[Rcpp-devel] use of auxiliary functions

[Dirk Eddelbuettel]

On 12 August 2010 at 20:59, baptiste auguie wrote:
| Point well taken, l'll try to use const and const& wherever appropriate.

I am coming to this late (sorry, too much other stuff going on...) but I had
meant to illustrate the good advise provided by Davor and Romain.  So here is
a stylized Armadillo examples -- not involving SEXPs as these really are
pointers which muddles everything.   So here I simply create a matrix and
ship it to two functions: one uses const ref , one uses copy:

   edd at max:~/src/progs/C++$ ./const_ref_vs_copy_cost
   t0: 2010-Aug-12 14:45:04.526660
   t1: 2010-Aug-12 14:45:04.526731 00:00:00.000071
   t2: 2010-Aug-12 14:45:05.054273 00:00:00.527542

So 71 microsecs versus 527 millisecs.  This uses Boost which gives us nice
granularity of time on Linux.  On another OS your mileage may vary. The Emacs
compile-command "works for me" with Boost and Armadillo in standard
places. Adjust as necessary.

And for what it is worth, "const whenever possible" is item #3 in "Effective
C++" by Meyers, a book Romain and I both cherish.

Cheers, Dirk


// -*- compile-command: "g++ -s -O3 -Wall -pedantic -pipe -o const_ref_vs_copy_cost const_ref_vs_copy_cost.cpp -lboost_date_time-mt" -*-

#include <boost/date_time/posix_time/posix_time.hpp>
#include <armadillo>
#include <iostream>

using namespace boost::posix_time; 		// boost timers
using namespace boost::gregorian;

using namespace arma;

double doSomeMindlessWork_const_ref(const mat & m) {
  int n = m.n_rows;
  int k = m.n_cols;

  return m(0,0) + m(n-1, k-1);
}

double doSomeMindlessWork_copy(mat m) {
  int n = m.n_rows;
  int k = m.n_cols;
  
  return m(0,0) + m(n-1, k-1);
}


int main() {
  const int n = 10000;
  
  mat m = mat(n, n);
  
  ptime t0 = microsec_clock::local_time();
  doSomeMindlessWork_const_ref(m);
  ptime t1 = microsec_clock::local_time();
  doSomeMindlessWork_copy(m);
  ptime t2 = microsec_clock::local_time();
  
  std::cout << "t0: " << to_simple_string(t0) << "\n"
	    << "t1: " << to_simple_string(t1) << " " << to_simple_string(t1-t0) << "\n"
	    << "t2: " << to_simple_string(t2) << " " << to_simple_string(t2-t1) << "\n"
	    << std::endl;

  return 0;
}

-- 
Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com