Calling fft from C

Hi

I have made a R function 'convolve2' for convolution of two real
valued vectors based on Rs 'convolve' with option type="open" - see
below.
(exp.length and irf.length are variables set in another part of the program)

I wish to implement the function convolve2 in C and use it in a
function used from R with .Call - e.g. I need to call fft in C.
All I can find in the source code is do_fft in Internals.h - but how
do I use do_fft? Or should I call another C routine (and how)?

How do I solve the problem in the most appropriate way?

convolve2=function2(x,y)
{
x<- c(rep.int(0,exp.length-1),x)
n <- length(y<-c(y, rep.int(0, irf.length -1)))
x <- fft(fft(x) * Conj(fft(y)), inverse=TRUE)
return(Re(x)/n)
}

--
Best wishes/bedste hilsner

Gunnar Hellmund

cand. scient., PhD
http://staff.pubhealth.ku.dk/~guhe/

---
?If everyone is thinking alike, then somebody isn't thinking.?

The Writing R Extensions manual specifically uses convolve as an
example of calling C from R.

On Sun, May 30, 2010 at 2:08 PM, Gunnar Hellmund <ghellmund at gmail.com> wrote:

Hi

I have made a R function 'convolve2' for convolution of two real
valued vectors based on Rs 'convolve' with option type="open" - see
below.
(exp.length and irf.length are variables set in another part of the program)

I wish to implement the function convolve2 in C and use it in a
function used from R with .Call - e.g. I need to call fft in C.
All I can find in the source code is do_fft in Internals.h - but how
do I use do_fft? Or should I call another C routine (and how)?

How do I solve the problem in the most appropriate way?

convolve2=function2(x,y)
{
x<- c(rep.int(0,exp.length-1),x)
n <- length(y<-c(y, rep.int(0, irf.length -1)))
x <- fft(fft(x) * Conj(fft(y)), inverse=TRUE)
return(Re(x)/n)
}

--
Best wishes/bedste hilsner

Gunnar Hellmund

cand. scient., PhD
http://staff.pubhealth.ku.dk/~guhe/

---
?If everyone is thinking alike, then somebody isn't thinking.?

Thanks for the reply, but ...

No, I am not interested in the convolution function defined in the
R-extensions manual.
I want to use a fast fourier transform version implemented in C
returning values to another C function.
So the question is: How do I call the fft supplied by R in C? (just
like it is possible to use other R functions in C using #include
<Rmath.h> etc)

Best,
Gunnar

On Sun, May 30, 2010 at 8:12 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:

The Writing R Extensions manual specifically uses convolve as an
example of calling C from R.

On Sun, May 30, 2010 at 2:08 PM, Gunnar Hellmund <ghellmund at gmail.com> wrote:

Hi

I have made a R function 'convolve2' for convolution of two real
valued vectors based on Rs 'convolve' with option type="open" - see
below.
(exp.length and irf.length are variables set in another part of the program)

I wish to implement the function convolve2 in C and use it in a
function used from R with .Call - e.g. I need to call fft in C.
All I can find in the source code is do_fft in Internals.h - but how
do I use do_fft? Or should I call another C routine (and how)?

How do I solve the problem in the most appropriate way?

convolve2=function2(x,y)
{
x<- c(rep.int(0,exp.length-1),x)
n <- length(y<-c(y, rep.int(0, irf.length -1)))
x <- fft(fft(x) * Conj(fft(y)), inverse=TRUE)
return(Re(x)/n)
}

--
Best wishes/bedste hilsner

Gunnar Hellmund

cand. scient., PhD
http://staff.pubhealth.ku.dk/~guhe/

---
?If everyone is thinking alike, then somebody isn't thinking.?