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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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Chris@10 1 <html lang="en">
Chris@10 2 <head>
Chris@10 3 <title>Fortran Examples - FFTW 3.3.3</title>
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Chris@10 48 <div class="node">
Chris@10 49 <a name="Fortran-Examples"></a>
Chris@10 50 <p>
Chris@10 51 Next:&nbsp;<a rel="next" accesskey="n" href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>,
Chris@10 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>,
Chris@10 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>
Chris@10 54 <hr>
Chris@10 55 </div>
Chris@10 56
Chris@10 57 <h3 class="section">8.4 Fortran Examples</h3>
Chris@10 58
Chris@10 59 <p>In C, you might have something like the following to transform a
Chris@10 60 one-dimensional complex array:
Chris@10 61
Chris@10 62 <pre class="example"> fftw_complex in[N], out[N];
Chris@10 63 fftw_plan plan;
Chris@10 64
Chris@10 65 plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
Chris@10 66 fftw_execute(plan);
Chris@10 67 fftw_destroy_plan(plan);
Chris@10 68 </pre>
Chris@10 69 <p>In Fortran, you would use the following to accomplish the same thing:
Chris@10 70
Chris@10 71 <pre class="example"> double complex in, out
Chris@10 72 dimension in(N), out(N)
Chris@10 73 integer*8 plan
Chris@10 74
Chris@10 75 call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
Chris@10 76 call dfftw_execute_dft(plan, in, out)
Chris@10 77 call dfftw_destroy_plan(plan)
Chris@10 78 </pre>
Chris@10 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>
Chris@10 80 Notice how all routines are called as Fortran subroutines, and the
Chris@10 81 plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
Chris@10 82 Notice also that we changed <code>fftw_execute</code> to
Chris@10 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
Chris@10 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:
Chris@10 85
Chris@10 86 <pre class="example"> integer iret
Chris@10 87 call dfftw_init_threads(iret)
Chris@10 88 call dfftw_plan_with_nthreads(8)
Chris@10 89 </pre>
Chris@10 90 <p><a name="index-dfftw_005finit_005fthreads-590"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-591"></a>
Chris@10 91 (You might want to check the value of <code>iret</code>: if it is zero, it
Chris@10 92 indicates an unlikely error during thread initialization.)
Chris@10 93
Chris@10 94 <p>To transform a three-dimensional array in-place with C, you might do:
Chris@10 95
Chris@10 96 <pre class="example"> fftw_complex arr[L][M][N];
Chris@10 97 fftw_plan plan;
Chris@10 98
Chris@10 99 plan = fftw_plan_dft_3d(L,M,N, arr,arr,
Chris@10 100 FFTW_FORWARD, FFTW_ESTIMATE);
Chris@10 101 fftw_execute(plan);
Chris@10 102 fftw_destroy_plan(plan);
Chris@10 103 </pre>
Chris@10 104 <p>In Fortran, you would use this instead:
Chris@10 105
Chris@10 106 <pre class="example"> double complex arr
Chris@10 107 dimension arr(L,M,N)
Chris@10 108 integer*8 plan
Chris@10 109
Chris@10 110 call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
Chris@10 111 &amp; FFTW_FORWARD, FFTW_ESTIMATE)
Chris@10 112 call dfftw_execute_dft(plan, arr, arr)
Chris@10 113 call dfftw_destroy_plan(plan)
Chris@10 114 </pre>
Chris@10 115 <p><a name="index-dfftw_005fplan_005fdft_005f3d-592"></a>
Chris@10 116 Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
Chris@10 117 and Fortran.
Chris@10 118
Chris@10 119 <p>To transform a one-dimensional real array in Fortran, you might do:
Chris@10 120
Chris@10 121 <pre class="example"> double precision in
Chris@10 122 dimension in(N)
Chris@10 123 double complex out
Chris@10 124 dimension out(N/2 + 1)
Chris@10 125 integer*8 plan
Chris@10 126
Chris@10 127 call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
Chris@10 128 call dfftw_execute_dft_r2c(plan, in, out)
Chris@10 129 call dfftw_destroy_plan(plan)
Chris@10 130 </pre>
Chris@10 131 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-593"></a><a name="index-dfftw_005fexecute_005fdft_005fr2c-594"></a>
Chris@10 132 To transform a two-dimensional real array, out of place, you might use
Chris@10 133 the following:
Chris@10 134
Chris@10 135 <pre class="example"> double precision in
Chris@10 136 dimension in(M,N)
Chris@10 137 double complex out
Chris@10 138 dimension out(M/2 + 1, N)
Chris@10 139 integer*8 plan
Chris@10 140
Chris@10 141 call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
Chris@10 142 call dfftw_execute_dft_r2c(plan, in, out)
Chris@10 143 call dfftw_destroy_plan(plan)
Chris@10 144 </pre>
Chris@10 145 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-595"></a>
Chris@10 146 <strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
Chris@10 147 complex output array that is cut in half in Fortran, rather than the
Chris@10 148 last dimension as in C. This is a consequence of the interface routines
Chris@10 149 reversing the order of the array dimensions passed to FFTW so that the
Chris@10 150 Fortran program can use its ordinary column-major order.
Chris@10 151 <a name="index-column_002dmajor-596"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-597"></a>
Chris@10 152 <!-- -->
Chris@10 153
Chris@10 154 </body></html>
Chris@10 155