Re: Strings and reading from a file

[jrcarter001@my-dejanews.com]

In article <7hf611$16i$1@cnn.Princeton.EDU>,
  mcc@entropy.cs.princeton.edu (Martin C. Carlisle) wrote:
> I can't imagine!  Probably something more like:
>
> function Next_Line(File : in Ada.Text_IO.File_Type :=
>    Ada.Text_Io.Standard_Input) return String is
>    Answer : String(1..256);
>    Last   : Natural;
> begin
>    Ada.Text_IO.Get_Line(File => File,
>       Item => Answer,
>       Last => Last);
>    if Last = Answer'Last then
>       return Answer & Next_Line;
>    else
>       return Answer(1..Last);
>    end if;
> end Next_Line;
>
> --Martin
>
> In article <7hf2bc$imm$1@nnrp1.deja.com>,  <dennison@telepath.com>
wrote:
> >In article <7hevh1$g08$1@nnrp1.deja.com>,
> >> Make the function recursive, and it can return a string of any
length,
> >> eliminating that nagging worry about strings that are longer than
> >> whatever magic number you used in the function.
> >
> >Surely you don't mean (forgive me if Deja decides to throw away my
> >formatting):
> >
> >function Next_Line return String is
> >   Char : Character;
> >   Next_Char : Character;
> >   EOL  : Boolean;
> >begin
> >   Get (Char);
> >   Look_Ahead (Next_Char, EOL);
> >   if EOL then
> >      return (1 => Char);
> >   else
> >      return Char & Next_Line;
> >   end if;
> >end Next_Line;
> >
> >I think we may have just smoked out a Lisp programmer...

I didn't expect to generate such a response...

The original inspiration for this is

Carter, J., "Variable-Length String Input in Ada", _Ada Letters_, 1989
May/Jun

which presents the following (Ada-83) algorithm:

with Text_Io;
function Get_Line (File : Text_Io.File_Type := Text_Io.Current_Input)
return String is
   Char : Character;
begin -- Get_Line
   if Text_Io.End_Of_Line (File) then
      Text_Io.Skip_Line (File);

      return "";
   else
      Text_Io.Get (File, Char);

      return Char & Get_Line (File);
   end if;
end Get_Line;

This is the output of a straightforward algorithm creation process,
which goes something like:

In Ada, it has to be a function so the result can be any length. Ok,
assume I have a function that will return the rest of the line, provided
I get the first character myself. Use that function to implement itself:

begin
   Text_Io.Get (Char);

   return Char & Get_Line;
end;

How do I terminate the recursion? When I'm at the end of the line. What
do I return then? A null string. End of exercise, and the function is
finished, and works correctly. Pretty it up by adding a File parameter
with appropriate default, and you're done.

However, most lines are short, and recursion for every character seems
wasteful, even though it ran as fast as I could want. My current
implementation has improved the algorithm:

with Ada.Text_Io;
use Ada;
function Get_Line (File : Text_Io.File_Type := Text_Io.Current_Input)
return String is
   Line : String (1 .. 100);
   Last : Natural;
begin -- Get_Line
   Text_IO.Get_Line (File => File, Item => Line, Last => Last);

   if Last < Line'Last then
      return Line (Line'First .. Last);
   else
      return Line & Get_Line (File);
   end if;
end Get_Line;

This is effectively the same as Carlisle's  version. This does no
recursion for most lines I've encountered, and gracefully handles the
"pathological" cases. It still has trouble with those lines
that are Positive'Last characters long, especially on 32-bit machines.


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