Re: Ada.Containers.Vectors - querying multiple elements

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

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

> On 02 May 2005 14:57:15 -0400, Robert A Duff wrote:
> 
> > "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:
> > 
> >> So "" is not allowed for any string type which index is not a
> >> subtype/derived type S of some base type T, such that S'First>T'First. With
> >> a nice consequence that:
> >> 
> >> type T1 is range -2147483647..0;
> >                     ^^^^^^^^^^
> > You misspelled "-2**31".  ;-)
> 
> Actually it is 1 - 2**31 (:-)),

Right.  Which proves my point, which is that your point would be clearer
if you used "2**31" or whatever instead of putting in a literal.
Without underscores, even!

>... but the point is that it can be any number!

Right.

> The standard is silent about powers of 2.

As well it should be, IMHO.

>... Can I write a compiler with a
> built-in integer type Short_Int which range is -5..5? Is this illegal?

Yes.  But the built-in types are irrelevant.  What's relevant is the
"base range" of a type.  The RM says it has to be (almost) symmetric
about zero.  So, yes, if you say "type T is range 2..3" the base range
could be -5..5 (according to the RM).

> >> type T1_Array is array (T range <>) of Character;
> >> 
> >> type T2 is range -2147483648..0;
> >> type T2_Array is array (T range <>) of Character;
> >> 
> >> type T3 is range -2147483649..0;
> >> type T3_Array is array (T range <>) of Character;
> >> 
> >> X : T1_Array := ""; -- Legal
> >> Y : T2_Array := ""; -- Illegal
> >> Z : T3_Array := ""; -- Again legal!!!!!
> > 
> > Well, Ada's integer types are intended to be hardware-oriented, for
> > better or worse.  This empty-array issue is an extremely rare case.
> > Consider:
> > 
> >     X1: T1 := T1'First;
> >     X2: T2 := T2'First;
> >     X3: T3 := T3'First;
> > 
> >     X1 := (X1-1)/2;
> >     X2 := (X2-1)/2;
> >     X3 := (X3-1)/2;
> > 
> > Which of the above will raise Constraint_Error (due to overflow on the
> > subtraction)?  Well it's entirely implementation defined, but given
> > typical hardware and compiler, you'll get the same behavior as with the
> > empty strings.
> 
> How would you formally, in the sense ARM 1.1.5, classify the error above
> and one of using empty strings? It seems to me that to rely on
> Constraint_Error above would be a bounded error. But programs using empty
> strings would be illegal Ada programs.

There's no "bounded error" here.  These things raise C_E, or return the
correct result.  If every intermediate result is in the base range, the
RM requires correct results; otherwise, it allows C_E or correct
results.  RM-4.9(34..35) might make some such things illegal -- I'm not
sure.  (Illegal = compile-time error.)

> > Seems to me that arithmetic is a more common case.  I mean, nobody makes
> > arrays starting near -2**31.
> 
> I don't think that statistical approach is appropriate here. Anyway it
> makes an implementation of many generic algorithms and container libraries
> very difficult if possible.

I don't see a big problem here.  If you're counting things, use signed
integers, and start counting at zero or one.  If you're using enums or
modulars, you're not "counting", and zero-length arrays/vectors/whatever
make no sense.

> > I admit this stuff is a little bit error prone.  That's what you get
> > when the language is hardware oriented (for efficiency, of course).
> 
> Is this the only way to achieve efficiency?

No.  I think I can design an efficent language that doesn't have these
problems.  But it ain't Ada.

- Bob