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<h3 class="section">8.4 Fortran Examples</h3>

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

<pre class="example">             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);
</pre>
   <p>In Fortran, you would use the following to accomplish the same thing:

<pre class="example">             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)
</pre>
   <p><a name="index-dfftw_005fplan_005fdft_005f1d-587"></a><a name="index-dfftw_005fexecute_005fdft-588"></a><a name="index-dfftw_005fdestroy_005fplan-589"></a>
Notice how all routines are called as Fortran subroutines, and the
plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
Notice also that we changed <code>fftw_execute</code> to
<code>dfftw_execute_dft</code> (see <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>).  To do
the same thing, but using 8 threads in parallel (see <a href="Multi_002dthreaded-FFTW.html#Multi_002dthreaded-FFTW">Multi-threaded FFTW</a>), you would simply prefix these calls with:

<pre class="example">             integer iret
             call dfftw_init_threads(iret)
             call dfftw_plan_with_nthreads(8)
</pre>
   <p><a name="index-dfftw_005finit_005fthreads-590"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-591"></a>
(You might want to check the value of <code>iret</code>: if it is zero, it
indicates an unlikely error during thread initialization.)

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

<pre class="example">             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);
</pre>
   <p>In Fortran, you would use this instead:

<pre class="example">             double complex arr
             dimension arr(L,M,N)
             integer*8 plan

             call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
            &amp;                       FFTW_FORWARD, FFTW_ESTIMATE)
             call dfftw_execute_dft(plan, arr, arr)
             call dfftw_destroy_plan(plan)
</pre>
   <p><a name="index-dfftw_005fplan_005fdft_005f3d-592"></a>
Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
and Fortran.

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

<pre class="example">             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)
</pre>
   <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-593"></a><a name="index-dfftw_005fexecute_005fdft_005fr2c-594"></a>
To transform a two-dimensional real array, out of place, you might use
the following:

<pre class="example">             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)
</pre>
   <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-595"></a>
<strong>Important:</strong> Notice that it is the <em>first</em> 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.
<a name="index-column_002dmajor-596"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-597"></a>
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author	Chris Cannam
date	Wed, 20 Mar 2013 15:35:50 +0000
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