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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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3 <!-- This manual is for FFTW
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6 Copyright (C) 2003 Matteo Frigo.
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35 <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
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72 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
73 <div class="header">
74 <p>
75 Previous: <a href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" accesskey="p" rel="prev">Dynamic Arrays in C</a>, Up: <a href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" accesskey="u" rel="up">Multi-dimensional Array Format</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="Dynamic-Arrays-in-C_002d_002d_002dThe-Wrong-Way"></a>
79 <h4 class="subsection">3.2.5 Dynamic Arrays in C&mdash;The Wrong Way</h4>
80
81 <p>A different method for allocating multi-dimensional arrays in C is
82 often suggested that is incompatible with FFTW: <em>using it will
83 cause FFTW to die a painful death</em>. We discuss the technique here,
84 however, because it is so commonly known and used. This method is to
85 create arrays of pointers of arrays of pointers of &hellip;etcetera.
86 For example, the analogue in this method to the example above is:
87 </p>
88 <div class="example">
89 <pre class="example">int i,j;
90 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
91
92 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
93 for (i = 0; i &lt; 5; ++i) {
94 a_bad_array[i] =
95 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
96 for (j = 0; j &lt; 12; ++j)
97 a_bad_array[i][j] =
98 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
99 }
100 </pre></div>
101
102 <p>As you can see, this sort of array is inconvenient to allocate (and
103 deallocate). On the other hand, it has the advantage that the
104 <em>(i,j,k)</em>-th element can be referenced simply by
105 <code>a_bad_array[i][j][k]</code>.
106 </p>
107 <p>If you like this technique and want to maximize convenience in accessing
108 the array, but still want to pass the array to FFTW, you can use a
109 hybrid method. Allocate the array as one contiguous block, but also
110 declare an array of arrays of pointers that point to appropriate places
111 in the block. That sort of trick is beyond the scope of this
112 documentation; for more information on multi-dimensional arrays in C,
113 see the <code>comp.lang.c</code>
114 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
115 </p>
116
117
118
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120 </html>