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fftw_plan_dft_1d F90

Started by bastide May 16, 2006
Hi everybody,
I have a Fortran90 program where I have to use FFTW librairies. fftw is
written in C but wrappers exist for Fortran. Thus ffwt_plan_dft_1d can
be applied as in C with slight differences (no ffwt_malloc, no
fftw_free, and no pointer).

I have an array of 0 everywhere except within the central part (=1),
real data that I put
into the real components of the complex input "in".  The imaginary
components of the complex input are all zeros.
my input "in" is defined as complex : the real part is equel to my real
array, and imaginary part is all 0.

this is how I call fftw_plan_dft_1d in Fortran90 :

call fftw_plan_dft_1d(plan,N, in, out, FFTW_FORWARD,
FFTW_ESTIMATE)

plan is defined as integer*8.

The operation is doing well when N is not so high (<100). When N = 2000
for example,
every component of the output, real and imaginary, is exactly zero !!


I would be grateful to anyone who could enlighten me as to why this is
the case.

Many thanks in advance for any help,
Franck

bastide wrote:
> The operation is doing well when N is not so high (<100). When N = 2000 > for example, every component of the output, real and imaginary, is exactly zero !!
Are you sure you're using FFTW_ESTIMATE? If you are using FFTW_MEASURE or FFTW_PATIENT, then you have to initialize your array after creating the plan (see Q3.16 in the FAQ) since the planner sets the data to zero. Cordially, Steven G. Johnson
Yes, I really use FFTW_ESTIMATE but I initialized the input before
creating plan !
It seems that even with FFTW_ESTIMATE, I have to initialize it after
creating the plan.
It's ok, now.
Thanks so much for your answer Steven !!
Franck.

For people who would have the same problem, this is what you have to do
in your code  (I spent too much time to undestrand why output was all
0) :

call fftw_plan_dft_1d(plan,N, in, out, FFTW_FORWARD,
FFTW_ESTIMATE)
in=0  ! <------------------ initialize the input here
call fftw_execute(plan)
call fftw_destroy_plan(plan)

Something is still wrong, because FFTW_ESTIMATE mode should not
overwrite the data.

For one thing, you're using the wrong subroutine name...the name of the
Fortran wrapper is dfftw_plan_dft_1d, not fftw_plan_dft_1d.

Steven

Yes, you're right. I use dfftw_ instaed of fftw as written above. Sorry
for this mistake.
For FFTW_ESTIMATE, I agree with you again. It seems that I have to
initialize my array only if I use FFTW_MEASURE or FFTW_PATIENT.
However, if I don't initialize this input array with the use od
FFTW_ESTIMATE, there are still 0 in output ! (I don't understand why).

Thanks again
Franck

Yes, you're right. I use dfftw_ instaed of fftw as written above. Sorry
for this mistake.
For FFTW_ESTIMATE, I agree with you again. It seems that I have to
initialize my array only if I use FFTW_MEASURE or FFTW_PATIENT.
However, if I don't initialize this input array with the use od
FFTW_ESTIMATE, there are still 0 in output ! (I don't understand why).

Thanks again
Franck

> However, if I don't initialize this input array with the use od > FFTW_ESTIMATE, there are still 0 in output ! (I don't understand why).
You compiler may initialize arrays to 0 by default? Anyway, that is a question for comp.lang.fortran