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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</title>
Chris@42 26
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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="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" rel="next" title="Dynamic Arrays in C-The Wrong Way">
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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"></a>
Chris@42 73 <div class="header">
Chris@42 74 <p>
Chris@42 75 Next: <a href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" accesskey="n" rel="next">Dynamic Arrays in C-The Wrong Way</a>, Previous: <a href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C" accesskey="p" rel="prev">Fixed-size 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-1"></a>
Chris@42 79 <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
Chris@42 80
Chris@42 81 <p>We recommend allocating most arrays dynamically, with
Chris@42 82 <code>fftw_malloc</code>. This isn&rsquo;t too hard to do, although it is not as
Chris@42 83 straightforward for multi-dimensional arrays as it is for
Chris@42 84 one-dimensional arrays.
Chris@42 85 </p>
Chris@42 86 <p>Creating the array is simple: using a dynamic-allocation routine like
Chris@42 87 <code>fftw_malloc</code>, allocate an array big enough to store N
Chris@42 88 <code>fftw_complex</code> values (for a complex DFT), where N is the product
Chris@42 89 of the sizes of the array dimensions (i.e. the total number of complex
Chris@42 90 values in the array). For example, here is code to allocate a
Chris@42 91 5&nbsp;&times;&nbsp;12&nbsp;&times;&nbsp;27 rank-3 array:
Chris@42 92 <a name="index-fftw_005fmalloc-2"></a>
Chris@42 93 </p>
Chris@42 94 <div class="example">
Chris@42 95 <pre class="example">fftw_complex *an_array;
Chris@42 96 an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
Chris@42 97 </pre></div>
Chris@42 98
Chris@42 99 <p>Accessing the array elements, however, is more tricky&mdash;you can&rsquo;t
Chris@42 100 simply use multiple applications of the &lsquo;<samp>[]</samp>&rsquo; operator like you
Chris@42 101 could for fixed-size arrays. Instead, you have to explicitly compute
Chris@42 102 the offset into the array using the formula given earlier for
Chris@42 103 row-major arrays. For example, to reference the <em>(i,j,k)</em>-th
Chris@42 104 element of the array allocated above, you would use the expression
Chris@42 105 <code>an_array[k + 27 * (j + 12 * i)]</code>.
Chris@42 106 </p>
Chris@42 107 <p>This pain can be alleviated somewhat by defining appropriate macros,
Chris@42 108 or, in C++, creating a class and overloading the &lsquo;<samp>()</samp>&rsquo; operator.
Chris@42 109 The recent C99 standard provides a way to reinterpret the dynamic
Chris@42 110 array as a &ldquo;variable-length&rdquo; multi-dimensional array amenable to
Chris@42 111 &lsquo;<samp>[]</samp>&rsquo;, but this feature is not yet widely supported by compilers.
Chris@42 112 <a name="index-C99"></a>
Chris@42 113 <a name="index-C_002b_002b-2"></a>
Chris@42 114 </p>
Chris@42 115
Chris@42 116
Chris@42 117
Chris@42 118 </body>
Chris@42 119 </html>