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<h4 class="subsection">3.2.4 Dynamic Arrays in C</h4>

<p>We recommend allocating most arrays dynamically, with
<code>fftw_malloc</code>.  This isn't too hard to do, although it is not as
straightforward for multi-dimensional arrays as it is for
one-dimensional arrays.

   <p>Creating the array is simple: using a dynamic-allocation routine like
<code>fftw_malloc</code>, allocate an array big enough to store N
<code>fftw_complex</code> values (for a complex DFT), where N is the product
of the sizes of the array dimensions (i.e. the total number of complex
values in the array).  For example, here is code to allocate a
5&nbsp;&times;&nbsp;12&nbsp;&times;&nbsp;27 rank-3 array:
<a name="index-fftw_005fmalloc-121"></a>
<pre class="example">     fftw_complex *an_array;
     an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex));
</pre>
   <p>Accessing the array elements, however, is more tricky&mdash;you can't
simply use multiple applications of the &lsquo;<samp><span class="samp">[]</span></samp>&rsquo; operator like you
could for fixed-size arrays.  Instead, you have to explicitly compute
the offset into the array using the formula given earlier for
row-major arrays.  For example, to reference the (i,j,k)-th
element of the array allocated above, you would use the expression
<code>an_array[k + 27 * (j + 12 * i)]</code>.

   <p>This pain can be alleviated somewhat by defining appropriate macros,
or, in C++, creating a class and overloading the &lsquo;<samp><span class="samp">()</span></samp>&rsquo; operator.
The recent C99 standard provides a way to reinterpret the dynamic
array as a &ldquo;variable-length&rdquo; multi-dimensional array amenable to
&lsquo;<samp><span class="samp">[]</span></samp>&rsquo;, but this feature is not yet widely supported by compilers.
<a name="index-C99-122"></a><a name="index-C_002b_002b-123"></a>
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author	Chris Cannam <cannam@all-day-breakfast.com>
date	Wed, 20 Mar 2013 15:35:50 +0000
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