# Sum a list of tables

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## Sum a list of tables

 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 ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.
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## Re: Sum a list of tables

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## Re: Sum a list of tables

 sorry, cancel that.  I will try again. On Mon, Aug 23, 2010 at 12:35 PM, RICHARD M. HEIBERGER <[hidden email]>wrote: > Reduce(`+`, k) >         [[alternative HTML version deleted]] ______________________________________________ [hidden email] mailing list https://stat.ethz.ch/mailman/listinfo/r-helpPLEASE do read the posting guide http://www.R-project.org/posting-guide.htmland provide commented, minimal, self-contained, reproducible code.