[R] Subscripting a matrix-like object

[jhallman at frb.gov]

I have an S3 class called "tis" (Time Indexed Series) which may or may
not have multiple columns.  I have a function "[<-.tis" that I've
reproduced below.  

My question is this: inside of "[<-.tis", how can I distinguish between
calls of the form 

x[i] <- someValue

and

x[i,] <- someValue ?

In either case, nargs() is 3, and looking at the values from sys.call()
and match.call() I could not tell them apart.  Am I missing something?


"[<-.tis" <- function(x, i, j, ..., value){
  tif <- tif(x)
  xStart <- start(x)
  x <- stripTis(x)
  if(missing(i)){
    if(missing(j)) x[]   <- value
    else           x[,j] <- value
  }
  else {
    i <- i[!is.na(i)]
    if(is.numeric(i)){
      if(!is.ti(i) && couldBeTi(i, tif = tif))
        i <- asTi(i)
      if(is.ti(i)){
        i <- i + 1 - xStart
        if(any(i < 1)){
          newRows <- 1 - min(i)
          xStart <- xStart - newRows
          if(is.null(m <- ncol(x)))  m <- 1
          i <- i + newRows
          if(is.matrix(x))
            x <- rbind(matrix(NA, newRows, m), x)
          else
            x <- c(rep(NA, newRows), x)
        }
      }
    }
    else if(!is.logical(i)) stop("non-numeric, non-logical row index")
   if(is.matrix(x)){
      if(any(i > nrow(x))){
        newRows <- max(i) - nrow(x)
        x <- rbind(x, matrix(NA, newRows, ncol(x)))
      }
      if(missing(j)){
        if(is.matrix(i))  x[i] <- value
        else {
          if(is.logical(i))
            x[i,] <- rep(value, length = sum(i)*ncol(x))
          else
            x[i,] <- rep(value, length = length(i)*ncol(x))
        }
      }
      else x[i,j] <- value
    }
    else x[i] <- value
  }
  start(x) <- xStart
  class(x) <- c("tis", oldClass(x))
  x
}