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1 <html lang="en">
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2 <head>
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3 <title>Dynamic Arrays in C-The Wrong Way - FFTW 3.3.4</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.4">
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7 <link title="Top" rel="start" href="index.html#Top">
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8 <link rel="up" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format" title="Multi-dimensional Array Format">
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9 <link rel="prev" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C" title="Dynamic Arrays in C">
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10 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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11 <!--
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12 This manual is for FFTW
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13 (version 3.3.4, 20 September 2013).
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14
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15 Copyright (C) 2003 Matteo Frigo.
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16
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17 Copyright (C) 2003 Massachusetts Institute of Technology.
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18
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19 Permission is granted to make and distribute verbatim copies of
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20 this manual provided the copyright notice and this permission
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21 notice are preserved on all copies.
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22
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23 Permission is granted to copy and distribute modified versions of
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24 this manual under the conditions for verbatim copying, provided
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25 that the entire resulting derived work is distributed under the
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26 terms of a permission notice identical to this one.
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27
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28 Permission is granted to copy and distribute translations of this
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29 manual into another language, under the above conditions for
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30 modified versions, except that this permission notice may be
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31 stated in a translation approved by the Free Software Foundation.
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32 -->
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34 <style type="text/css"><!--
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43 span.sansserif { font-family:sans-serif; font-weight:normal; }
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44 --></style>
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45 </head>
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46 <body>
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47 <div class="node">
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48 <a name="Dynamic-Arrays-in-C-The-Wrong-Way"></a>
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49 <a name="Dynamic-Arrays-in-C_002dThe-Wrong-Way"></a>
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50 <p>
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51 Previous: <a rel="previous" accesskey="p" href="Dynamic-Arrays-in-C.html#Dynamic-Arrays-in-C">Dynamic Arrays in C</a>,
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52 Up: <a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>
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53 <hr>
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54 </div>
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55
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56 <h4 class="subsection">3.2.5 Dynamic Arrays in C—The Wrong Way</h4>
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57
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58 <p>A different method for allocating multi-dimensional arrays in C is
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59 often suggested that is incompatible with FFTW: <em>using it will
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60 cause FFTW to die a painful death</em>. We discuss the technique here,
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61 however, because it is so commonly known and used. This method is to
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62 create arrays of pointers of arrays of pointers of <small class="dots">...</small>etcetera.
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63 For example, the analogue in this method to the example above is:
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64
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65 <pre class="example"> int i,j;
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66 fftw_complex ***a_bad_array; /* <span class="roman">another way to make a 5x12x27 array</span> */
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67
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68 a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
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69 for (i = 0; i < 5; ++i) {
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70 a_bad_array[i] =
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71 (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
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72 for (j = 0; j < 12; ++j)
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73 a_bad_array[i][j] =
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74 (fftw_complex *) malloc(27 * sizeof(fftw_complex));
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75 }
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76 </pre>
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77 <p>As you can see, this sort of array is inconvenient to allocate (and
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78 deallocate). On the other hand, it has the advantage that the
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79 (i,j,k)-th element can be referenced simply by
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80 <code>a_bad_array[i][j][k]</code>.
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81
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82 <p>If you like this technique and want to maximize convenience in accessing
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83 the array, but still want to pass the array to FFTW, you can use a
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84 hybrid method. Allocate the array as one contiguous block, but also
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85 declare an array of arrays of pointers that point to appropriate places
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86 in the block. That sort of trick is beyond the scope of this
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87 documentation; for more information on multi-dimensional arrays in C,
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88 see the <code>comp.lang.c</code>
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89 <a href="http://c-faq.com/aryptr/dynmuldimary.html">FAQ</a>.
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90
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91 <!-- -->
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92 </body></html>
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93
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