[R] convenient way to combine Surv objects?

[Heinz Tuechler]

Dear All,

is there a convenient way to combine Surv objects?
Imagine two Surv objects as in the following example.
Is it possible to combine them into one _Surv_ object?
I tried rbind(), cbind(), c(), merge(), but none of these function does,
what I would like.
So I drafted a method for rbind.Surv (see below), but I would be happy
about a better solution.

Ideas? Comments?

Thanks,
Heinz

R version 2.4.0 Patched (2006-11-03 r39792)
Windows XP

library(survival)
## create example data
so1 <- Surv(1:5, c(0, 0, 1, 0, 1))
so2 <- Surv(6:8, c(1, 0, 0))
### "combinefunction"(so1, so2)
## desired result:
[1] 1+ 2+ 3  4+ 5  6  7+ 8+

### Surv method for rbind - untested  
rbind.Surv <- function(..., deparse.level = 1)
{
  so <- list(...)
  ## check objects for number of columns
  so.dims <- sapply(so, dim)[2, ] # colums of each so
  min.so.cols <- min(so.dims)
  max.so.cols <- max(so.dims)
  ## if max range == 3 insert zero column in so with only 2 columns
  if (min.so.cols == 2 & max.so.cols == 3)
    for (i in seq(along=so)) {
      if (dim(so[[i]])[2] == 2) so[[i]] <- cbind(0, so[[i]])
    }
  ## function to get column in list element
  elco <- function(element,column) {element[,column]}
  if (max.so.cols == 2)
    return(Surv(unlist(sapply(so, elco, 1)), unlist(sapply(so, elco, 2))))
  else
    return(Surv(unlist(sapply(so, elco, 1)), unlist(sapply(so, elco, 2)),
                unlist(sapply(so, elco, 3))))
}

rbind(so1, so2)