[Rd] .Call in R

Raymond gwgc5 at mail.missouri.edu
Thu Nov 17 18:09:30 CET 2011

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){
.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!

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