[R] function arguments: name of an object vs. call producing the object?

[Duncan Murdoch]

On 16/03/2010 8:26 AM, Michael Friendly wrote:
> In a function, say foo.glm for glm objects I want to use the name of the 
> object as a label for some output,
> but *only* if a glm object was passed as an argument, not a call to 
> glm() producing that object.
> How can I distinguish these two cases?

Look at typeof(substitute(object)).  For example,

 > f <- function(object) typeof(substitute(object))
 > f(aname)
[1] "symbol"
 > f(aname+1)
[1] "language"

That will tell you if an object was passed by name, or if you got an 
expression to produce the object.

One problem with this approach is that it only looks at the direct call 
to f, for example

 > g <- function(x) f(x)
 > g(aname + 1)
[1] "symbol"

because substitute looks at the f(x) call, not the g(aname + 1) call.

Duncan Murdoch


> 
> For example, I can use the following to get the name of the argument:
> 
> foo.glm <- function(object) {
>     oname <- as.character(sys.call())[2]
>     oname
> }
> 
>  > indep <- glm(Freq ~ mental + ses, family = poisson, data = Mental)
>  > foo.glm(indep)
> [1] "indep"
> 
> But in foo.glm() I want to avoid using this as oname:
>  > foo.glm(glm(formula = Freq ~ mental + ses, family = poisson, data = 
> Mental))
> [1] "glm(formula = Freq ~ mental + ses, family = poisson, data = Mental)"
> 
> Here is Mental, if it matters.
> 
>  dput(Mental)
> structure(list(ses = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L,
> 2L, 3L, 3L, 3L, 3L, 4L, 4L, 4L, 4L, 5L, 5L, 5L, 5L, 6L, 6L, 6L,
> 6L), .Label = c("1", "2", "3", "4", "5", "6"), class = c("ordered",
> "factor")), mental = structure(c(1L, 2L, 3L, 4L, 1L, 2L, 3L,
> 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L,
> 4L), .Label = c("Well", "Mild", "Moderate", "Impaired"), class = 
> c("ordered",
> "factor")), Freq = c(64L, 94L, 58L, 46L, 57L, 94L, 54L, 40L,
> 57L, 105L, 65L, 60L, 72L, 141L, 77L, 94L, 36L, 97L, 54L, 78L,
> 21L, 71L, 54L, 71L)), .Names = c("ses", "mental", "Freq"), row.names = 
> c(NA,
> -24L), class = "data.frame")
>