Chris@19: Next: Dynamic Arrays in C-The Wrong Way, Chris@19: Previous: Fixed-size Arrays in C, Chris@19: Up: Multi-dimensional Array Format Chris@19:
We recommend allocating most arrays dynamically, with
Chris@19: fftw_malloc. This isn't too hard to do, although it is not as
Chris@19: straightforward for multi-dimensional arrays as it is for
Chris@19: one-dimensional arrays.
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Creating the array is simple: using a dynamic-allocation routine like
Chris@19: fftw_malloc, allocate an array big enough to store N
Chris@19: fftw_complex values (for a complex DFT), where N is the product
Chris@19: of the sizes of the array dimensions (i.e. the total number of complex
Chris@19: values in the array). For example, here is code to allocate a
Chris@19: 5 × 12 × 27 rank-3 array:
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fftw_complex *an_array; Chris@19: an_array = (fftw_complex*) fftw_malloc(5*12*27 * sizeof(fftw_complex)); Chris@19:Chris@19:
Accessing the array elements, however, is more tricky—you can't
Chris@19: simply use multiple applications of the ‘[]’ operator like you
Chris@19: could for fixed-size arrays. Instead, you have to explicitly compute
Chris@19: the offset into the array using the formula given earlier for
Chris@19: row-major arrays. For example, to reference the (i,j,k)-th
Chris@19: element of the array allocated above, you would use the expression
Chris@19: an_array[k + 27 * (j + 12 * i)].
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This pain can be alleviated somewhat by defining appropriate macros, Chris@19: or, in C++, creating a class and overloading the ‘()’ operator. Chris@19: The recent C99 standard provides a way to reinterpret the dynamic Chris@19: array as a “variable-length” multi-dimensional array amenable to Chris@19: ‘[]’, but this feature is not yet widely supported by compilers. Chris@19: Chris@19: Chris@19: Chris@19: Chris@19: