R help - Jan 2011 - Extracting a (*specific*) variable name from a list of function arguments (not "...")

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...




Thanks,
Tal



----------------Contact
Details:-------------------------------------------------------
Contact me: Tal.Galili@gmail.com |  972-52-7275845
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On 30.01.2011 17:38, 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())
Replace by

   print(substitute(b))

Uwe Ligges

> }
> 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...
>
>
>
>
> Thanks,
> Tal
>
>
>
> ----------------Contact
> Details:-------------------------------------------------------
> Contact me: Tal.Galili at gmail.com |  972-52-7275845
> Read me: www.talgalili.com (Hebrew) | www.biostatistics.co.il (Hebrew) |
> www.r-statistics.com (English)
>
----------------------------------------------------------------------------------------------
>
> 	[[alternative HTML version deleted]]
>
> ______________________________________________
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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