> r <- functionReturningTwoValues()
R> a <- r[1]; b <- r[2]

or would the R programmer write something more like this:

R> functionReturningTwoValues <- function() {return(list(first=1, second=2))}
R> r <- functionReturningTwoValues()
R> r$first
[1] 1
R> r$second
[1] 2

--- edited to answer Shane's questions ---

I don't really need giving names to the result value parts. I am applying one aggregate function to the first component and an other to the second component (min and max. if it was the same function for both components I would not need splitting them).

Answer 1

Yes to your second and third questions -- that's what you need to do as you cannot have multiple 'lvalues' on the left of an assignment.

Answer 2

There's no right answer to this question. I really depends on what you're doing with the data. In the simple example above, I would strongly suggest:

1. Keep things as simple as possible.
2. Wherever possible, it's a best practice to keep your functions vectorized. That provides the greatest amount of flexibility and speed in the long run.

Is it important that the values 1 and 2 above have names? In other words, why is it important in this example that 1 and 2 be named a and b, rather than just r[1] and r[2]? One important thing to understand in this context is that a and b are also both vectors of length 1. So you're not really changing anything in the process of making that assignment, other than having 2 new vectors that don't need subscripts to be referenced:

> r <- c(1,2)
> a <- r[1]
> b <- r[2]
> class(r)
[1] "numeric"
> class(a)
[1] "numeric"
> a
[1] 1
> a[1]
[1] 1

You can also assign the names to the original vector if you would rather reference the letter than the index:

> names(r) <- c("a","b")
> names(r)
[1] "a" "b"
> r["a"]
a 
1

[Edit] Given that you will be applying min and max to each vector separately, I would suggest either using a matrix (if a and b will be the same length and the same data type) or data frame (if a and b will be the same length but can be different data types) or else use a list like in your last example (if they can be of differing lengths and data types).

> r <- data.frame(a=1:4, b=5:8)
> r
  a b
1 1 5
2 2 6
3 3 7
4 4 8
> min(r$a)
[1] 1
> max(r$b)
[1] 8

Answer 3

Usually I wrap the output into a list, which is very flexible (you can have any combination of numbers, strings, vectors, matrices, arrays, lists, objects int he output)

so like:

func2<-function(input) {
   a<-input+1
   b<-input+2
   output<-list(a,b)
   return(output)
}

output<-func2(5)

for (i in output) {
   print(i)
}

[1] 6
[1] 7

Answer 4

I somehow stumbled on this clever hack on the internet somehow ... I'm not sure if it's nasty or beautiful, but it lets you create a "magical" operator that almost allows you to unpack multiple return values into their own variable. The := function is defined here.

With that in hand, you can do what you're after:

functionReturningTwoValues <- function() { return(list(1, matrix(0, 2, 2))) }
c(a, b) := functionReturningTwoValues()
a
#[1] 1
b
#     [,1] [,2]
# [1,]    0    0
# [2,]    0    0

I don't know how I fell about that. Perhaps you might find it helpful in your interactive workspace. Using it to build (re-)usable libraries (for mass consumption) might not be the best idea, but I guess that's up to you.

... you know what they say about responsibility and power ...