R help - May 2010 - 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.?
>
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