On 17 November 2011 at 09:09, Raymond wrote:
| Hi R developers,
|
| I am new to this forum and hope someone can help me with .Call in R.
| Greatly appreciate any help!
|
| Say, I have a vector called "vecA" of length 10000, I generate a vector
| called "vecR" with elements randomly generated from Uniform[0,1]. Both vecA
| and vecR are of double type. I want to replace elements vecA by elements in
| vecR only if sum of elements in vecR is greater than or equal to 5000.
| Otherwise, vecR remain unchanged. This is easy to do in R, which reads
| vecA<-something;
| vecR<-runif(10000);
| if (sum(vecR)>=5000)){
| vecA<-vecR;
| }
|
|
| Now my question is, if I am going to do the same thing in R using .Call.
| How can I achieve it in a more efficient way (i.e. less computation time
| compared with pure R code above.). My c code (called "change_vecA.c") using
| .Call is like this:
Here is my take on it, using about the same number of commands in C++ thanks
to Rcpp and its vectorised sum() and runif() commands (which mimick the R
commands):
R> library(inline)
R> library(Rcpp)
R>
R> set.seed(42) # fix RNG seed
R> vecA <- rt(10000, 6) # 'something' in vecA: t-dist with 6 df
R>
R> fun <- cxxfunction(signature(va="numeric"), # pass in a vector
+ plugin="Rcpp", # use Rcpp, and code below
+ body='
+
+ Rcpp::NumericVector vA(va);
+ Rcpp::RNGScope tmp; // make sure RNG is set up
+ Rcpp::NumericVector vR = runif(10000); // 10k of a U(0,1)
+
+ if (sum(vR) >= 5000) { // sum is an Rcpp sugar op.
+ vA = vR; // swap vR into vA
+ }
+ return(vA); // return vA
+ ')
R>
R> sum( fun( vecA ) )
[1] 5033
R> sum( fun( vecA ) )
[1] 5015
R> sum( fun( vecA ) )
[1] 66
R> sum( fun( vecA ) )
[1] 66
R> sum( fun( vecA ) )
[1] 5015
R> sum( fun( vecA ) )
[1] 5024
R> sum( fun( vecA ) )
[1] 5020
R>
R> sum(vecA)
[1] 66
R>
You can learn about Rcpp from the vignettes in the package, at my page at
http://dirk.eddelbuettel.com/code/rcpp.html as well different posts on my
blog, and of course the rcpp-devel mailing list. The example above uses
cxxfunction() from the wonderful inline package you may find useful too as it
compiles, links and loads your C or C++ snippets.
Dirk
--
"Outside of a dog, a book is a man's best friend. Inside of a dog, it is too
dark to read." -- Groucho Marx
______________________________________________
R-devel@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel
