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Add FFTW3
author Chris Cannam <cannam@all-day-breakfast.com>
date Wed, 20 Mar 2013 15:35:50 +0000
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cannam@95 3 <title>Fortran Examples - FFTW 3.3.3</title>
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cannam@95 13 This manual is for FFTW
cannam@95 14 (version 3.3.3, 25 November 2012).
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cannam@95 16 Copyright (C) 2003 Matteo Frigo.
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cannam@95 18 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@95 48 <div class="node">
cannam@95 49 <a name="Fortran-Examples"></a>
cannam@95 50 <p>
cannam@95 51 Next:&nbsp;<a rel="next" accesskey="n" href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>,
cannam@95 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>,
cannam@95 53 Up:&nbsp;<a rel="up" accesskey="u" href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>
cannam@95 54 <hr>
cannam@95 55 </div>
cannam@95 56
cannam@95 57 <h3 class="section">8.4 Fortran Examples</h3>
cannam@95 58
cannam@95 59 <p>In C, you might have something like the following to transform a
cannam@95 60 one-dimensional complex array:
cannam@95 61
cannam@95 62 <pre class="example"> fftw_complex in[N], out[N];
cannam@95 63 fftw_plan plan;
cannam@95 64
cannam@95 65 plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
cannam@95 66 fftw_execute(plan);
cannam@95 67 fftw_destroy_plan(plan);
cannam@95 68 </pre>
cannam@95 69 <p>In Fortran, you would use the following to accomplish the same thing:
cannam@95 70
cannam@95 71 <pre class="example"> double complex in, out
cannam@95 72 dimension in(N), out(N)
cannam@95 73 integer*8 plan
cannam@95 74
cannam@95 75 call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
cannam@95 76 call dfftw_execute_dft(plan, in, out)
cannam@95 77 call dfftw_destroy_plan(plan)
cannam@95 78 </pre>
cannam@95 79 <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>
cannam@95 80 Notice how all routines are called as Fortran subroutines, and the
cannam@95 81 plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
cannam@95 82 Notice also that we changed <code>fftw_execute</code> to
cannam@95 83 <code>dfftw_execute_dft</code> (see <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>). To do
cannam@95 84 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:
cannam@95 85
cannam@95 86 <pre class="example"> integer iret
cannam@95 87 call dfftw_init_threads(iret)
cannam@95 88 call dfftw_plan_with_nthreads(8)
cannam@95 89 </pre>
cannam@95 90 <p><a name="index-dfftw_005finit_005fthreads-590"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-591"></a>
cannam@95 91 (You might want to check the value of <code>iret</code>: if it is zero, it
cannam@95 92 indicates an unlikely error during thread initialization.)
cannam@95 93
cannam@95 94 <p>To transform a three-dimensional array in-place with C, you might do:
cannam@95 95
cannam@95 96 <pre class="example"> fftw_complex arr[L][M][N];
cannam@95 97 fftw_plan plan;
cannam@95 98
cannam@95 99 plan = fftw_plan_dft_3d(L,M,N, arr,arr,
cannam@95 100 FFTW_FORWARD, FFTW_ESTIMATE);
cannam@95 101 fftw_execute(plan);
cannam@95 102 fftw_destroy_plan(plan);
cannam@95 103 </pre>
cannam@95 104 <p>In Fortran, you would use this instead:
cannam@95 105
cannam@95 106 <pre class="example"> double complex arr
cannam@95 107 dimension arr(L,M,N)
cannam@95 108 integer*8 plan
cannam@95 109
cannam@95 110 call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
cannam@95 111 &amp; FFTW_FORWARD, FFTW_ESTIMATE)
cannam@95 112 call dfftw_execute_dft(plan, arr, arr)
cannam@95 113 call dfftw_destroy_plan(plan)
cannam@95 114 </pre>
cannam@95 115 <p><a name="index-dfftw_005fplan_005fdft_005f3d-592"></a>
cannam@95 116 Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
cannam@95 117 and Fortran.
cannam@95 118
cannam@95 119 <p>To transform a one-dimensional real array in Fortran, you might do:
cannam@95 120
cannam@95 121 <pre class="example"> double precision in
cannam@95 122 dimension in(N)
cannam@95 123 double complex out
cannam@95 124 dimension out(N/2 + 1)
cannam@95 125 integer*8 plan
cannam@95 126
cannam@95 127 call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
cannam@95 128 call dfftw_execute_dft_r2c(plan, in, out)
cannam@95 129 call dfftw_destroy_plan(plan)
cannam@95 130 </pre>
cannam@95 131 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-593"></a><a name="index-dfftw_005fexecute_005fdft_005fr2c-594"></a>
cannam@95 132 To transform a two-dimensional real array, out of place, you might use
cannam@95 133 the following:
cannam@95 134
cannam@95 135 <pre class="example"> double precision in
cannam@95 136 dimension in(M,N)
cannam@95 137 double complex out
cannam@95 138 dimension out(M/2 + 1, N)
cannam@95 139 integer*8 plan
cannam@95 140
cannam@95 141 call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
cannam@95 142 call dfftw_execute_dft_r2c(plan, in, out)
cannam@95 143 call dfftw_destroy_plan(plan)
cannam@95 144 </pre>
cannam@95 145 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-595"></a>
cannam@95 146 <strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
cannam@95 147 complex output array that is cut in half in Fortran, rather than the
cannam@95 148 last dimension as in C. This is a consequence of the interface routines
cannam@95 149 reversing the order of the array dimensions passed to FFTW so that the
cannam@95 150 Fortran program can use its ordinary column-major order.
cannam@95 151 <a name="index-column_002dmajor-596"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-597"></a>
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