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<title>FFTW 3.3.8: Fortran Examples</title>
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<a name="Fortran-Examples"></a>
<div class="header">
<p>
Next: <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Fortran-Examples-1"></a>
<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:
</p>
<div class="example">
<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></div>
<p>In Fortran, you would use the following to accomplish the same thing:
</p>
<div class="example">
<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></div>
<a name="index-dfftw_005fplan_005fdft_005f1d"></a>
<a name="index-dfftw_005fexecute_005fdft-1"></a>
<a name="index-dfftw_005fdestroy_005fplan"></a>
<p>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:
</p>
<div class="example">
<pre class="example"> integer iret
call dfftw_init_threads(iret)
call dfftw_plan_with_nthreads(8)
</pre></div>
<a name="index-dfftw_005finit_005fthreads"></a>
<a name="index-dfftw_005fplan_005fwith_005fnthreads"></a>
<p>(You might want to check the value of <code>iret</code>: if it is zero, it
indicates an unlikely error during thread initialization.)
</p>
<p>To transform a three-dimensional array in-place with C, you might do:
</p>
<div class="example">
<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></div>
<p>In Fortran, you would use this instead:
</p>
<div class="example">
<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_dft(plan, arr, arr)
call dfftw_destroy_plan(plan)
</pre></div>
<a name="index-dfftw_005fplan_005fdft_005f3d"></a>
<p>Note that we pass the array dimensions in the “natural” order in both C
and Fortran.
</p>
<p>To transform a one-dimensional real array in Fortran, you might do:
</p>
<div class="example">
<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></div>
<a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d"></a>
<a name="index-dfftw_005fexecute_005fdft_005fr2c"></a>
<p>To transform a two-dimensional real array, out of place, you might use
the following:
</p>
<div class="example">
<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></div>
<a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d"></a>
<p><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-3"></a>
<a name="index-r2c_002fc2r-multi_002ddimensional-array-format-3"></a>
</p>
<hr>
<div class="header">
<p>
Next: <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
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