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author Chris Cannam <cannam@all-day-breakfast.com>
date Mon, 02 Mar 2020 14:03:47 +0000
parents 7867fa7e1b6b
children
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cannam@127 24 <head>
cannam@127 25 <title>FFTW 3.3.5: Dynamic Arrays in C</title>
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cannam@127 72 <a name="Dynamic-Arrays-in-C"></a>
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cannam@127 74 <p>
cannam@127 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>
cannam@127 76 </div>
cannam@127 77 <hr>
cannam@127 78 <a name="Dynamic-Arrays-in-C-1"></a>
cannam@127 79 <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
cannam@127 80
cannam@127 81 <p>We recommend allocating most arrays dynamically, with
cannam@127 82 <code>fftw_malloc</code>. This isn&rsquo;t too hard to do, although it is not as
cannam@127 83 straightforward for multi-dimensional arrays as it is for
cannam@127 84 one-dimensional arrays.
cannam@127 85 </p>
cannam@127 86 <p>Creating the array is simple: using a dynamic-allocation routine like
cannam@127 87 <code>fftw_malloc</code>, allocate an array big enough to store N
cannam@127 88 <code>fftw_complex</code> values (for a complex DFT), where N is the product
cannam@127 89 of the sizes of the array dimensions (i.e. the total number of complex
cannam@127 90 values in the array). For example, here is code to allocate a
cannam@127 91 5&nbsp;&times;&nbsp;12&nbsp;&times;&nbsp;27 rank-3 array:
cannam@127 92 <a name="index-fftw_005fmalloc-2"></a>
cannam@127 93 </p>
cannam@127 94 <div class="example">
cannam@127 95 <pre class="example">fftw_complex *an_array;
cannam@127 96 an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
cannam@127 97 </pre></div>
cannam@127 98
cannam@127 99 <p>Accessing the array elements, however, is more tricky&mdash;you can&rsquo;t
cannam@127 100 simply use multiple applications of the &lsquo;<samp>[]</samp>&rsquo; operator like you
cannam@127 101 could for fixed-size arrays. Instead, you have to explicitly compute
cannam@127 102 the offset into the array using the formula given earlier for
cannam@127 103 row-major arrays. For example, to reference the <em>(i,j,k)</em>-th
cannam@127 104 element of the array allocated above, you would use the expression
cannam@127 105 <code>an_array[k + 27 * (j + 12 * i)]</code>.
cannam@127 106 </p>
cannam@127 107 <p>This pain can be alleviated somewhat by defining appropriate macros,
cannam@127 108 or, in C++, creating a class and overloading the &lsquo;<samp>()</samp>&rsquo; operator.
cannam@127 109 The recent C99 standard provides a way to reinterpret the dynamic
cannam@127 110 array as a &ldquo;variable-length&rdquo; multi-dimensional array amenable to
cannam@127 111 &lsquo;<samp>[]</samp>&rsquo;, but this feature is not yet widely supported by compilers.
cannam@127 112 <a name="index-C99"></a>
cannam@127 113 <a name="index-C_002b_002b-2"></a>
cannam@127 114 </p>
cannam@127 115
cannam@127 116
cannam@127 117
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