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Current fftw source
author Chris Cannam
date Tue, 18 Oct 2016 13:40:26 +0100
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Chris@42 3 <!-- This manual is for FFTW
Chris@42 4 (version 3.3.5, 30 July 2016).
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way</title>
Chris@42 26
Chris@42 27 <meta name="description" content="FFTW 3.3.5: Dynamic Arrays in C-The Wrong Way">
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Chris@42 36 <link href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" rel="up" title="Multi-dimensional Array Format">
Chris@42 37 <link href="Words-of-Wisdom_002dSaving-Plans.html#Words-of-Wisdom_002dSaving-Plans" rel="next" title="Words of Wisdom-Saving Plans">
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Chris@42 70
Chris@42 71 <body lang="en" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000">
Chris@42 72 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 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>
Chris@42 76 </div>
Chris@42 77 <hr>
Chris@42 78 <a name="Dynamic-Arrays-in-C_002d_002d_002dThe-Wrong-Way"></a>
Chris@42 79 <h4 class="subsection">3.2.5 Dynamic Arrays in C&mdash;The Wrong Way</h4>
Chris@42 80
Chris@42 81 <p>A different method for allocating multi-dimensional arrays in C is
Chris@42 82 often suggested that is incompatible with FFTW: <em>using it will
Chris@42 83 cause FFTW to die a painful death</em>. We discuss the technique here,
Chris@42 84 however, because it is so commonly known and used. This method is to
Chris@42 85 create arrays of pointers of arrays of pointers of &hellip;etcetera.
Chris@42 86 For example, the analogue in this method to the example above is:
Chris@42 87 </p>
Chris@42 88 <div class="example">
Chris@42 89 <pre class="example">int i,j;
Chris@42 90 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
Chris@42 91
Chris@42 92 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
Chris@42 93 for (i = 0; i &lt; 5; ++i) {
Chris@42 94 a_bad_array[i] =
Chris@42 95 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
Chris@42 96 for (j = 0; j &lt; 12; ++j)
Chris@42 97 a_bad_array[i][j] =
Chris@42 98 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
Chris@42 99 }
Chris@42 100 </pre></div>
Chris@42 101
Chris@42 102 <p>As you can see, this sort of array is inconvenient to allocate (and
Chris@42 103 deallocate). On the other hand, it has the advantage that the
Chris@42 104 <em>(i,j,k)</em>-th element can be referenced simply by
Chris@42 105 <code>a_bad_array[i][j][k]</code>.
Chris@42 106 </p>
Chris@42 107 <p>If you like this technique and want to maximize convenience in accessing
Chris@42 108 the array, but still want to pass the array to FFTW, you can use a
Chris@42 109 hybrid method. Allocate the array as one contiguous block, but also
Chris@42 110 declare an array of arrays of pointers that point to appropriate places
Chris@42 111 in the block. That sort of trick is beyond the scope of this
Chris@42 112 documentation; for more information on multi-dimensional arrays in C,
Chris@42 113 see the <code>comp.lang.c</code>
Chris@42 114 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
Chris@42 115 </p>
Chris@42 116
Chris@42 117
Chris@42 118
Chris@42 119 </body>
Chris@42 120 </html>