Re: [Chicken-users] array-lib : empty arrays advertise indices that they cannot be indexed by

[Kon Lovett]

On Oct 4, 2007, at 2:59 AM, Terrence Brannon wrote:

> Re: http://www.call-with-current-continuation.org/eggs/array-lib.html
>
> We read: (make-array [<prototype>]) will construct an empty array, not
> a rank 0 array. Such arrays cannot be used with any setter or getter.
> However, property queries will work, and they can be used as a
> PROTOTYPE.
>
> So I have several questions based on this information:
>
> 1 - what is meant by getter or setter? what particular functions? I'm
> guessing array-ref might be one. And the one of most interest to me,
> array-for-each-index

A getter is a field or element reference procedure. A setter is a  
field or element assignment procedure. Since the 'for-each' procedure  
is a "higher-order" operation it is not really a "getter" as it deals  
w/ the array in-aggregate. But, since it is implemented as a loop  
around a "getter" I did consider it, along w/ every other operation  
that accesses the elements of an array, as unusable w/ empty arrays.

> 2 - Since the docs make the distinction between an empty array and a
> rank-0 array:
>   a - how do these two things differ?
>   b - how can a rank-0 array be created since (make-array) does not  
> do it?

Example: (array '#() 0 1)

And 'array-split' can create rank-0 arrays.

> 3 - It seems unusual to me that the array created by (make-array)
> produces its indices, yet throws an error when you use the indices it
> produces.

Actually it procedure no indices. The internals of the 'for-each'  
procedure is generating an index of 0, assuming the array is rank-0.

> I'm trying to create some generic code which works for
> arrays of any rank and it works fine for arrays of rank 1 and greater,
> but for "rank-0" arrays, it is failing similar to below. Rank-0 is in
> quotes because my empty arrays are not rank-0 per the docs. But I
> would definitely like a rank-0 array, or at least something which does
> not write it's own death sentence when queried for its indices :)

An empty array doesn't have indices to "query".

However, the 'for-each' procedure (and its' various siblings) should  
ignore an empty array. This is fixed in the 2.114 release.

> (define e (make-array))
> (define (a vec)
>   (array-for-each-index
>    (lambda (*indices*)
>      (display *indices*)
>      )
>    vec
>    ))
>
> (define (b vec)
>   (array-for-each-index
>    (lambda (*indices*)
>      (display (array-ref vec *indices*))
>      )
>    vec
>    ))
>
> (a e) ; displays 0
> (b e) ; Error: (array-ref) out of range: (0)
>
>
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Best Wishes,
Kon