[R] Calling fft from C

[biostatmatt]

The fft in R has the following C call chain

file                      purpose
src/main/names.c          links R function "fft" to do_fft
src/main/fourier.c        do_fft calls fft_factor, fft_work
src/appl/fft.c            home of fft_factor, fft_work

If you want to use the fft at a low level, you should use fft_factor and
fft_work. You can read about these functions in the fft.c file. In order
to use these functions in an R package, you must use the definitions in
the R/R_ext/Applic.h header. 

However, I believe the time limiting factor in an fft call is the work
performed by the fft_factor and fft_work functions. Hence, you are not
likely to save much time by calling them directly.

BTW, I think this sort of question is better suited for the R-devel
mailing list.

Matt Shotwell
Graduate Student
Division of Biostatistics and Bioinformatics 
Medical University of South Carolina

On Sun, 2010-05-30 at 21:35 +0200, Gunnar Hellmund wrote:
> 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.”
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >>
> >
> 
> 
>