Gene Wirchenko wrote:
> I just got bit in a minor way with the way that VFP handles array
> elements. Consider the following code:
>
> local twod(10,2)
> for i=1 to 10
> for j=1 to 2
> twod(i,j)=10*i+j
> endfor
> endfor
> ? twod(4,3)
>
> Note the array dimensions and the reference. The reference is
> treated as being the third element starting in row 4 with VFP not
> checking against the declared dimension. The reference is treated as
> referring to twod(5,1), not as being an error.
>
> The docs do say that 2-D arrays can be accessed as 1-D. The
> implementation appears to be more general than that. Forewarned is
> forearmed and all that.
>
>
The docs would be more accurate to say that 1-D arrays can be accessed
as 2-D, because 1-D is all that Fox can handle. That is, every array is
one dimensional. Declaring twod(10,2) creates the same 20 element array
as twod(20) [or as twod(5,4)]. The 2-D conceit is a logical 2-D
addressing scheme, but does not affect the actual array structure. It
appears that Fox locates twod[4,3] by locating the element corresponding
to the start of the logical fourth row by multiplying the declared
number of columns (2) times 4 - which is element number 7 - and then
moves the pointer by two more elements, to element number 9. This, as
you noted, is the result of ASUBSCRIPT(9).
The 1-D structure also explains the behavior of re-dimensioning an
array. If aFoo(3, 3) is:
1 2 3
4 5 6
7 8 9
DIMENSION aFoo(2, 2) will result in:
1 2
3 4
as opposed to:
1 2
4 5
As you said, forewarned is forearmed.
- Rush
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