Re: Surprise in array concatenation

["Dr. Adrian Wrigley" <amtw@linuxchip.demon.co.uk.uk.uk>]

On Thu, 01 Sep 2005 14:42:22 -0700, Gene wrote:

...
> I completely agree with the marginal utility of other-than-1 least
> array indices.

Like others, I'm surprised to find this is controvertial.

People have given some examples already.  My favourite
examples are:

Compatibility with languages that have 0-based arrays.
it would be really annoying if indices couldn't be shared
between C and Ada arrays, for example.
(most C/C++ users see litle utility in other-than-0 least indices!)

Ability for subprograms to receive slices of arrays as
parameters.  C code often has to pass start and stop
indices as separate function parameters, in addition to
the pointer to the first element.  Three parameters instead
of one adds clutter.  Particularly common in recursive calls.

Arrays indexed by modular types. (I know these have been criticised
elsewhere).  Modular indices are really handy for circular
buffers, as well as hash tables and things (pre Ada 200Y!).

Arrays indexed by characters (eg a letter frequency table,
'a' .. 'z').

I also use symmetric array indices (eg -10 .. 10) quite a
lot for tables indexed by differences.

Sometimes negative indices are useful too, (with 0 at the top, usually)
(eg -100 .. 0).  This is useful for analysing events through
relative time.  Negative indices are preconditions, positive
indices would be postconditions or responses. (
(If you only have positive indices, you find some arrays in
your program run forwards through time, and some run backwards.
Tracking which is which, and getting the '-' in the right
place is particularly error prone)

And of course, plenty of contrived examples can be made up,
some of which so actually occur in real problems.
It's just another area of coding where Ada matches the
problem domain, and other languages match the solution domain.
Like so many things in Ada, you may not use them every day
in some applications, but they're there when you need them!

By the way... why aren't fixed point types discrete?
The values are integral multiples of small, which can be
supplied by the user.  So I'd expect to be able to use
fixed point types as array indices etc.  You cant :(
Wouldn't fixed point be *much* more useful if it was discrete?
-- 
Adrian