cannam@127: Next: Wisdom of Fortran?, Previous: FFTW Execution in Fortran, Up: Calling FFTW from Legacy Fortran [Contents][Index]
cannam@127:In C, you might have something like the following to transform a cannam@127: one-dimensional complex array: cannam@127:
cannam@127:fftw_complex in[N], out[N]; cannam@127: fftw_plan plan; cannam@127: cannam@127: plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE); cannam@127: fftw_execute(plan); cannam@127: fftw_destroy_plan(plan); cannam@127:
In Fortran, you would use the following to accomplish the same thing: cannam@127:
cannam@127:double complex in, out cannam@127: dimension in(N), out(N) cannam@127: integer*8 plan cannam@127: cannam@127: call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE) cannam@127: call dfftw_execute_dft(plan, in, out) cannam@127: call dfftw_destroy_plan(plan) cannam@127:
Notice how all routines are called as Fortran subroutines, and the
cannam@127: plan is returned via the first argument to dfftw_plan_dft_1d.
cannam@127: Notice also that we changed fftw_execute to
cannam@127: dfftw_execute_dft (see FFTW Execution in Fortran). To do
cannam@127: the same thing, but using 8 threads in parallel (see Multi-threaded FFTW), you would simply prefix these calls with:
cannam@127:
integer iret cannam@127: call dfftw_init_threads(iret) cannam@127: call dfftw_plan_with_nthreads(8) cannam@127:
(You might want to check the value of iret: if it is zero, it
cannam@127: indicates an unlikely error during thread initialization.)
cannam@127:
To transform a three-dimensional array in-place with C, you might do: cannam@127:
cannam@127:fftw_complex arr[L][M][N]; cannam@127: fftw_plan plan; cannam@127: cannam@127: plan = fftw_plan_dft_3d(L,M,N, arr,arr, cannam@127: FFTW_FORWARD, FFTW_ESTIMATE); cannam@127: fftw_execute(plan); cannam@127: fftw_destroy_plan(plan); cannam@127:
In Fortran, you would use this instead: cannam@127:
cannam@127:double complex arr cannam@127: dimension arr(L,M,N) cannam@127: integer*8 plan cannam@127: cannam@127: call dfftw_plan_dft_3d(plan, L,M,N, arr,arr, cannam@127: & FFTW_FORWARD, FFTW_ESTIMATE) cannam@127: call dfftw_execute_dft(plan, arr, arr) cannam@127: call dfftw_destroy_plan(plan) cannam@127:
Note that we pass the array dimensions in the “natural” order in both C cannam@127: and Fortran. cannam@127:
cannam@127:To transform a one-dimensional real array in Fortran, you might do: cannam@127:
cannam@127:double precision in cannam@127: dimension in(N) cannam@127: double complex out cannam@127: dimension out(N/2 + 1) cannam@127: integer*8 plan cannam@127: cannam@127: call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE) cannam@127: call dfftw_execute_dft_r2c(plan, in, out) cannam@127: call dfftw_destroy_plan(plan) cannam@127:
To transform a two-dimensional real array, out of place, you might use cannam@127: the following: cannam@127:
cannam@127:double precision in cannam@127: dimension in(M,N) cannam@127: double complex out cannam@127: dimension out(M/2 + 1, N) cannam@127: integer*8 plan cannam@127: cannam@127: call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE) cannam@127: call dfftw_execute_dft_r2c(plan, in, out) cannam@127: call dfftw_destroy_plan(plan) cannam@127:
Important: Notice that it is the first dimension of the cannam@127: complex output array that is cut in half in Fortran, rather than the cannam@127: last dimension as in C. This is a consequence of the interface routines cannam@127: reversing the order of the array dimensions passed to FFTW so that the cannam@127: Fortran program can use its ordinary column-major order. cannam@127: cannam@127: cannam@127:
cannam@127:cannam@127: Next: Wisdom of Fortran?, Previous: FFTW Execution in Fortran, Up: Calling FFTW from Legacy Fortran [Contents][Index]
cannam@127: