Re: variable lenght strings

From: Matthew Heaney (mheaney_at_on2.com)
Date: 10/22/04 

Date: Thu, 21 Oct 2004 19:14:19 -0400

"Marius Amado Alves" <amado.alves@netcabo.pt> wrote in message
news:mailman.46.1098398641.10401.comp.lang.ada@ada-france.org...
>> 1) Is it possible to use Get_Line with Unbounded and/or Bounded
>> Strings?
>
> Not in the standard, but subprograms like those are usually around, e.g.
> in the GNAT Library, or end up being written in house.
>
>> 2) If not, how should usei input be managed when lines length isn't
>> known a priori?
>
> There's a way using the standard Get_Line, explained in AdaPower.

Mario is probably referring to an article I posted to CLA a few years' ago,
and which is now archived at the adapower website.

The basic idea is this: algorithms that consume input from a stream need a
way a identify when all of the input has been consumed. Typically this is
done using a special value that you know is outside the range of normal
values, e.g.

declare
   I : Natural;
begin
   loop
     Get (Stream, I);
     exit when I = 0; --the special value
     ... -- do something with I
  end loop;
end;

Text_IO.Get_Line works just like this, except that it's not obvious what the
termination condition is:

declare
   Line : String (1 .. 81);
   Last : Natural;
begin
   loop
     Get_Line (Line, Last);
     .. -- do something with Line (1 .. Last);
    exit when ???
   end loop;
end;

The termination condition is as follows: you know that you've consumed the
entire line when Last < Line'Last. We can now write:

declare
   Line : String (1 .. 81);
   Last : Natural;
begin
   loop
     Get_Line (Line, Last);
     .. -- do something with Line (1 .. Last);
     exit when Last < Line'Last;
   end loop;
end;

Typically you know what a normal line length is, so you declare Line with a
length of Max + 1. (That's why Line'Last is 81 in the examples above.)

Now that you know the termination condition, the next problem is deciding
how to handle a line of input that is longer than your buffer. You can
either handle it as an error, or just copy the buffer into an expandable
string buffer such as Unbounded_String.

If you want, you can use recursion to read the line, and return the string
as a function result:

declare
   function Get_Line (N : Positive := 80) return String is
      Line : String (1 .. N + 1);
      Last : Natural;
   begin
      Get_Line (Line, Last);

      if Last < Line'Last then
        return Line (1 .. Last);
      else
         return Line & Get_Line (2 * N);
      end if;
   end Get_Line;

   Line : constant String := Get_Line;
begin

This avoids the loop, and having to store intermediate results in a buffer.

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