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Add FFTW3
author Chris Cannam
date Wed, 20 Mar 2013 15:35:50 +0000
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1 <html lang="en">
2 <head>
3 <title>Dynamic Arrays in C-The Wrong Way - FFTW 3.3.3</title>
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8 <link rel="up" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format">
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11 <!--
12 This manual is for FFTW
13 (version 3.3.3, 25 November 2012).
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15 Copyright (C) 2003 Matteo Frigo.
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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48 <a name="Dynamic-Arrays-in-C-The-Wrong-Way"></a>
49 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
50 <p>
51 Previous:&nbsp;<a rel="previous" accesskey="p" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>,
52 Up:&nbsp;<a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>
53 <hr>
54 </div>
55
56 <h4 class="subsection">3.2.5 Dynamic Arrays in C&mdash;The Wrong Way</h4>
57
58 <p>A different method for allocating multi-dimensional arrays in C is
59 often suggested that is incompatible with FFTW: <em>using it will
60 cause FFTW to die a painful death</em>. We discuss the technique here,
61 however, because it is so commonly known and used. This method is to
62 create arrays of pointers of arrays of pointers of <small class="dots">...</small>etcetera.
63 For example, the analogue in this method to the example above is:
64
65 <pre class="example"> int i,j;
66 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
67
68 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
69 for (i = 0; i &lt; 5; ++i) {
70 a_bad_array[i] =
71 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
72 for (j = 0; j &lt; 12; ++j)
73 a_bad_array[i][j] =
74 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
75 }
76 </pre>
77 <p>As you can see, this sort of array is inconvenient to allocate (and
78 deallocate). On the other hand, it has the advantage that the
79 (i,j,k)-th element can be referenced simply by
80 <code>a_bad_array[i][j][k]</code>.
81
82 <p>If you like this technique and want to maximize convenience in accessing
83 the array, but still want to pass the array to FFTW, you can use a
84 hybrid method. Allocate the array as one contiguous block, but also
85 declare an array of arrays of pointers that point to appropriate places
86 in the block. That sort of trick is beyond the scope of this
87 documentation; for more information on multi-dimensional arrays in C,
88 see the <code>comp.lang.c</code>
89 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
90
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93