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Tidy up for ROLI
author Geogaddi\David <d.m.ronan@qmul.ac.uk>
date Tue, 17 May 2016 18:50:19 +0100
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1 <html lang="en">
2 <head>
3 <title>Fortran Examples - FFTW 3.2.1</title>
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48 <div class="node">
49 <p>
50 <a name="Fortran-Examples"></a>
51 Next:&nbsp;<a rel="next" accesskey="n" href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f">Wisdom of Fortran?</a>,
52 Previous:&nbsp;<a rel="previous" accesskey="p" href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>,
53 Up:&nbsp;<a rel="up" accesskey="u" href="Calling-FFTW-from-Fortran.html#Calling-FFTW-from-Fortran">Calling FFTW from Fortran</a>
54 <hr>
55 </div>
56
57 <h3 class="section">7.4 Fortran Examples</h3>
58
59 <p>In C, you might have something like the following to transform a
60 one-dimensional complex array:
61
62 <pre class="example"> fftw_complex in[N], out[N];
63 fftw_plan plan;
64
65 plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
66 fftw_execute(plan);
67 fftw_destroy_plan(plan);
68 </pre>
69 <p>In Fortran, you would use the following to accomplish the same thing:
70
71 <pre class="example"> double complex in, out
72 dimension in(N), out(N)
73 integer*8 plan
74
75 call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
76 call dfftw_execute_dft(plan, in, out)
77 call dfftw_destroy_plan(plan)
78 </pre>
79 <p><a name="index-dfftw_005fplan_005fdft_005f1d-341"></a><a name="index-dfftw_005fexecute_005fdft-342"></a><a name="index-dfftw_005fdestroy_005fplan-343"></a>
80 Notice how all routines are called as Fortran subroutines, and the
81 plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
82 Notice also that we changed <code>fftw_execute</code> to
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
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:
85
86 <pre class="example"> call dfftw_init_threads
87 call dfftw_plan_with_nthreads(8)
88 </pre>
89 <p><a name="index-dfftw_005finit_005fthreads-344"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-345"></a>
90 To transform a three-dimensional array in-place with C, you might do:
91
92 <pre class="example"> fftw_complex arr[L][M][N];
93 fftw_plan plan;
94
95 plan = fftw_plan_dft_3d(L,M,N, arr,arr,
96 FFTW_FORWARD, FFTW_ESTIMATE);
97 fftw_execute(plan);
98 fftw_destroy_plan(plan);
99 </pre>
100 <p>In Fortran, you would use this instead:
101
102 <pre class="example"> double complex arr
103 dimension arr(L,M,N)
104 integer*8 plan
105
106 call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
107 &amp; FFTW_FORWARD, FFTW_ESTIMATE)
108 call dfftw_execute_dft(plan, arr, arr)
109 call dfftw_destroy_plan(plan)
110 </pre>
111 <p><a name="index-dfftw_005fplan_005fdft_005f3d-346"></a>
112 Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
113 and Fortran.
114
115 <p>To transform a one-dimensional real array in Fortran, you might do:
116
117 <pre class="example"> double precision in
118 dimension in(N)
119 double complex out
120 dimension out(N/2 + 1)
121 integer*8 plan
122
123 call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
124 call dfftw_execute_dft_r2c(plan, in, out)
125 call dfftw_destroy_plan(plan)
126 </pre>
127 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-347"></a><a name="index-dfftw_005fexecute_005fdft_005fr2c-348"></a>
128 To transform a two-dimensional real array, out of place, you might use
129 the following:
130
131 <pre class="example"> double precision in
132 dimension in(M,N)
133 double complex out
134 dimension out(M/2 + 1, N)
135 integer*8 plan
136
137 call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
138 call dfftw_execute_dft_r2c(plan, in, out)
139 call dfftw_destroy_plan(plan)
140 </pre>
141 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-349"></a>
142 <strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
143 complex output array that is cut in half in Fortran, rather than the
144 last dimension as in C. This is a consequence of the interface routines
145 reversing the order of the array dimensions passed to FFTW so that the
146 Fortran program can use its ordinary column-major order.
147 <a name="index-column_002dmajor-350"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-351"></a>
148 <!-- -->
149
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151