Reduce(`+`, k)
On Mon, Aug 23, 2010 at 12:18 PM, Thaler, Thorn, LAUSANNE, Applied
Mathematics <thorn.tha...@rdls.nestle.com> wrote:
> Hi all,
>
> In R it is possible to sum tables:
>
> > (a <- table(rep(1:3, sample(10,3))))
>
> 1 2 3
> 2 5 7
>
> > a+a
>
> 1 2 3
> 4 10 14
>
> Now suppose that I have a list of tables, where each table counts the
> same things
>
> > k <- list(a,a,a)
>
> How can I sum all tables in k?
>
> > do.call(sum, k)
> [1] 42
>
> does not work since it sums over each table.
>
> > do.call(`+`, list(a,a))
>
> 1 2 3
> 4 10 14
>
> works not with lists containing not exactly two values (since `+` takes
> exactly 2 values). So I think I should write something like
>
> Summary.table <- function(..., na.rm) {
> if (.Generic == "sum") {
> ...
> } else { # use the default method
> NextMethod()
> }
> }
>
> So first question: where is the `+` operation defined for tables? Is it
> S4? How can I see the source code of S4 functions (I'm not very
> comfortable with S4)? Or in general how do I find all generic functions
> for a specific class? I can get all S3 implementations of plot with
> methods(plot), I can get all S4 functions with getMethods(plot). But
> I've no idea of how to find all methods defined for a class? (e.g. all
> functions that operate on tables, say)
>
> Second question: my first dirty hack would be something like
>
> args <- list(...)
> table.sum <- args[[1]]
> for (i in 2:length(args)) table.sum <- table.sum + args[[i]]
>
>
> making use of the fact that `+` is defined for tables (and forgetting
> about cases where two tables don't feature the same names). It works,
> but isn't there a more elegant way to get the same?
>
> Last question: is there a way to determine the call stack, such that I
> can see the names of the function which are actually executed when I
> commit a command? I know a little about R's dispatching mechanism for S3
> classes (plot(a) actually calls plot.table) but I've no clue which
> function is called if I type a + a (especially since `+` belongs to the
> generic function group Ops and I do not know at all whether its S4 or
> S3). I read the documentation about S3 Group Generic Functions and tried
> to delve into S4, but obviously I was not able to understand everything
> completely. So it would be great if somebody could help me out with this
> specific topic and point me to some resources where I can learn more.
>
> Thanks for your help in advance.
>
> BR,
>
> Thorn
>
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