### 6.3 Fortran Examples

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

                 call dfftw_init_threads
                 call dfftw_plan_with_nthreads(8)

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(plan)
                 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(plan)
                 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(plan)
                 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.