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Replace these with versions built using an older toolset (so as to avoid ABI compatibilities when linking on Ubuntu 14.04 for packaging purposes)
author Chris Cannam
date Fri, 07 Feb 2020 11:51:13 +0000
parents 2cd0e3b3e1fd
children
rev   line source
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Chris@42 24 <head>
Chris@42 25 <title>FFTW 3.3.5: Dynamic Arrays in C</title>
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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 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>