I've not seen any reposnse to this, probably because
-- this is not the list for C programming questions (use R-devel)
-- there is no example provided for the times quoted.
-- the code is not compilable.
But I got a compiler warning on
int *ipiv = (int *) R_alloc((int) nrA, sizeof(int));
foo.c:8: warning: cast from pointer to integer of different size
(it should be *nrA) and that's most likely your error. (BTW, using
liberal casts like this is just stopping your tools helping you!)
The overhead of C function call is measured in nanosecs on modern systems,
especially to functions in the same compilation unit.
On Thu, 10 Apr 2008, Søren Højsgaard wrote:
Dear list,
I am a little puzzled by computing time in connection with calling C
functions. With the function mysolve1 given below I solve Ax=B, where
the actual matrix operation takes place in mysolve2. Doing this 5000
times takes 3.51 secs. However, if I move the actual matrix inversion
part into mysolve1 (by uncommenting the two commented lines and skip the
call to mysolve2) then the computations take only 0.03 secs. Can anyone
tell me why there is such a "time-overhead" in introducing the extra
function call to mysolve2?
I am on windows XP using R 2.6.1
Best regards
Søren
SEXP mysolve1(SEXP Ain, SEXP Bin, SEXP tolin)
{
int *Adims, *Bdims;
double tol = asReal(tolin);
SEXP A, B;
PROTECT(A = duplicate(Ain));
PROTECT(B = duplicate(Bin));
Adims = INTEGER(coerceVector(getAttrib(A, R_DimSymbol), INTSXP));
Bdims = INTEGER(coerceVector(getAttrib(B, R_DimSymbol), INTSXP));
int nrA, ncB;
double *Ap, *Bp;
nrA = Adims[0];
ncB = Bdims[1];
Ap = REAL(A);
Bp = REAL(B);
/* int info, *ipiv = (int *) R_alloc(nrA, sizeof(int)); */
/* F77_CALL(dgesv)(&nrA, &ncB, Ap, &nrA, ipiv, Bp, &nrA, &info); */
mysolve2(Ap, &nrA, Bp, &ncB, &tol);
UNPROTECT(2);
return B;
}
void mysolve2(double *A, int *nrA, double *B, int *ncB, double *tolin)
{
int info;
int *ipiv = (int *) R_alloc((int) nrA, sizeof(int));
F77_CALL(dgesv)(nrA, ncB, A, nrA, ipiv, B, nrA, &info);
}
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Brian D. Ripley, [EMAIL PROTECTED]
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