Does anyone has a clue what went wrong in the
following attempt?
I am trying to call the R built-in function Rdqags()
from my C
program for numerical integration. Following are the C
program
and the corresponding R program:
C program
---------
void test(double *a,
double *b,
double *epsabs,
double *epsrel,
double *result,
double *abserr,
int *neval,
int *ier,
int *limit,
int *lenw,
int *last,
int *iwork,
double *work,
double *exx)
{
void *ex;
ex = exx;
Rdqags(tmpfun, ex, a, b, epsabs, epsrel,
result, abserr, neval, ier,
limit, lenw, last, iwork, work);
}
// User supplied function
void tmpfun(double *x, int n, void *ex)
{
int i;
double *tmp;
tmp = (double *)ex;
// for(i=1;i<=n;i++) {x[i] = pow(x[i], *tmp);}
for(i=1;i<=n;i++) {x[i] = 2.0*x[i];}
return;
}
R program
---------
test <- function(a, b, epsabs .Machine$double.eps^0.25, epsrel = epsabs,
limit = 100, lenw = 400, ex = -0.5)
{
val <- .C("test",
as.double(a),
as.double(b),
as.double(epsabs),
as.double(epsrel),
result = double(1),
abserr = as.double(-1),
neval = as.integer(-1),
ier = as.integer(-1),
as.integer(limit),
as.integer(lenw),
last = as.integer(-1),
integer(limit+1),
double(lenw+1),
as.double(ex),
NAOK=T,
specialsok=T
)
val
}
I followed the instructions in Writing R Extensions ->
The R API
-> Integration, and the program was complied and
loaded into R
successfully. However, calling R function test(0,
20)$result
gives me 385.0554, not 400! The returned ier value of
test(0, 20)
is 0! But if I call integrate(function(x) 2 * x, lower
= 0, upper
= 20), I got the correct result of 400. Apparently,
there must be
something wrong in my programs. But I cannot see where
is the
mistake. Do I miss anything obvious here?
The programs above were tested in R-1.9.0 for
MS-Windows and the
compiler is MSC++ V6. I have used similar calls to the
built-in
random generators and never had any problems.
Paul.
