# Re: octothorpe array syntax; other comments

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```bear wrote:

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```  Do I understand correctly?  You mean An array of no elements, whose
every element is an array of N elements?
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No.  I means a 0*N array.

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A 2-dimensional (rank 2) array is *not* the same thing as a vector of vectors. The implementation is obviously different, but the semantics are also different, at least when it comes to empty arrays.
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```  Why is this different from or more useful than any other kind of
array of no elements?
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If you transpose a 0*N array you get a N*0 array, which is corresponds to a N-element vector, each of which is empty. Sort-of.
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```  I am aware that you can get such entities as a result in matrix
operations, but I had always (perhaps mistakenly?)  regarded them as a
degenerate case not useful for further processing, and so didn't feel
that they needed a source syntax.
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It is probably not a high priority to make a nice syntax for degeneate arrays, as long as we have the #, syntax to fall-back on.
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Common Lisp (and APL) also allow rank-0 arrays, which have a single element. (The total number of elements of an array is the product of each dimension; if there are no dimensions, the product is 1.) In APL, a rank-0 array is equivalent to a scalar. Common Lisp has the read/write syntax #0a() for a rank-zero array. A 3*0 array is #a2(() () ()) while (if I'm reading the HperSpec right) a 0*3 is written #a2(()) - which is ambiguous.
```--
--Per Bothner
per@xxxxxxxxxxx   http://per.bothner.com/

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