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(fftw.info)Dynamic Arrays in C

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Dynamic Arrays in C
-------------------

   Often, especially for large arrays, it is desirable to allocate the
arrays dynamically, at runtime.  This isn't too hard to do, although it
is not as straightforward for multi-dimensional arrays as it is for
one-dimensional arrays.

   Creating the array is simple: using a dynamic-allocation routine like
`malloc', allocate an array big enough to store N `fftw_complex'
values, 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 5x12x27 rank 3 array:

     fftw_complex *an_array;
     
     an_array = (fftw_complex *) malloc(5 * 12 * 27 * sizeof(fftw_complex));

   Accessing the array elements, however, is more tricky--you can't
simply use multiple applications of the `[]' operator like you could for
static 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 `an_array[k + 27 * (j + 12 * i)]'.

   This pain can be alleviated somewhat by defining appropriate macros,
or, in C++, creating a class and overloading the `()' operator.