fftw3: Dynamic Arrays in C
3.2.4 Dynamic Arrays in C
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We recommend allocating most arrays dynamically, with 'fftw_malloc'.
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
'fftw_malloc', allocate an array big enough to store N 'fftw_complex'
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 x 12 x 27 rank-3
array:
fftw_complex *an_array;
an_array = (fftw_complex*) fftw_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
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 '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. The
recent C99 standard provides a way to reinterpret the dynamic array as a
"variable-length" multi-dimensional array amenable to '[]', but this
feature is not yet widely supported by compilers.
Info Catalog
fftw3: Fixed-size Arrays in C
fftw3: Multi-dimensional Array Format
fftw3: Dynamic Arrays in C-The Wrong Way