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108 lines
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108 lines
4.9 KiB
HTML
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<!-- This manual is for FFTW
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(version 3.3.10, 10 December 2020).
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Copyright (C) 2003 Matteo Frigo.
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Copyright (C) 2003 Massachusetts Institute of Technology.
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Permission is granted to make and distribute verbatim copies of this
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manual provided the copyright notice and this permission notice are
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preserved on all copies.
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Permission is granted to copy and distribute modified versions of this
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entire resulting derived work is distributed under the terms of a
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permission notice identical to this one.
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Permission is granted to copy and distribute translations of this manual
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approved by the Free Software Foundation. -->
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<title>Dynamic Arrays in C (FFTW 3.3.10)</title>
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<meta name="description" content="Dynamic Arrays in C (FFTW 3.3.10)">
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<link href="index.html" rel="start" title="Top">
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Multi_002ddimensional-Array-Format.html" rel="up" title="Multi-dimensional Array Format">
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<link href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html" rel="next" title="Dynamic Arrays in C-The Wrong Way">
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<link href="Fixed_002dsize-Arrays-in-C.html" rel="prev" title="Fixed-size Arrays in C">
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<span id="Dynamic-Arrays-in-C"></span><div class="header">
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<p>
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Next: <a href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html" accesskey="n" rel="next">Dynamic Arrays in C-The Wrong Way</a>, Previous: <a href="Fixed_002dsize-Arrays-in-C.html" accesskey="p" rel="prev">Fixed-size Arrays in C</a>, Up: <a href="Multi_002ddimensional-Array-Format.html" accesskey="u" rel="up">Multi-dimensional Array Format</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<span id="Dynamic-Arrays-in-C-1"></span><h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
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<p>We recommend allocating most arrays dynamically, with
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<code>fftw_malloc</code>. This isn’t too hard to do, although it is not as
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straightforward for multi-dimensional arrays as it is for
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one-dimensional arrays.
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</p>
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<p>Creating the array is simple: using a dynamic-allocation routine like
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<code>fftw_malloc</code>, allocate an array big enough to store N
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<code>fftw_complex</code> values (for a complex DFT), where N is the product
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of the sizes of the array dimensions (i.e. the total number of complex
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values in the array). For example, here is code to allocate a
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5 × 12 × 27
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rank-3 array:
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<span id="index-fftw_005fmalloc-2"></span>
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</p>
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<div class="example">
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<pre class="example">fftw_complex *an_array;
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an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
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</pre></div>
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<p>Accessing the array elements, however, is more tricky—you can’t
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simply use multiple applications of the ‘<samp>[]</samp>’ operator like you
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could for fixed-size arrays. Instead, you have to explicitly compute
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the offset into the array using the formula given earlier for
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row-major arrays. For example, to reference the <em>(i,j,k)</em>-th
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element of the array allocated above, you would use the expression
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<code>an_array[k + 27 * (j + 12 * i)]</code>.
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</p>
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<p>This pain can be alleviated somewhat by defining appropriate macros,
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or, in C++, creating a class and overloading the ‘<samp>()</samp>’ operator.
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The recent C99 standard provides a way to reinterpret the dynamic
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array as a “variable-length” multi-dimensional array amenable to
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‘<samp>[]</samp>’, but this feature is not yet widely supported by compilers.
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<span id="index-C99"></span>
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<span id="index-C_002b_002b-2"></span>
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</p>
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