109 lines
4.7 KiB
HTML
109 lines
4.7 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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<title>Dynamic Arrays in C-The Wrong Way (FFTW 3.3.10)</title>
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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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<span id="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></span><div class="header">
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<p>
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Previous: <a href="Dynamic-Arrays-in-C.html" accesskey="p" rel="prev">Dynamic 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_002d_002d_002dThe-Wrong-Way"></span><h4 class="subsection">3.2.5 Dynamic Arrays in C—The Wrong Way</h4>
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<p>A different method for allocating multi-dimensional arrays in C is
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often suggested that is incompatible with FFTW: <em>using it will
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cause FFTW to die a painful death</em>. We discuss the technique here,
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however, because it is so commonly known and used. This method is to
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create arrays of pointers of arrays of pointers of …etcetera.
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For example, the analogue in this method to the example above is:
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</p>
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<div class="example">
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<pre class="example">int i,j;
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fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
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a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
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for (i = 0; i < 5; ++i) {
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a_bad_array[i] =
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(fftw_complex **) malloc(12 * sizeof(fftw_complex *));
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for (j = 0; j < 12; ++j)
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a_bad_array[i][j] =
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(fftw_complex *) malloc(27 * sizeof(fftw_complex));
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}
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</pre></div>
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<p>As you can see, this sort of array is inconvenient to allocate (and
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deallocate). On the other hand, it has the advantage that the
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<em>(i,j,k)</em>-th element can be referenced simply by
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<code>a_bad_array[i][j][k]</code>.
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</p>
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<p>If you like this technique and want to maximize convenience in accessing
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the array, but still want to pass the array to FFTW, you can use a
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hybrid method. Allocate the array as one contiguous block, but also
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declare an array of arrays of pointers that point to appropriate places
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in the block. That sort of trick is beyond the scope of this
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documentation; for more information on multi-dimensional arrays in C,
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see the <code>comp.lang.c</code>
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<a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
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</p>
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