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

[Serguei Sokol]

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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