[Rd] Attributes of 1st argument in ...

From: Daniel Murphy <chiefmurphy_at_gmail.com>
Date: Fri, 02 Jul 2010 14:26:04 -0700


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?):


   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?

Dan Murphy

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