How to pass a list to a function in R?

Question

I have been having difficulty finding information about how to pass a list to a function in R.

I have used this approach before, e.g.

plot(list(x=1,y=1))

but the following example gives me an error:

foo <- function(a, b) c <- a + b
foo(list(a=1,b=1))

Error in foo(list(a = 1, b = 1)) : 
   argument "b" is missing, with no default

Furthermore, ?function doesn't work and help('function') does not provide information on passing a list to a function.

update

To clarify, I understand how I can use a list as a single argument, but I was confused because I was under the impression that a property of functions was that multiple arguments could be passed as a list. It appears that this impression was incorrect. Rather, many functions are written specifically to handle lists, as described in the comments and answers below.

You are passing a single argument to your function which expects 2. The correct call would be: foo(a=list(1), b=list(2)). If you wanted to pass to your function a single list then you have to declare it as function(a) and then call it the same way you did. Your function logic is also off since you can't add non-numeric arguments in such a way and you will have to index your lists within your function body to achieve what you want (guessing here) i.e. a[[1]] + b[[1]]. — diliop
– diliop, Commented Jun 27, 2011 at 17:59
I haven't looked into it, but I would guess that plot has a method that handles lists. To do something similar with foo, you would need to make it generic and write appropriate methods. — Joshua Ulrich
– Joshua Ulrich, Commented Jun 27, 2011 at 18:00
@Joshua, thank you for the clarification. I was under the impression that a list could be passed without special handling. — David LeBauer
– David LeBauer, Commented Jun 27, 2011 at 18:06
plot(list(x=1,y=1)) calls plot.default, which uses xy.coords. The source of xy.coords has an if/else branch to handle multiple types of x arguments (ts, matrix, data.frame, formula, complex, list). So it's not a generic using methods. The analogous solution would be to define foo(a, b=NULL) and have an if/else branch when b is null to handle multiple classes of a. — Joshua Ulrich
– Joshua Ulrich, Commented Jun 27, 2011 at 18:14

Sameer · Accepted Answer · 2011-06-27 17:56:49Z

43

Use do.call

foo <- function(a, b)  a + b

do.call(foo, list(a=1,b=1))

Alternatively you can do

foo <- function(l) l$a + l$b

foo(list(a=1,b=1))

answered Jun 27, 2011 at 17:56

Sameer

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Maiasaura · Accepted Answer · 2011-06-27 18:12:58Z

7

Your function has two arguments but you are only passing one, hence the error.

You can modify your code like so:

foo <- function(a) c <- a[[1]] + a[[2]]
foo(list(a=1,b=1))

edited Jun 27, 2011 at 18:12

answered Jun 27, 2011 at 18:00

Maiasaura

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