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author Chris Cannam
date Wed, 07 Sep 2016 10:40:32 +0100
parents 26056e866c29
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Chris@19 1 <html lang="en">
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Chris@19 3 <title>Dynamic Arrays in C - FFTW 3.3.4</title>
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Chris@19 49 <a name="Dynamic-Arrays-in-C"></a>
Chris@19 50 <p>
Chris@19 51 Next:&nbsp;<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>,
Chris@19 52 Previous:&nbsp;<a rel="previous" accesskey="p" href="Fixed_002dsize-Arrays-in-C.html#Fixed_002dsize-Arrays-in-C">Fixed-size Arrays in C</a>,
Chris@19 53 Up:&nbsp;<a rel="up" accesskey="u" href="Multi_002ddimensional-Array-Format.html#Multi_002ddimensional-Array-Format">Multi-dimensional Array Format</a>
Chris@19 54 <hr>
Chris@19 55 </div>
Chris@19 56
Chris@19 57 <h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>
Chris@19 58
Chris@19 59 <p>We recommend allocating most arrays dynamically, with
Chris@19 60 <code>fftw_malloc</code>. This isn't too hard to do, although it is not as
Chris@19 61 straightforward for multi-dimensional arrays as it is for
Chris@19 62 one-dimensional arrays.
Chris@19 63
Chris@19 64 <p>Creating the array is simple: using a dynamic-allocation routine like
Chris@19 65 <code>fftw_malloc</code>, allocate an array big enough to store N
Chris@19 66 <code>fftw_complex</code> values (for a complex DFT), where N is the product
Chris@19 67 of the sizes of the array dimensions (i.e. the total number of complex
Chris@19 68 values in the array). For example, here is code to allocate a
Chris@19 69 5&nbsp;&times;&nbsp;12&nbsp;&times;&nbsp;27 rank-3 array:
Chris@19 70 <a name="index-fftw_005fmalloc-121"></a>
Chris@19 71 <pre class="example"> fftw_complex *an_array;
Chris@19 72 an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
Chris@19 73 </pre>
Chris@19 74 <p>Accessing the array elements, however, is more tricky&mdash;you can't
Chris@19 75 simply use multiple applications of the &lsquo;<samp><span class="samp">[]</span></samp>&rsquo; operator like you
Chris@19 76 could for fixed-size arrays. Instead, you have to explicitly compute
Chris@19 77 the offset into the array using the formula given earlier for
Chris@19 78 row-major arrays. For example, to reference the (i,j,k)-th
Chris@19 79 element of the array allocated above, you would use the expression
Chris@19 80 <code>an_array[k + 27 * (j + 12 * i)]</code>.
Chris@19 81
Chris@19 82 <p>This pain can be alleviated somewhat by defining appropriate macros,
Chris@19 83 or, in C++, creating a class and overloading the &lsquo;<samp><span class="samp">()</span></samp>&rsquo; operator.
Chris@19 84 The recent C99 standard provides a way to reinterpret the dynamic
Chris@19 85 array as a &ldquo;variable-length&rdquo; multi-dimensional array amenable to
Chris@19 86 &lsquo;<samp><span class="samp">[]</span></samp>&rsquo;, but this feature is not yet widely supported by compilers.
Chris@19 87 <a name="index-C99-122"></a><a name="index-C_002b_002b-123"></a>
Chris@19 88 <!-- =========> -->
Chris@19 89
Chris@19 90 </body></html>
Chris@19 91