R-Devel:

I am trying to get an attribute of the first argument in a call to a
function whose formal arguments consist of dots only and do something, e.g.,
call 'cbind', based on the attribute
f<- function(...) {get first attribute; maybe or maybe not call 'cbind'}

I thought of (ignoring "deparse.level" for the moment)

f<-function(...) {x <- attr(list(...)[[1L]], "foo"); if (x=="bar")
cbind(...) else x}

but I feared my solution might do some extra copying, with a performance
penalty if the dotted objects in the actual call to "f' are very large.

I thought the following alternative might avoid a potential performance hit
by evaluating the attribute in the parent.frame (and therefore avoid extra
copying?):

f<-function(...)
{
   L<-match.call(expand.dots=FALSE)[[2L]]
   x <- eval(substitute(attr(x,"foo"), list(x=L[[1L]])))
   if (x=="bar") cbind(...) else x
}

system.time tests showed this second form to be only marginally faster.

Is my fear about extra copying unwarranted? If not, is there a better way to
get the "foo" attribute of the first argument other than my two
alternatives?

Thanks,
Dan Murphy

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