R help - Aug 2004 - dots expansion

Hi list,

I'm trying to write a function similar to rbind, except that needs to 
add a factor to each component array before rbinding them together so 
that the rows from different arrays are distinguishable.

The problem that arose is how to loop through arguments in the dots 
"..." list. I need to get a hand on each of them but don't know
how many
of them there are and what their names are.

It'd be useful if I could look at how rbind(...) or c(...) do this but 
they are both Internal functions.

Thanks in anticipation of help!

Regards,
viet

On Tue, 03 Aug 2004 14:37:56 +1000, Viet Nguyen
<vietnguyen at fastmail.fm> wrote:
>Hi list,
>
>I'm trying to write a function similar to rbind, except that needs to 
>add a factor to each component array before rbinding them together so 
>that the rows from different arrays are distinguishable.
>
>The problem that arose is how to loop through arguments in the dots 
>"..." list. I need to get a hand on each of them but don't
know how many
>of them there are and what their names are.
>
>It'd be useful if I could look at how rbind(...) or c(...) do this but 
>they are both Internal functions.
list(...) will create a list, one item per argument.  If the args are
named then the corresponding list item will be named.

Duncan Murdoch

Duncan already gave the answer.  Here's a bit more detail:

rbindGroup <- function(...) {
    theList <- list(...)
    nGroup <- length(theList)
    size <- sapply(theList, nrow)
    result <- rbind(...)
    result <- cbind(result, rep(1:nGroup, each=size)
    result
}

You probably want to add more error checks (like making sure all arrays have
the same number of columns).

HTH,
Andy
> From: Viet Nguyen
> 
> Hi list,
> 
> I'm trying to write a function similar to rbind, except that needs to 
> add a factor to each component array before rbinding them together so 
> that the rows from different arrays are distinguishable.
> 
> The problem that arose is how to loop through arguments in the dots 
> "..." list. I need to get a hand on each of them but don't 
> know how many 
> of them there are and what their names are.
> 
> It'd be useful if I could look at how rbind(...) or c(...) do 
> this but 
> they are both Internal functions.
> 
> Thanks in anticipation of help!
> 
> Regards,
> viet
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>

In addition to Gabor's comments:

There's a reason why I didn't coerce the grouping variable to a factor.
rbind()ing data frames is much more expensive than rbind()ing
arrays/matrices.  Unless your data really have different data types in
different columns, it would mostly likely be better to work with the matrix
version of them.  If you really want a data frame with the grouping variable
as a factor, you can do the coercion afterward.

Andy
> From: Gabor Grothendieck
> 
> Viet Nguyen <vietnguyen <at> fastmail.fm> writes:
> 
> > 
> > Thanks to all who helped.
> > 
> > I used your ideas and code samples to write the following (for the 
> > benefit of people who will search this list later):
> > 
> > rbind.case <- function(..., name="case", values) {
> >     dots <- list(...);
> >     if (missing(values)) values <- 1:length(dots);
> >     if (length(values)!=length(dots))
> >       stop("length(values)!=length(list(...))");
> > 
> >     eval(parse(text> >               
paste("cbind(rbind(...), ",name,
> >                      "=rep(values, sapply(dots,
nrow)))",sep="")));
> > }
> > 
> > The function is to be used with data frames. It's not as 
> good as it can 
> > be but it works for my purpose.
> 
> Regarding improvements, eliminate the semicolons at the end 
> of statements,
> place the default value for values= in the arg list to make 
> it more readable,
> use stopifnot to check args (also for readability), add a check for 
> data frames (which is mentioned after the code but not checked for, 
> and eliminate the eval and rep calculations by simply 
> lapplying over an 
> index and appending the name column to each data frame in turn:
> 
> 
> rbind.case <- function(..., name="case", values = seq(along = 
> list(...))) 
> # rbind the ... data frames together adding a column named name whose 
> # value for rows from ith argument is values[i]
> {
>    dots <- list(...)
>    stopifnot(length(dots) == length(values),
> 	   all(sapply(dots, inherits, "data.frame")))
> 
>    f <- function(i) { x <- dots[[i]]; x[,name] <- values[i]; x }
>    do.call("rbind", lapply(seq(along = dots), f))
> }
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>