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 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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