Re: need help learning Ada for a modula-2 programmer

[Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>]

On 02/02/2014 08:02 AM, agent@drrob1.com wrote:
>
> ArgStr and CmdlineFrag are defined at the module level.  The working
> version of this code declares them to be bounded strings.  Brian said
> that he uses fixed length strings almost always.  I am interested in
> how that could be done.
>
> Is it correct that having a function return a fixed string is more
> flexible than a string param in a procedure?  I am getting the sense
> that having a function return the string on the stack works by not
> setting its dimensions in advance, but a param has to set them in
> advance.  So the trick is to use the stack for strings by recursion,
> as you just did?

A String object always has a fixed length, and an [in] out parameter is always 
an object. So if you want the flexibility of a variable-length result, you 
either need an unbounded parameter or a function. When a function result is 
itself made up of an unknown number of pieces concatenated together, recursion 
is a useful technique.

> I don't use recursion in my code.  This is probably because the first
> language I learned was Fortran 66, and I never needed it before.

An iterative version would collect the pieces in an Unbounded_String:

function Command_Line return String is
    Result : Ada.Strings.Unbounded.Unbounded_String;

    use type Ada.Strings.Unbounded.Unbounded_String;
begin -- Command_Line
    All_Arguments : for Arg in 1 .. Ada.Command_Line.Argument_Count loop
       Result := Result & Ada.Command_Line.Argument (Arg);

       if Arg < Ada.Command_Line.Argument_Count then
          Result := Result & ' ';
       end if;
    end loop All_Arguments;

    return Ada.Strings.Unbounded.To_String (Result);
end Command_Line;

> With the single exception of the quicksort algorithm.  Which I copied
> from a book.

I first encountered quick sort in /Software Tools/, which implemented it without 
recursion.

-- 
Jeff Carter
"We'll make Rock Ridge think it's a chicken
that got caught in a tractor's nuts!"
Blazing Saddles
87