Le 09/12/2018 à 09:35, Mark Leeds a écrit :
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
>
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