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(fftw.info)Row-major Format

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Row-major Format
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   The multi-dimensional arrays passed to `fftwnd' are expected to be
stored as a single contiguous block in "row-major" order (sometimes
called "C order").  Basically, this means that as you step through
adjacent memory locations, the first dimension's index varies most
slowly and the last dimension's index varies most quickly.

   To be more explicit, let us consider an array of rank d whose
dimensions are n1 x n2 x n3 x ... x nd.  Now, we specify a location in
the array by a sequence of (zero-based) indices, one for each dimension:
(i1, i2, ..., id).  If the array is stored in row-major order, then
this element is located at the position id + nd * (id-1 + nd-1 * (... +
n2 * i1)).

   Note that each element of the array must be of type `fftw_complex';
i.e. a (real, imaginary) pair of (double-precision) numbers. Note also
that, in `fftwnd', the expression above is multiplied by the stride to
get the actual array index--this is useful in situations where each
element of the multi-dimensional array is actually a data structure or
another array, and you just want to transform a single field. In most
cases, however, you use a stride of 1.