[Rcpp-devel] fill a NumericMatrix row by row

Thanks for the reply and great libraries Conrad and Romain.

Conrad, my example is a bit too simple maybe, I am really doing:

for i from 1 to m:
   do computation
   store a row of results into a tall m x n matrix

...where i am pretty sure that the time required for computation is more
than storing the results.

Out of curiosity, I tried to do some profiling for row filling of tall
Armadillo matrices vs. column filling. Row filling becomes worse when the
rows are long.  I guess the memory allocation could obscure some speed
differences:

library(Rcpp)
library(inline)

# fill a matrix by row with m0 rows and n0 columns
row.code <- '
   int m = Rcpp::as<int>(m0);
   int n = Rcpp::as<int>(n0);
   arma::mat mat = arma::zeros(m,n);
   arma::rowvec vec = Rcpp::as<arma::rowvec>(vec0);
   for (int i = 0; i < m; i++) {
      mat.row(i) = vec;
   }
   return Rcpp::wrap(mat);
'
row.f <-
cxxfunction(signature(m0="integer",n0="integer",vec0="numeric"),row.code,plugin="RcppArmadillo")

# so here we swap, fill by column, m0 is the number of columns and n0 is
the number of rows
col.code <- '
   int m = Rcpp::as<int>(m0);
   int n = Rcpp::as<int>(n0);
   arma::mat mat = arma::zeros(n,m);
   arma::colvec vec = Rcpp::as<arma::colvec>(vec0);
   for (int i = 0; i < m; i++) {
      mat.col(i) = vec;
   }
   return Rcpp::wrap(mat);
'
col.f <- cxxfunction(signature(m0="
integer",n0="integer",vec0="numeric"),col.code,plugin="RcppArmadillo")

m <- 1e7
n <- 10
vec0 <- seq_len(n)
system.time({row.f(m,n,vec0)})

user  system elapsed
  0.779   1.022   1.801

system.time({col.f(m,n,vec0)})

user  system elapsed
  0.957   0.966   1.922

all.equal(row.f(m,n,vec0), t(col.f(m,n,vec0)))

[1] TRUE

m <- 1e6
n <- 100
vec0 <- seq_len(n)
system.time({row.f(m,n,vec0)})

user  system elapsed
  1.925   1.425   3.351

system.time({col.f(m,n,vec0)})

user  system elapsed
  1.244   1.423   2.667

all.equal(row.f(m,n,vec0), t(col.f(m,n,vec0)))

[1] TRUE

There's an even simpler way when using Armadillo, so that no loop is
required:
http://arma.sourceforge.net/docs.html#each_colrow

For example:

mat X(4,5);
rowvec r(5);

X.each_row() = r;


(btw, in general it's more efficient to access matrices as columns
rather than rows).


On Wed, Mar 27, 2013 at 6:23 PM, Michael Love
<michaelisaiahlove at gmail.com> wrote:

As recommended, I have switched to using Armadillo matrices, which

doesn't

print out these lines.
...
Example with Armadillo:

arma.code <- '
   arma::mat mat = Rcpp::as<arma::mat>(mat0);
   arma::rowvec vec = Rcpp::as<arma::rowvec>(vec0);
   for (int i = 0; i < mat.n_rows; i++) {
      mat.row(i) = vec;
   }
   return Rcpp::wrap(mat);
'
arma.f <-

cxxfunction(signature(mat0="numeric",vec0="numeric"),arma.code,plugin="RcppArmadillo")

arma.f(mat0,vec0)

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