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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 37bf6b4a2645
children
rev   line source
Chris@10 1 <html lang="en">
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Chris@10 3 <title>Fortran Examples - FFTW 3.3.3</title>
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Chris@10 13 This manual is for FFTW
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Chris@10 16 Copyright (C) 2003 Matteo Frigo.
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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
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Chris@10 155