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In C, you might have something like the following to transform a Chris@19: one-dimensional complex array: Chris@19: Chris@19:
fftw_complex in[N], out[N]; Chris@19: fftw_plan plan; Chris@19: Chris@19: plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE); Chris@19: fftw_execute(plan); Chris@19: fftw_destroy_plan(plan); Chris@19:Chris@19:
In Fortran, you would use the following to accomplish the same thing: Chris@19: Chris@19:
double complex in, out Chris@19: dimension in(N), out(N) Chris@19: integer*8 plan Chris@19: Chris@19: call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE) Chris@19: call dfftw_execute_dft(plan, in, out) Chris@19: call dfftw_destroy_plan(plan) Chris@19:Chris@19:
Chris@19: Notice how all routines are called as Fortran subroutines, and the
Chris@19: plan is returned via the first argument to dfftw_plan_dft_1d.
Chris@19: Notice also that we changed fftw_execute to
Chris@19: dfftw_execute_dft (see FFTW Execution in Fortran). To do
Chris@19: the same thing, but using 8 threads in parallel (see Multi-threaded FFTW), you would simply prefix these calls with:
Chris@19:
Chris@19:
integer iret Chris@19: call dfftw_init_threads(iret) Chris@19: call dfftw_plan_with_nthreads(8) Chris@19:Chris@19:
Chris@19: (You might want to check the value of iret: if it is zero, it
Chris@19: indicates an unlikely error during thread initialization.)
Chris@19:
Chris@19:
To transform a three-dimensional array in-place with C, you might do: Chris@19: Chris@19:
fftw_complex arr[L][M][N]; Chris@19: fftw_plan plan; Chris@19: Chris@19: plan = fftw_plan_dft_3d(L,M,N, arr,arr, Chris@19: FFTW_FORWARD, FFTW_ESTIMATE); Chris@19: fftw_execute(plan); Chris@19: fftw_destroy_plan(plan); Chris@19:Chris@19:
In Fortran, you would use this instead: Chris@19: Chris@19:
double complex arr Chris@19: dimension arr(L,M,N) Chris@19: integer*8 plan Chris@19: Chris@19: call dfftw_plan_dft_3d(plan, L,M,N, arr,arr, Chris@19: & FFTW_FORWARD, FFTW_ESTIMATE) Chris@19: call dfftw_execute_dft(plan, arr, arr) Chris@19: call dfftw_destroy_plan(plan) Chris@19:Chris@19:
Chris@19: Note that we pass the array dimensions in the “natural” order in both C Chris@19: and Fortran. Chris@19: Chris@19:
To transform a one-dimensional real array in Fortran, you might do: Chris@19: Chris@19:
double precision in Chris@19: dimension in(N) Chris@19: double complex out Chris@19: dimension out(N/2 + 1) Chris@19: integer*8 plan Chris@19: Chris@19: call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE) Chris@19: call dfftw_execute_dft_r2c(plan, in, out) Chris@19: call dfftw_destroy_plan(plan) Chris@19:Chris@19:
Chris@19: To transform a two-dimensional real array, out of place, you might use Chris@19: the following: Chris@19: Chris@19:
double precision in Chris@19: dimension in(M,N) Chris@19: double complex out Chris@19: dimension out(M/2 + 1, N) Chris@19: integer*8 plan Chris@19: Chris@19: call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE) Chris@19: call dfftw_execute_dft_r2c(plan, in, out) Chris@19: call dfftw_destroy_plan(plan) Chris@19:Chris@19:
Chris@19: Important: Notice that it is the first dimension of the Chris@19: complex output array that is cut in half in Fortran, rather than the Chris@19: last dimension as in C. This is a consequence of the interface routines Chris@19: reversing the order of the array dimensions passed to FFTW so that the Chris@19: Fortran program can use its ordinary column-major order. Chris@19: Chris@19: Chris@19: Chris@19: Chris@19: