[Rcpp-devel] Matrix: column-major storage?

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Interesting thread.  I had no idea the speed differences were that
significant.
I'm curious -- are there obvious use-cases where staying in column-major is
*not* possible via careful setup?

I realize it's a minimal test-case, but the cost of memory allocation in
the above example dominates.

Compare with:

byRow_inplace <- cxxfunction(signature(xm_ = "numeric"),
                     body = '
//int m = as<int>(dm);
//NumericMatrix xm(m, m);
NumericMatrix xm(xm_);
int m = xm.nrow();
int i;
int j;
for(i = 0; i < m; i++) {
   for(j = 0; j < m; j++) {
     xm(i, j) = i * m + j;
  }
}
',
                     plugin = "RcppArmadillo",
                     verbose = TRUE)

nn <- 10000
aa <- matrix(nn, nrow=nn, ncol=nn)

res <- benchmark(byRow(nn), byRow_inplace(aa),
          columns=c("test", "replications", "elapsed", "relative",
          "user.self", "sys.self"),
          order="relative", replications=10)

best,
Christian

 I thought that, for any of the Matrix types available, if we have

x(i, j)
then
x(i, j + 1)

would be contiguous in memory (whereas x(i + 1, j) could be extremely far
away, depending on the size of the matrix).

However, it seems that with Matrix objects it is the other way around, so
that x(i, j) is next to x(i + 1, j), not to x(i, j + 1):

-- 
A man, a plan, a cat, a ham, a yak, a yam, a hat, a canal ? Panama!
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