Re: 'first of strings returned from a function should be 1?

[mheaney@ni.net (Matthew Heaney)]

In article <01bc9a76$459c2250$4c8371a5@dhoossr>, "David C. Hoos, Sr."
<david.c.hoos.sr@ada95.com> wrote:


>From my own experience, I know that it is easy to write a poor function
>returning a string result of which the first subscript is not 1, 

Poor is a relative term.  For some abstractions, returning a string whose
lower index is not 1 might make more sense.

In the example I gave, 

function To_Uppercase (S : String) return String;

one could argue that the return value should have the same bounds as the
input string S (which doesn't necessarily have 1 as the lower bound).

Consider a sequence:

generic
   type Sequence_Item is private;
   type Sequence_Item_Array is 
      array (Positive range <>) of Sequence_Item;
package Sequences_G is

   type Root_Sequence is abstract tagged private;

   function Subsequence
     (Sequence : Root_Sequence;
      First           : Positive;
      Last            : Positive) return Sequence_Item_Array;

Surely, the array returned by this function would have First, not 1, as its
lower bound.
     

>and I
>always make the effort to slice (if necessary) the result into the range 1
>. result'length, but if you're using
>something you can't change, then something like 
>New_Result (1 .. Result_String'length) :=
>Result_String(Result_String'first .. result_String'first +
>Result_String'length -1)
>will do the job.

Why not

Result_String'Last, 

instead of 

Result_String'First + Result_String'Length - 1 ?

And why not

   := Result_String 

instead of

   := Result_String (Result_STring'First .. Result_String'Last) ?

Indeed, why bother with the copy at all:

declare
   Result_String : constant String := f (...);
   subtype Slid is String (1 .. Result_String'Length);
begin
   if Slid (Result_String)(1) then

Even if you do decide to make a copy, why not do it in the declarative region:

declare
   Result_String_Unslid : constant String := f (...);
   subtype Slid is String (1 .. Result_String_Unslid'Length);
   Result_String : constant String := Slid (Result_String_Unslid);
begin
   if Result_String (1) ...

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Matthew Heaney
Software Development Consultant
<mailto:matthew_heaney@acm.org>
(818) 985-1271