Re: Processing array subsections, a newbie question.

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

Ludovic Brenta wrote:
> "Peter C. Chapin" <pcc482719@gmail.com> writes:
>> Ok        : Boolean := True;
>> Index_Fst : Natural := Buffer'First;
>> Index_Lst : Natural;
>> ...
>>
>> while Ok and Index_Fst <= Buffer'Last loop
>>   Do_Something(Buffer, Index_Fst, Index_Lst, Ok);
>>   Index_Fst := Index_Lst + 1;
>> end loop;
>>
>> The problem with this code is that the assignment to Index_Fst inside the loop
>> might raise Constraint_Error if Index_Lst = Buffer'Last after Do_Something
>> finishes. I can work around this problem but my solutions tend to be rather
>> ungainly looking. Surely there must be an easy way to handle this.

Constraint_Error would be raised only if Index_Lst = Natural'Last after 
Do_Something. By subtyping the index type of Buffer you could ensure 
that Buffer'Last < Natural'Last, which would avoid the risk of 
Constraint_Error. (Perhaps this is the ungainly work-around that you 
mention.)

> How about:

A few nitpicks on Ludovic's suggestion:

> Ok        : Boolean := True;

The initial value for Ok is unneccessary, as Do_Something will assign Ok 
before it it used.

> Index_Fst : Natural := Buffer'First;
> Index_Lst : Natural;
> ...
> loop
>    Do_Something(Buffer, Index_Fst, Index_Lst, Ok);

In this call of Do_Something, Index_Fst may not be a valid index for 
Buffer, since there is no preceding check that Index_Fst <= Buffer'Last, 
as there was in the original while-loop. But perhaps it is known, in the 
original context, that Buffer is not empty.

>    exit when not Ok;
>    exit when Index_Fst = Buffer'Last;

For robustness I would write Index_Fst >= Buffer'Last. (Perhaps this 
could also help some SPARK range analysis.)

>    Index_Fst := Index_Lst + 1;
> end loop;

So my suggestion is:

Ok        : Boolean;
Index_Fst : Natural := Buffer'First;
Index_Lst : Natural;
...

if Index_Fst <= Buffer'Last then
    -- Buffer is not empty, something to be done.
    loop
       Do_Something(Buffer, Index_Fst, Index_Lst, Ok);
       exit when not Ok;
       exit when Index_Fst >= Buffer'Last;
       Index_Fst := Index_Lst + 1;
    end loop;
else
    -- Buffer is empty.
    ...
end if;

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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