Re: unsigned type

[anon@anon.org (anon)]

Adam. 
 
RM 3.6.1 ( 17..18 ) proves your point about strings with subscripts of ( 1..0 ). 
But it kinds of flys in the face of RM 0 ( 29 ) [ see below ]. Since we must use the keyword "reverse" to transverse a loop index 
in deceasing order, it would seam logical that ( 1 .. 0 ) or S ( S'First .. ( S'First - 1 ) ) 
should be illegal for Strings because the 0 is not within the range of the 
predefined type Positive. But a String with length greater than 1 the we could 
use: S ( ( S'First + 1 ) .. S'First ) => S ( 2 ..1 ) would be legal because 
both index values are legal subscripts and these values could be all the 
way to S ( S'Last .. S'First ) to get a null array (String).
 
 
Note : Around line 291 of RM 95 Text version

RM  0. (29) 
29   Execution of a program unit may encounter error situations in which
normal program execution cannot continue.  For example, an arithmetic
computation may exceed the maximum allowed value of a number, or an attempt
                                                              ^^^^^^^^^^^^^
may be made to access an array component by using an incorrect index value.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
To deal with such error situations, the statements of a program unit can be
textually followed by exception handlers that specify the actions to be taken
when the error situation arises.  Exceptions can be raised explicitly by a
raise statement.
  




In <273dedb7-8d68-42d2-8602-aa44c79f3708@b9g2000yqm.googlegroups.com>, Adam Beneschan <adam@irvine.com> writes:
>On Jun 30, 4:48=A0pm, a...@anon.org (anon) wrote:
>> Read RM 3.5 =A0( 4 )
>>
>> Is "A ( 0 .. 0 )" an example of null range. =A0By the definition in RM 3.=
>5
>> ( 4 ), the Right side range (index) must be less than the Left side, so
>> "A ( 0.. 0 )" is not a valid null range statement. So, this statement
>> should generate a compiler or runtime error, because either range is not
>> a subset of the range for Strings.
>
>OK, I think I've finally figured out why we're having this confusing
>argument.  It goes way back to this post of yours:
>
>>> For Strings:
>>>                          --  'A' is a zero length string, A'Last =3D 0, =
>and
>>>                          --  put_line  ( A ( A'First .. A'Last ) ) ;
>>>                          --  does not raise an Constraint_Error even tho=
>ugh in
>>>                          --  this case it translate to:
>>>                          --  put_line  ( A ( 0 .. 0 ) ) ;
>>>  A : String :=3D ""        ;
>
>It does not translate to A (0..0); it translates to A (1..0).  If A is
>declared as in your example above, A'First will be 1 and A'Last will
>be 0.  Try it (try declaring A like that and outputting A'First and
>A'Last).  It looks like everyone else missed this original error of
>yours, which has apparently led to some confusion.
>
>In this case, A'First..A'Last, which is 1..0, is compatible with the
>subtype because it's a null range, and null ranges are compatible with
>the subtype even when the range bounds don't actually belong to the
>subtype.  0..0 is not compatible with the subtype, but you cannot
>declare a string with that index range unless you try to do it
>explicitly:
>
>   A : String (0..0);
>
>and then you *will* get a Constraint_Error.
>
>So your later assertion that follows:
>
>>> Since you can have zero length string , the index is Natual instead of P=
>ositive,
>>> because zero is in Natural but is not define in Positive. Even though th=
>e
>>> Standard package define String index as a Positive type. (GNAT)
>
>is wrong.  The index range is Positive, but null ranges don't have to
>meet that constraint.  They don't have to be Natural, either.  This is
>legal and will not raise an exception at runtime:
>
>   B : String (-9 .. -10) :=3D "";
>
>Hope this clears everything up.
>
>                                  -- Adam
>