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<title>Fortran Examples - FFTW 3.1.2</title>
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<p>
<a name="Fortran-Examples"></a>
Next: <a rel="next" accesskey="n" href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>,
Previous: <a rel="previous" accesskey="p" href="FFTW-Constants-in-Fortran.html#FFTW-Constants-in-Fortran">FFTW Constants in Fortran</a>,
Up: <a rel="up" accesskey="u" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>
<hr>
</div>
<h3 class="section">6.3 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(plan)
call dfftw_destroy_plan(plan)
</pre>
<p><a name="index-dfftw_005fplan_005fdft_005f1d-326"></a><a name="index-dfftw_005fexecute-327"></a><a name="index-dfftw_005fdestroy_005fplan-328"></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>. 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"> call dfftw_init_threads
call dfftw_plan_with_nthreads(8)
</pre>
<p><a name="index-dfftw_005finit_005fthreads-329"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-330"></a>
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,
& FFTW_FORWARD, FFTW_ESTIMATE)
call dfftw_execute(plan)
call dfftw_destroy_plan(plan)
</pre>
<p><a name="index-dfftw_005fplan_005fdft_005f3d-331"></a>
Note that we pass the array dimensions in the “natural” 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(plan)
call dfftw_destroy_plan(plan)
</pre>
<p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-332"></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(plan)
call dfftw_destroy_plan(plan)
</pre>
<p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-333"></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-334"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-335"></a>
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