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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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manual under the conditions for verbatim copying, provided that the
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Fortran Examples (FFTW 3.3.10)</title>
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<meta name="description" content="Fortran Examples (FFTW 3.3.10)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Calling-FFTW-from-Legacy-Fortran.html" rel="up" title="Calling FFTW from Legacy Fortran">
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<link href="Wisdom-of-Fortran_003f.html" rel="next" title="Wisdom of Fortran?">
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<link href="FFTW-Execution-in-Fortran.html" rel="prev" title="FFTW Execution in Fortran">
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</head>
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<body lang="en">
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<span id="Fortran-Examples"></span><div class="header">
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<p>
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Next: <a href="Wisdom-of-Fortran_003f.html" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Fortran-Examples-1"></span><h3 class="section">8.4 Fortran Examples</h3>
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<p>In C, you might have something like the following to transform a
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one-dimensional complex array:
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</p>
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<div class="example">
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<pre class="example"> fftw_complex in[N], out[N];
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fftw_plan plan;
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plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
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fftw_execute(plan);
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fftw_destroy_plan(plan);
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</pre></div>
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<p>In Fortran, you would use the following to accomplish the same thing:
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</p>
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<div class="example">
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<pre class="example"> double complex in, out
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dimension in(N), out(N)
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integer*8 plan
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call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
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call dfftw_execute_dft(plan, in, out)
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call dfftw_destroy_plan(plan)
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</pre></div>
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<span id="index-dfftw_005fplan_005fdft_005f1d"></span>
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<span id="index-dfftw_005fexecute_005fdft-1"></span>
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<span id="index-dfftw_005fdestroy_005fplan"></span>
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<p>Notice how all routines are called as Fortran subroutines, and the
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plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
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Notice also that we changed <code>fftw_execute</code> to
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<code>dfftw_execute_dft</code> (see <a href="FFTW-Execution-in-Fortran.html">FFTW Execution in Fortran</a>). To do
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the same thing, but using 8 threads in parallel (see <a href="Multi_002dthreaded-FFTW.html">Multi-threaded FFTW</a>), you would simply prefix these calls with:
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</p>
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<div class="example">
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<pre class="example"> integer iret
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call dfftw_init_threads(iret)
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call dfftw_plan_with_nthreads(8)
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</pre></div>
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<span id="index-dfftw_005finit_005fthreads"></span>
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<span id="index-dfftw_005fplan_005fwith_005fnthreads"></span>
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<p>(You might want to check the value of <code>iret</code>: if it is zero, it
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indicates an unlikely error during thread initialization.)
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</p>
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<p>To check the number of threads currently being used by the planner, you
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can do the following:
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</p>
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<div class="example">
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<pre class="example"> integer iret
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call dfftw_planner_nthreads(iret)
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</pre></div>
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<span id="index-dfftw_005fplanner_005fnthreads"></span>
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<p>To transform a three-dimensional array in-place with C, you might do:
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</p>
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<div class="example">
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<pre class="example"> fftw_complex arr[L][M][N];
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fftw_plan plan;
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plan = fftw_plan_dft_3d(L,M,N, arr,arr,
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FFTW_FORWARD, FFTW_ESTIMATE);
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fftw_execute(plan);
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fftw_destroy_plan(plan);
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</pre></div>
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<p>In Fortran, you would use this instead:
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</p>
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<div class="example">
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<pre class="example"> double complex arr
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dimension arr(L,M,N)
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integer*8 plan
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call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
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& FFTW_FORWARD, FFTW_ESTIMATE)
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call dfftw_execute_dft(plan, arr, arr)
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call dfftw_destroy_plan(plan)
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</pre></div>
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<span id="index-dfftw_005fplan_005fdft_005f3d"></span>
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<p>Note that we pass the array dimensions in the “natural” order in both C
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and Fortran.
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</p>
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<p>To transform a one-dimensional real array in Fortran, you might do:
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</p>
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<div class="example">
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<pre class="example"> double precision in
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dimension in(N)
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double complex out
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dimension out(N/2 + 1)
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integer*8 plan
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call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
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call dfftw_execute_dft_r2c(plan, in, out)
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call dfftw_destroy_plan(plan)
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</pre></div>
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<span id="index-dfftw_005fplan_005fdft_005fr2c_005f1d"></span>
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<span id="index-dfftw_005fexecute_005fdft_005fr2c"></span>
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<p>To transform a two-dimensional real array, out of place, you might use
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the following:
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</p>
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<div class="example">
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<pre class="example"> double precision in
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dimension in(M,N)
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double complex out
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dimension out(M/2 + 1, N)
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integer*8 plan
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call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
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call dfftw_execute_dft_r2c(plan, in, out)
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call dfftw_destroy_plan(plan)
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</pre></div>
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<span id="index-dfftw_005fplan_005fdft_005fr2c_005f2d"></span>
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<p><strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
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complex output array that is cut in half in Fortran, rather than the
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last dimension as in C. This is a consequence of the interface routines
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reversing the order of the array dimensions passed to FFTW so that the
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Fortran program can use its ordinary column-major order.
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<span id="index-column_002dmajor-3"></span>
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<span id="index-r2c_002fc2r-multi_002ddimensional-array-format-3"></span>
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</p>
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<hr>
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<div class="header">
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<p>
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Next: <a href="Wisdom-of-Fortran_003f.html" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html" 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" title="Index" rel="index">Index</a>]</p>
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</div>
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</body>
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</html>
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