[Rcpp-devel] trying to insert a number as first element of already existing vector

Small addendum: A large part of the performance gain in my example comes 
from using NumericVector instead of std::vector<double>. Which avoids a 
conversion. An example using std::copy with Numeric vector runs in the 
same time as the version using memcpy.

Jan



On 10-12-18 12:28, Jan van der Laan wrote:

For performance memcpy is probably fastest. This gives the same 
performance a c().

// [[Rcpp::export]]
NumericVector mybar3(NumericVector x, double firstelem) {
?? NumericVector result(x.size() + 1);
?? result[0] = firstelem;
?? std::memcpy(result.begin()+1, x.begin(), x.size()*sizeof(double));
?? return result;
}


Or a more general version concatenating vector of arbitrary lengths:


// [[Rcpp::export]]
NumericVector mybar4(NumericVector x, NumericVector y) {
?? NumericVector result(x.size() + y.size());
?? std::memcpy(result.begin(), x.begin(), x.size()*sizeof(double));
?? std::memcpy(result.begin()+x.size(), y.begin(), 
y.size()*sizeof(double));
?? return result;
}



?> n=1E7
?> testvec = c(1,seq_len(n))
?> testelem <- 7
?> microbenchmark(c(testelem, testvec), mybar(testvec,testelem),
+?? mybar2(testvec,testelem),
+?? mybar3(testvec,testelem),
+?? mybar4(testvec,testelem)
+?? )
Unit: milliseconds
?????????????????????? expr?????? min??????? lq????? mean??? median   
uq ?????? max neval
?????? c(testelem, testvec)? 36.48577? 36.93754? 41.10550? 43.76742 
44.20709? 46.09741?? 100
?? mybar(testvec, testelem) 102.54042 103.21756 106.88749 104.32033 
110.31527 119.55512?? 100
??mybar2(testvec, testelem)? 95.64696? 96.19447 100.24691 102.61380 
103.58189 109.28290?? 100
??mybar3(testvec, testelem)? 36.45794? 36.87915? 40.43486? 37.18063 
43.49643? 95.49049?? 100
??mybar4(testvec, testelem)? 36.51334? 37.05409? 41.39680? 43.20627 
43.57958? 94.95482?? 100


Best,
Jan



On 10-12-18 12:10, Serguei Sokol wrote:

Le 09/12/2018 ? 09:35, Mark Leeds a ?crit?:

Hi All: I wrote below and it works but I have a strong feeling 
there's a better way to do it.

If performance is an issue, you can save few percents of cpu time by 
using std::copy() instead of explicit for loop. Yet, for this 
operation R's c() remains the best bet. It is more then twice faster 
than both Rcpp versions below:

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
std::vector<double> mybar(const std::vector<double>& x, double 
firstelem) {
?? std::vector<double> tmp(x.size() + 1);
?? tmp[0] = firstelem;
?? for (int i = 1; i < (x.size()+1); i++)
???? tmp[i] = x[i-1];
?? return tmp;
}
// [[Rcpp::export]]
std::vector<double> mybar2(const std::vector<double>& x, double 
firstelem) {
?? std::vector<double> tmp(x.size() + 1);
?? tmp[0] = firstelem;
?? std::copy(x.begin(), x.end(), tmp.begin()+1);
?? return tmp;
}

/*** R
library(microbenchmark)
n=100000
testvec = c(1,seq_len(n))
testelem <- 7
microbenchmark(c(testelem, testvec), mybar(testvec,testelem), 
mybar2(testvec,testelem))
*/

# Ouput
Unit: microseconds
?????????????????????? expr???? min?????? lq????? mean   
median??????? uq
?????? c(testelem, testvec) 247.098 248.5655? 444.8657 257.3300  
630.7725
?? mybar(testvec, testelem) 594.978 622.3560 1226.5683 637.0230 
1386.8385
??mybar2(testvec, testelem) 576.191 604.7565 1029.2124 616.1055 
1351.6740
??????? max neval
?? 7587.977?? 100
??22149.605?? 100
??11651.831?? 100


Best,
Serguei.

I looked on the net and found some material from back in ~2014 about 
concatenating
vectors but I didn't see anything final about it. Thanks for any 
insights.

Also, the documentation for Rcpp is beyond incredible (thanks to 
dirk, romain, kevin and all the other people I'm leaving out )? but 
is there a general methodology for finding equivalents of R 
functions. For example, if I want a cumsum function in Rcpp, how do 
I know whether to use the stl with accumulate or if there's already 
one built in so
that I just call cumsum.

Thanks.

#=======================================================

#include <Rcpp.h>
using namespace Rcpp;

// [[Rcpp::export]]
std::vector<double> mybar(const std::vector<double>& x, double 
firstelem) {
?? std::vector<double> tmp(x.size() + 1);
?? tmp[0] = firstelem;
?? for (int i = 1; i < (x.size()+1); i++)
???? tmp[i] = x[i-1];
?? return tmp;
}

/*** R

testvec = c(1,2,3)
testelem <- 7
mybar(testvec,testelem)

*/

#===============================
# OUTPUT FROM RUNNING ABOVE
#=================================
?> testvec <-? c(1,2,3)
?> testelem <- 7
?> mybar(testvec,testelem)
[1] 7 1 2 3
?>