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Current fftw source
author Chris Cannam <cannam@all-day-breakfast.com>
date Tue, 18 Oct 2016 13:40:26 +0100
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3 <!-- This manual is for FFTW
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25 <title>FFTW 3.3.5: Fortran Examples</title>
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36 <link href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" rel="up" title="Calling FFTW from Legacy Fortran">
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72 <a name="Fortran-Examples"></a>
73 <div class="header">
74 <p>
75 Next: <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
76 </div>
77 <hr>
78 <a name="Fortran-Examples-1"></a>
79 <h3 class="section">8.4 Fortran Examples</h3>
80
81 <p>In C, you might have something like the following to transform a
82 one-dimensional complex array:
83 </p>
84 <div class="example">
85 <pre class="example"> fftw_complex in[N], out[N];
86 fftw_plan plan;
87
88 plan = fftw_plan_dft_1d(N,in,out,FFTW_FORWARD,FFTW_ESTIMATE);
89 fftw_execute(plan);
90 fftw_destroy_plan(plan);
91 </pre></div>
92
93 <p>In Fortran, you would use the following to accomplish the same thing:
94 </p>
95 <div class="example">
96 <pre class="example"> double complex in, out
97 dimension in(N), out(N)
98 integer*8 plan
99
100 call dfftw_plan_dft_1d(plan,N,in,out,FFTW_FORWARD,FFTW_ESTIMATE)
101 call dfftw_execute_dft(plan, in, out)
102 call dfftw_destroy_plan(plan)
103 </pre></div>
104 <a name="index-dfftw_005fplan_005fdft_005f1d"></a>
105 <a name="index-dfftw_005fexecute_005fdft-1"></a>
106 <a name="index-dfftw_005fdestroy_005fplan"></a>
107
108 <p>Notice how all routines are called as Fortran subroutines, and the
109 plan is returned via the first argument to <code>dfftw_plan_dft_1d</code>.
110 Notice also that we changed <code>fftw_execute</code> to
111 <code>dfftw_execute_dft</code> (see <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran">FFTW Execution in Fortran</a>). To do
112 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:
113 </p>
114 <div class="example">
115 <pre class="example"> integer iret
116 call dfftw_init_threads(iret)
117 call dfftw_plan_with_nthreads(8)
118 </pre></div>
119 <a name="index-dfftw_005finit_005fthreads"></a>
120 <a name="index-dfftw_005fplan_005fwith_005fnthreads"></a>
121
122 <p>(You might want to check the value of <code>iret</code>: if it is zero, it
123 indicates an unlikely error during thread initialization.)
124 </p>
125 <p>To transform a three-dimensional array in-place with C, you might do:
126 </p>
127 <div class="example">
128 <pre class="example"> fftw_complex arr[L][M][N];
129 fftw_plan plan;
130
131 plan = fftw_plan_dft_3d(L,M,N, arr,arr,
132 FFTW_FORWARD, FFTW_ESTIMATE);
133 fftw_execute(plan);
134 fftw_destroy_plan(plan);
135 </pre></div>
136
137 <p>In Fortran, you would use this instead:
138 </p>
139 <div class="example">
140 <pre class="example"> double complex arr
141 dimension arr(L,M,N)
142 integer*8 plan
143
144 call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
145 &amp; FFTW_FORWARD, FFTW_ESTIMATE)
146 call dfftw_execute_dft(plan, arr, arr)
147 call dfftw_destroy_plan(plan)
148 </pre></div>
149 <a name="index-dfftw_005fplan_005fdft_005f3d"></a>
150
151 <p>Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
152 and Fortran.
153 </p>
154 <p>To transform a one-dimensional real array in Fortran, you might do:
155 </p>
156 <div class="example">
157 <pre class="example"> double precision in
158 dimension in(N)
159 double complex out
160 dimension out(N/2 + 1)
161 integer*8 plan
162
163 call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
164 call dfftw_execute_dft_r2c(plan, in, out)
165 call dfftw_destroy_plan(plan)
166 </pre></div>
167 <a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d"></a>
168 <a name="index-dfftw_005fexecute_005fdft_005fr2c"></a>
169
170 <p>To transform a two-dimensional real array, out of place, you might use
171 the following:
172 </p>
173 <div class="example">
174 <pre class="example"> double precision in
175 dimension in(M,N)
176 double complex out
177 dimension out(M/2 + 1, N)
178 integer*8 plan
179
180 call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
181 call dfftw_execute_dft_r2c(plan, in, out)
182 call dfftw_destroy_plan(plan)
183 </pre></div>
184 <a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d"></a>
185
186 <p><strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
187 complex output array that is cut in half in Fortran, rather than the
188 last dimension as in C. This is a consequence of the interface routines
189 reversing the order of the array dimensions passed to FFTW so that the
190 Fortran program can use its ordinary column-major order.
191 <a name="index-column_002dmajor-3"></a>
192 <a name="index-r2c_002fc2r-multi_002ddimensional-array-format-3"></a>
193 </p>
194 <hr>
195 <div class="header">
196 <p>
197 Next: <a href="Wisdom-of-Fortran_003f.html#Wisdom-of-Fortran_003f" accesskey="n" rel="next">Wisdom of Fortran?</a>, Previous: <a href="FFTW-Execution-in-Fortran.html#FFTW-Execution-in-Fortran" accesskey="p" rel="prev">FFTW Execution in Fortran</a>, Up: <a href="Calling-FFTW-from-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran" accesskey="u" rel="up">Calling FFTW from Legacy Fortran</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html#Concept-Index" title="Index" rel="index">Index</a>]</p>
198 </div>
199
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