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3 <title>Dynamic Arrays in C - FFTW 3.3.3</title>
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4 <meta http-equiv="Content-Type" content="text/html">
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5 <meta name="description" content="FFTW 3.3.3">
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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="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C" title="Fixed-size Arrays in C">
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10 <link rel="next" href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way" title="Dynamic Arrays in C-The Wrong Way">
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11 <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
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12 <!--
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13 This manual is for FFTW
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14 (version 3.3.3, 25 November 2012).
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15
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16 Copyright (C) 2003 Matteo Frigo.
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17
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18 Copyright (C) 2003 Massachusetts Institute of Technology.
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19
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20 Permission is granted to make and distribute verbatim copies of
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21 this manual provided the copyright notice and this permission
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22 notice are preserved on all copies.
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23
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24 Permission is granted to copy and distribute modified versions of
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25 this manual under the conditions for verbatim copying, provided
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26 that the entire resulting derived work is distributed under the
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27 terms of a permission notice identical to this one.
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28
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29 Permission is granted to copy and distribute translations of this
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30 manual into another language, under the above conditions for
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31 modified versions, except that this permission notice may be
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32 stated in a translation approved by the Free Software Foundation.
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35 <style type="text/css"><!--
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45 --></style>
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46 </head>
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47 <body>
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48 <div class="node">
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49 <a name="Dynamic-Arrays-in-C"></a>
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50 <p>
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51 Next: <a rel="next" accesskey="n" href="Dynamic-Arrays-in-C_002dThe-Wrong-Way.html#Dynamic-Arrays-in-C_002dThe-Wrong-Way">Dynamic Arrays in C-The Wrong Way</a>,
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52 Previous: <a rel="previous" accesskey="p" href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C">Fixed-size Arrays in C</a>,
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53 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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54 <hr>
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55 </div>
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56
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57 <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
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58
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59 <p>We recommend allocating most arrays dynamically, with
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60 <code>fftw_malloc</code>. This isn't too hard to do, although it is not as
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61 straightforward for multi-dimensional arrays as it is for
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62 one-dimensional arrays.
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63
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64 <p>Creating the array is simple: using a dynamic-allocation routine like
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65 <code>fftw_malloc</code>, allocate an array big enough to store N
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66 <code>fftw_complex</code> values (for a complex DFT), where N is the product
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67 of the sizes of the array dimensions (i.e. the total number of complex
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68 values in the array). For example, here is code to allocate a
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69 5 × 12 × 27 rank-3 array:
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70 <a name="index-fftw_005fmalloc-121"></a>
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71 <pre class="example"> fftw_complex *an_array;
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72 an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
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73 </pre>
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74 <p>Accessing the array elements, however, is more tricky—you can't
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75 simply use multiple applications of the ‘<samp><span class="samp">[]</span></samp>’ operator like you
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76 could for fixed-size arrays. Instead, you have to explicitly compute
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77 the offset into the array using the formula given earlier for
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78 row-major arrays. For example, to reference the (i,j,k)-th
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79 element of the array allocated above, you would use the expression
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80 <code>an_array[k + 27 * (j + 12 * i)]</code>.
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81
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82 <p>This pain can be alleviated somewhat by defining appropriate macros,
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83 or, in C++, creating a class and overloading the ‘<samp><span class="samp">()</span></samp>’ operator.
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84 The recent C99 standard provides a way to reinterpret the dynamic
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85 array as a “variable-length” multi-dimensional array amenable to
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86 ‘<samp><span class="samp">[]</span></samp>’, but this feature is not yet widely supported by compilers.
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87 <a name="index-C99-122"></a><a name="index-C_002b_002b-123"></a>
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88 <!-- =========> -->
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89
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90 </body></html>
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91
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