Re: Surprise in array concatenation

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On 31 Aug 2005 20:16:43 -0700, Gene wrote:
> 
> > Nonetheless the rule seems silly:  When a leading zero-length array is
> > catenated to another array the result takes on the starting index of
> > the _second_ operand.  This doesn't make sense to me.  Why not use the
> > starting index of the zero-length array?
> 
> Because that might be ill-defined, I guess.
> 
> A more interesting question is why Empty'First does not raise any
> exception.

Heh?  You want this:

    procedure Put(S: String) is
    begin
        for I in S'First..S'Last loop -- equivalent to S'Range
            Put_Char(S(I));

to crash when S = ""?

>... After all, there is no any lower bound of an empty index range.

In Ada, every range, and every array, has both a lower and an upper
bound.

> Provided, that empty arrays are all same, of course.

Depends on what you mean by "same".  ;-)  Different empty arrays can
have different bounds -- but "=" returns True!

>... If not, then another
> interesting question would appear: how to make an empty array with the
> lower bound Integer'First?

You can't.

You should always make sure Index'First > Index'Base'First when using
the "Index range <>" syntax.  (But you don't need to do that when you
have a constrained array -- "...array(Index) of ...".)

- Bob