On 2/26/2015 16:37, Pierre GLOAGUEN wrote:
Thanks, it works!
I'm not familiar with C++, is it necessary to always use such a
function in C++ or is it because of R/C++ interface?
Hi Pierre!
You're witnessing yet another fascinating difference between (usual in
the world of C++) value semantics and (not that usual in the world of
C++) reference semantics:
Compare the difference in behavior of `vector_reference` (using
`Rcpp::NumericVector`) and `vector_value` (using `std::vector<double>`)
in the code attached below.
To make things clear: no, in C++ you're not (not by default and not in
the standard world) supposed to care about (nor use) a function such as
`clone` -- this is idiomatic in programming languages with reference
semantics, like Java: https://en.wikipedia.org/wiki/Clone_%28Java_method%29
If this makes you feel better: If you're coming from C++ your
expectations are perfectly reasonable and this is how any well-designed
(as in: not violating the POLS) C++ library (including the C++ standard
library) will behave by default.
Which incidentally brings me to the advice I usually give in these
situations: unless you're absolutely dependent on the "features" of
`Rcpp::NumericVector` just forget about it and replace all uses with the
standard container `std::vector<double>`.
// Arguably, NumericVector's POLS violation -- as illustrated by the
numerous posts raising similar questions -- perhaps isn't necessarily
the optimal design choice?
Everything will behave just as you expect and as well-documented in the
usual places -- e.g., http://en.cppreference.com/w/cpp/container/vector
In contrast, `help.search("Rcpp::NumericVector")` happily announces "No
results found" at the moment -- and, as mentioned in another reply,
you're apparently expected to Google around to find methods for solving
problems you wouldn't even encounter otherwise.
As fun as it can be to learn-via-a-search-engine, if you don't find the
curiosities like the aforementioned reference-vs-value-semantics
differences _all_ _that_ fascinating, I think you may find yourself more
productive and better served by sticking to the standard containers :-)
HTH!
#include <Rcpp.h>
// [[Rcpp::export]]
Rcpp::NumericVector vector_reference(Rcpp::NumericVector input)
{
Rcpp::NumericVector output(input);
if (output.size() >= 1) output[0] = 123;
return output;
}
// [[Rcpp::export]]
std::vector<double> vector_value(const std::vector<double> & input)
{
std::vector<double> output(input);
if (output.size() >= 1) output[0] = 123;
return output;
}
/*** R
v = rep(1, 6)
vector_reference(v)
show(v)
v = rep(1, 6)
vector_value(v)
show(v)
#Output:
#> v = rep(1, 6)
#> vector_reference(v)
#[1] 123 1 1 1 1 1
#> show(v)
#[1] 123 1 1 1 1 1
#> v = rep(1, 6)
#> vector_value(v)
#[1] 123 1 1 1 1 1
#> show(v)
#[1] 1 1 1 1 1 1
*/
Best,
Matt
Thanks again for your help,
Pierre
Le 26/02/2015 16:30, Jeffrey Pollock a écrit :
Perhaps use the clone() function?
library(Rcpp)
cppFunction("
NumericVector par_CMAtR(NumericVector vec_CMA) {
NumericVector out = clone(vec_CMA);
out[5] = exp(out[5]);
return out;
}
")
vec_C <- rep(1, 6)
par_CMAtR(vec_C)
print(vec_C)
On Thu, Feb 26, 2015 at 3:16 PM, Pierre GLOAGUEN
<pierre.gloag...@ifremer.fr <mailto:pierre.gloag...@ifremer.fr>> wrote:
Hello everybody,
I have a very simple example
I have a vector vec_CMA which length is a multiple of 6.
I want to get the exact same vector, except the last element
which is the exponential of the last element of vec_CMA
The code is the following
//myfun.cpp
#include <Rcpp.h>
using namespace Rcpp;
// [[Rcpp::export]]
NumericVector par_CMAtR(NumericVector vec_CMA){
int K = (vec_CMA.size())/6;
NumericVector out(6*K);
out = vec_CMA;
out[6*K-1] = exp(vec_CMA[6*K-1]);
return out;
}
I apply the function with the R code
sourceCpp("myfun.cpp")
vec_C <- rep(1,6)
par_CMAtR(vec_C)
[1] 1 1 1 1 1 2.718282
8
works fine. Except the vec_C is modified too!
vec_C
[1] 1 1 1 1 1 2.718282
It's the first time I have this kind of problem. What is wrong in
my code?
Thanks for your help,
Pierre Gloaguen
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