On Jan 30, 2011, at 10:38 AM, Tal Galili wrote:
> Hello all,
>
> I would like to extract the variable name passed into a specific argument of
> a function.
> Using match.call() will give me a list of arguments and what was passed into
> them, but I couldn't find how to get a particular argument out of it (at
> least without using some string manipulation).
>
> Here is an example (in it, my objective is to know what is the variable name
> assigned to the argument "b"):
>
> foo <- function(a, b, c)
> {
> print(match.call())
> }
> x = 1 ; y = 1
> foo(x, y) # here the answer should be "y"
> foo(c= x, y) # here the answer should be "" (or NULL/NA)
>
>
> Any suggestion on how to do this?
>
>
> BTW, the purpose of this is to be able to avoid the error produced by, for
> example, this:
>
> yy <- data.frame(y = 1:10, x = 1:10)
> plot(y, data = yy) # error
> plot(y~x, data = yy) # no error...
Tal,
Try this:
x <- 1
y <- 1
foo <- function(a, b, c) {as.list(match.call())}
> foo(x, y)
[[1]]
foo
$a
x
$b
y
> foo(x, y)$b
y
> foo(c = x, y)$b
NULL
If you really only want 'b' and not be able to get the other arguments, you can
use:
foo2 <- function(a, b, c) {as.list(match.call())$b}
> foo2(x, y)
y
> foo2(c = x, y)
NULL
In your plot example above, you are first dispatching plot.default(), which of
course has no 'data' argument. In the second case, you are dispatching
plot.formula() which does have a 'data' argument, as do most, if not all, R
functions that take a formula as an argument.
HTH,
Marc Schwartz
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