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7.4 Fortran Examples

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In C, you might have something like the following to transform a d@0: one-dimensional complex array: d@0: d@0: 

             fftw_complex in[N], out[N];
d@0:              fftw_plan plan;
d@0:      
d@0:              plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
d@0:              fftw_execute(plan);
d@0:              fftw_destroy_plan(plan);
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In Fortran, you would use the following to accomplish the same thing: d@0: d@0: 

             double complex in, out
d@0:              dimension in(N), out(N)
d@0:              integer*8 plan
d@0:      
d@0:              call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
d@0:              call dfftw_execute_dft(plan, in, out)
d@0:              call dfftw_destroy_plan(plan)
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d@0: Notice how all routines are called as Fortran subroutines, and the d@0: plan is returned via the first argument to dfftw_plan_dft_1d. d@0: Notice also that we changed fftw_execute to d@0: dfftw_execute_dft (see FFTW Execution in Fortran). To do d@0: the same thing, but using 8 threads in parallel (see Multi-threaded FFTW), you would simply prefix these calls with: d@0: d@0: 

             call dfftw_init_threads
d@0:              call dfftw_plan_with_nthreads(8)
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d@0: To transform a three-dimensional array in-place with C, you might do: d@0: d@0: 

             fftw_complex arr[L][M][N];
d@0:              fftw_plan plan;
d@0:      
d@0:              plan = fftw_plan_dft_3d(L,M,N, arr,arr,
d@0:                                      FFTW_FORWARD, FFTW_ESTIMATE);
d@0:              fftw_execute(plan);
d@0:              fftw_destroy_plan(plan);
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In Fortran, you would use this instead: d@0: d@0: 

             double complex arr
d@0:              dimension arr(L,M,N)
d@0:              integer*8 plan
d@0:      
d@0:              call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
d@0:             &                       FFTW_FORWARD, FFTW_ESTIMATE)
d@0:              call dfftw_execute_dft(plan, arr, arr)
d@0:              call dfftw_destroy_plan(plan)
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d@0: Note that we pass the array dimensions in the “natural” order in both C d@0: and Fortran. d@0: d@0:

To transform a one-dimensional real array in Fortran, you might do: d@0: d@0: 

             double precision in
d@0:              dimension in(N)
d@0:              double complex out
d@0:              dimension out(N/2 + 1)
d@0:              integer*8 plan
d@0:      
d@0:              call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
d@0:              call dfftw_execute_dft_r2c(plan, in, out)
d@0:              call dfftw_destroy_plan(plan)
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d@0: To transform a two-dimensional real array, out of place, you might use d@0: the following: d@0: d@0: 

             double precision in
d@0:              dimension in(M,N)
d@0:              double complex out
d@0:              dimension out(M/2 + 1, N)
d@0:              integer*8 plan
d@0:      
d@0:              call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
d@0:              call dfftw_execute_dft_r2c(plan, in, out)
d@0:              call dfftw_destroy_plan(plan)
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d@0: Important: Notice that it is the first dimension of the d@0: complex output array that is cut in half in Fortran, rather than the d@0: last dimension as in C. This is a consequence of the interface routines d@0: reversing the order of the array dimensions passed to FFTW so that the d@0: Fortran program can use its ordinary column-major order. d@0: d@0: d@0: d@0: d@0: