Hi,
A probably very naive remark, but I believe that the probability of sum(
runif(10000) ) >= 50000 is exactly 0.5. So why not just test that, and
generate the uniform values only if needed ?
Karl Forner
On Thu, Nov 17, 2011 at 6:09 PM, Raymond <gw...@mail.missouri.edu> 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:
>
> SEXP change_vecA(SEXP vecA){
> int i,vecA_len;
> double sum,*res_ptr,*vecR_ptr,*vecA_ptr;
>
> vecA_ptr=REAL(vecA);
> vecA_len=length(vecA);
> SEXP res_vec,vecR;
>
> PROTECT(res_vec=allocVector(REALSXP, vec_len));
> PROTECT(vecR=allocVector(REALSXP, vec_len));
> res_ptr=REAL(res_vec);
> vecR_ptr=REAL(vecR);
> GetRNGstate();
> sum=0.0;
> for (i=0;i<vecA_len;i++){
> vecR_ptr[i]=runif(0,1);
> sum+=vecR_ptr[i];
> }
> if (sum>=5000){
> /*copy vecR to the vector to be returned*/
> for (i=0;i<vecA_len;i++){
> res_ptr[i]=vecR_ptr[i];
> }
> }
> else{
> /*copy vecA to the vector to be returned*/
> for (i=0;i<vecA_len;i++){
> res_ptr[i]=vecA_ptr[i];
> }
> }
>
> PutRNGstate();
> UNPROTECT(2);
> resturn(res);
> }
> My R wrapper function is
> change_vecA<-function(vecA){
> dyn.load("change_vecA.so");
> .Call("change_vecA",vecA);
> }
>
> Now my question is, due to two loops (one generates the random
> vector and one determines the vector to be returned), can .Call still be
> faster than pure R code (only one loop to copy vecR to vecA given condition
> is met)? Or, how can I improve my c code to avoid redundant loops if any.
> My
> concern is if vecA is large (say of length 1000000 or even bigger), loops
> in
> C code can slow things down. Thanks for any help!
>
>
>
>
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/Call-in-R-tp4080721p4080721.html
> Sent from the R devel mailing list archive at Nabble.com.
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>
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