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In C, you might have something like the following to transform a one-dimensional complex array:

fftw_complex in[N], out[N]; fftw_plan plan; plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE); fftw_execute(plan); fftw_destroy_plan(plan);

In Fortran, you would use the following to accomplish the same thing:

double complex in, out dimension in(N), out(N) integer*8 plan call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE) call dfftw_execute_dft(plan, in, out) call dfftw_destroy_plan(plan)

Notice how all routines are called as Fortran subroutines, and the
plan is returned via the first argument to `dfftw_plan_dft_1d`

.
Notice also that we changed `fftw_execute`

to
`dfftw_execute_dft`

(see FFTW Execution in Fortran). To do
the same thing, but using 8 threads in parallel (see Multi-threaded FFTW), you would simply prefix these calls with:

integer iret call dfftw_init_threads(iret) call dfftw_plan_with_nthreads(8)

(You might want to check the value of `iret`

: if it is zero, it
indicates an unlikely error during thread initialization.)

To transform a three-dimensional array in-place with C, you might do:

fftw_complex arr[L][M][N]; fftw_plan plan; plan = fftw_plan_dft_3d(L,M,N, arr,arr, FFTW_FORWARD, FFTW_ESTIMATE); fftw_execute(plan); fftw_destroy_plan(plan);

In Fortran, you would use this instead:

double complex arr dimension arr(L,M,N) integer*8 plan call dfftw_plan_dft_3d(plan, L,M,N, arr,arr, & FFTW_FORWARD, FFTW_ESTIMATE) call dfftw_execute_dft(plan, arr, arr) call dfftw_destroy_plan(plan)

Note that we pass the array dimensions in the “natural” order in both C and Fortran.

To transform a one-dimensional real array in Fortran, you might do:

double precision in dimension in(N) double complex out dimension out(N/2 + 1) integer*8 plan call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE) call dfftw_execute_dft_r2c(plan, in, out) call dfftw_destroy_plan(plan)

To transform a two-dimensional real array, out of place, you might use the following:

double precision in dimension in(M,N) double complex out dimension out(M/2 + 1, N) integer*8 plan call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE) call dfftw_execute_dft_r2c(plan, in, out) call dfftw_destroy_plan(plan)

**Important:** Notice that it is the *first* dimension of the
complex output array that is cut in half in Fortran, rather than the
last dimension as in C. This is a consequence of the interface routines
reversing the order of the array dimensions passed to FFTW so that the
Fortran program can use its ordinary column-major order.