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date Wed, 20 Mar 2013 15:35:50 +0000
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1 <html lang="en">
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3 <title>Fortran Examples - FFTW 3.3.3</title>
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49 <a name="Fortran-Examples"></a>
50 <p>
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-Legacy-Fortran.html#Calling-FFTW-from-Legacy-Fortran">Calling FFTW from Legacy Fortran</a>
54 <hr>
55 </div>
56
57 <h3 class="section">8.4 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-587"></a><a name="index-dfftw_005fexecute_005fdft-588"></a><a name="index-dfftw_005fdestroy_005fplan-589"></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"> integer iret
87 call dfftw_init_threads(iret)
88 call dfftw_plan_with_nthreads(8)
89 </pre>
90 <p><a name="index-dfftw_005finit_005fthreads-590"></a><a name="index-dfftw_005fplan_005fwith_005fnthreads-591"></a>
91 (You might want to check the value of <code>iret</code>: if it is zero, it
92 indicates an unlikely error during thread initialization.)
93
94 <p>To transform a three-dimensional array in-place with C, you might do:
95
96 <pre class="example"> fftw_complex arr[L][M][N];
97 fftw_plan plan;
98
99 plan = fftw_plan_dft_3d(L,M,N, arr,arr,
100 FFTW_FORWARD, FFTW_ESTIMATE);
101 fftw_execute(plan);
102 fftw_destroy_plan(plan);
103 </pre>
104 <p>In Fortran, you would use this instead:
105
106 <pre class="example"> double complex arr
107 dimension arr(L,M,N)
108 integer*8 plan
109
110 call dfftw_plan_dft_3d(plan, L,M,N, arr,arr,
111 &amp; FFTW_FORWARD, FFTW_ESTIMATE)
112 call dfftw_execute_dft(plan, arr, arr)
113 call dfftw_destroy_plan(plan)
114 </pre>
115 <p><a name="index-dfftw_005fplan_005fdft_005f3d-592"></a>
116 Note that we pass the array dimensions in the &ldquo;natural&rdquo; order in both C
117 and Fortran.
118
119 <p>To transform a one-dimensional real array in Fortran, you might do:
120
121 <pre class="example"> double precision in
122 dimension in(N)
123 double complex out
124 dimension out(N/2 + 1)
125 integer*8 plan
126
127 call dfftw_plan_dft_r2c_1d(plan,N,in,out,FFTW_ESTIMATE)
128 call dfftw_execute_dft_r2c(plan, in, out)
129 call dfftw_destroy_plan(plan)
130 </pre>
131 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f1d-593"></a><a name="index-dfftw_005fexecute_005fdft_005fr2c-594"></a>
132 To transform a two-dimensional real array, out of place, you might use
133 the following:
134
135 <pre class="example"> double precision in
136 dimension in(M,N)
137 double complex out
138 dimension out(M/2 + 1, N)
139 integer*8 plan
140
141 call dfftw_plan_dft_r2c_2d(plan,M,N,in,out,FFTW_ESTIMATE)
142 call dfftw_execute_dft_r2c(plan, in, out)
143 call dfftw_destroy_plan(plan)
144 </pre>
145 <p><a name="index-dfftw_005fplan_005fdft_005fr2c_005f2d-595"></a>
146 <strong>Important:</strong> Notice that it is the <em>first</em> dimension of the
147 complex output array that is cut in half in Fortran, rather than the
148 last dimension as in C. This is a consequence of the interface routines
149 reversing the order of the array dimensions passed to FFTW so that the
150 Fortran program can use its ordinary column-major order.
151 <a name="index-column_002dmajor-596"></a><a name="index-r2c_002fc2r-multi_002ddimensional-array-format-597"></a>
152 <!-- -->
153
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