[Rd] Subsetting "dspMatrix" without coercion to "matrix"

[Mikael Jagan]

> This seems entirely avoidable, given that there is a relatively simple 
> formula for converting 2-ary indices [i,j] of S to 1-ary indices k of 
> S[lower.tri(S, TRUE)]:
> 
> k <- i + round(0.5 * (2L * n - j) * (j - 1L)) # for i >= j

I ought to be slightly more precise here: _coercion_ is avoidable, 
because we can always map [i,j] to [k], but memory limits are not. 
Certainly S using x[k] cannot be arbitrarily long...

At the very least, it would be convenient if the subset were performed 
efficiently whenever dimensions would be dropped anyway:

* S[i, ] and S[, j] where i and j are vectors indexing exactly zero or 
one rows/columns
* S[i] where i is a matrix of the form cbind(i, j)

This would support, e.g., a memory-efficient 'apply' analogue without 
any need for MARGIN...

applySymmetric <- function(X, FUN, ..., simplify = TRUE, check = TRUE) {
   if (check && !isSymmetric(X)) {
     stop("'X' is not a symmetric matrix.")
   }
   ## preprocessing
   ans <- vector("list", n)
   for (i in seq_len(n)) {
     ans[[i]] <- forceAndCall(1L, FUN, S[i, ], ...)
   }
   ## postprocessing
   ans
}