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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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cannam@127 3 <!-- This manual is for FFTW
cannam@127 4 (version 3.3.5, 30 July 2016).
cannam@127 5
cannam@127 6 Copyright (C) 2003 Matteo Frigo.
cannam@127 7
cannam@127 8 Copyright (C) 2003 Massachusetts Institute of Technology.
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way</title>
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cannam@127 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
cannam@127 72 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
cannam@127 73 <div class="header">
cannam@127 74 <p>
cannam@127 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Dynamic-Arrays-in-C_002d_002d_002dThe-Wrong-Way"></a>
cannam@127 79 <h4 class="subsection">3.2.5 Dynamic Arrays in C&mdash;The Wrong Way</h4>
cannam@127 80
cannam@127 81 <p>A different method for allocating multi-dimensional arrays in C is
cannam@127 82 often suggested that is incompatible with FFTW: <em>using it will
cannam@127 83 cause FFTW to die a painful death</em>. We discuss the technique here,
cannam@127 84 however, because it is so commonly known and used. This method is to
cannam@127 85 create arrays of pointers of arrays of pointers of &hellip;etcetera.
cannam@127 86 For example, the analogue in this method to the example above is:
cannam@127 87 </p>
cannam@127 88 <div class="example">
cannam@127 89 <pre class="example">int i,j;
cannam@127 90 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
cannam@127 91
cannam@127 92 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
cannam@127 93 for (i = 0; i &lt; 5; ++i) {
cannam@127 94 a_bad_array[i] =
cannam@127 95 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
cannam@127 96 for (j = 0; j &lt; 12; ++j)
cannam@127 97 a_bad_array[i][j] =
cannam@127 98 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
cannam@127 99 }
cannam@127 100 </pre></div>
cannam@127 101
cannam@127 102 <p>As you can see, this sort of array is inconvenient to allocate (and
cannam@127 103 deallocate). On the other hand, it has the advantage that the
cannam@127 104 <em>(i,j,k)</em>-th element can be referenced simply by
cannam@127 105 <code>a_bad_array[i][j][k]</code>.
cannam@127 106 </p>
cannam@127 107 <p>If you like this technique and want to maximize convenience in accessing
cannam@127 108 the array, but still want to pass the array to FFTW, you can use a
cannam@127 109 hybrid method. Allocate the array as one contiguous block, but also
cannam@127 110 declare an array of arrays of pointers that point to appropriate places
cannam@127 111 in the block. That sort of trick is beyond the scope of this
cannam@127 112 documentation; for more information on multi-dimensional arrays in C,
cannam@127 113 see the <code>comp.lang.c</code>
cannam@127 114 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
cannam@127 115 </p>
cannam@127 116
cannam@127 117
cannam@127 118
cannam@127 119 </body>
cannam@127 120 </html>