cannam@127: Previous: Dynamic Arrays in C, Up: Multi-dimensional Array Format [Contents][Index]
cannam@127:A different method for allocating multi-dimensional arrays in C is cannam@127: often suggested that is incompatible with FFTW: using it will cannam@127: cause FFTW to die a painful death. We discuss the technique here, cannam@127: however, because it is so commonly known and used. This method is to cannam@127: create arrays of pointers of arrays of pointers of …etcetera. cannam@127: For example, the analogue in this method to the example above is: cannam@127:
cannam@127:int i,j;
cannam@127: fftw_complex ***a_bad_array; /* another way to make a 5x12x27 array */
cannam@127:
cannam@127: a_bad_array = (fftw_complex ***) malloc(5 * sizeof(fftw_complex **));
cannam@127: for (i = 0; i < 5; ++i) {
cannam@127: a_bad_array[i] =
cannam@127: (fftw_complex **) malloc(12 * sizeof(fftw_complex *));
cannam@127: for (j = 0; j < 12; ++j)
cannam@127: a_bad_array[i][j] =
cannam@127: (fftw_complex *) malloc(27 * sizeof(fftw_complex));
cannam@127: }
cannam@127: As you can see, this sort of array is inconvenient to allocate (and
cannam@127: deallocate). On the other hand, it has the advantage that the
cannam@127: (i,j,k)-th element can be referenced simply by
cannam@127: a_bad_array[i][j][k].
cannam@127:
If you like this technique and want to maximize convenience in accessing
cannam@127: the array, but still want to pass the array to FFTW, you can use a
cannam@127: hybrid method. Allocate the array as one contiguous block, but also
cannam@127: declare an array of arrays of pointers that point to appropriate places
cannam@127: in the block. That sort of trick is beyond the scope of this
cannam@127: documentation; for more information on multi-dimensional arrays in C,
cannam@127: see the comp.lang.c
cannam@127: FAQ.
cannam@127: