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From: eachus@spectre.mitre.org (Robert I. Eachus)
Subject: Iterators (was Re: some questions re. Ada/GNAT from a C++/GCC user)
Date: 1996/03/30
Message-ID: <EACHUS.96Mar29191146@spectre.mitre.org>#1/1
X-Deja-AN: 144937450
references: <wnewmanDoxrCp.DKv@netcom.com> <Dp1oAw.7Cz@world.std.com>
organization: The Mitre Corp., Bedford, MA.
newsgroups: comp.lang.ada,comp.lang.c++
Date: 1996-03-30T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <Dp1oAw.7Cz@world.std.com> bobduff@world.std.com (Robert A Duff) writes:

  > > Is there any way in Ada to iterate abstractly over the contents of a 
  > > container,...

  > Several ways:

  >     - Write a generic procedure Iterate, which takes a formal
  >	 procedure representing the body of the loop -- that is,
  >	 the formal procedure says what to do for each element.

  Funny you should mention it...  Right now I have two projects where
I am dealing with a large mass (1000s) of source files.  I need to run
some tools over the code.  The choice was between Ada and a shell
script...I chose Ada:

generic
  with procedure To_Do(File: in String);
procedure Iterate_Files(Pattern: in String := "*";
                        Directory: in String := "");
-- This generic iterates over all files in a directory matching Pattern and
-- calls To_Do once for each such directory entry.  If a file has several
-- links in the directory To_Do will be called once for each.

  (The body for SunAda/VADS is tacked on at the end of this
message. It isn't all that complex. If someone wants to rewrite for
GNAT and post, feel free, but there may be a lot of GNAT versions
needed,  I don't know if it is possible to share code between
different operating systems.)

   Two observations.  First, the generic procedure approach is
certainly right for this case.  Second, the VADS run-time provided a
call for freeing A_Strings, but not for find_files.find_file_rec, so I
put in the call for the A_Strings case only.  If I felt that there
would be hundreds of calls with different patterns, I probably would
have gone through the pain of "fixing" the find_files package by
adding the missing functionality.  But in my case, the one call to
A_Strings.Free(Next) is enough garbage collection for all practical
purposes...the call in the exception handler is for style more than
anything else.

    And I know that this is something I have said before, but this
particular example is a good one.  The primary argument in favor of
compilers which support general garbage collection is that the end
user shouldn't have to worry about garbage collection.  But in this
example 1) the user of the package sees no garbage ever and 2)
managing memory "right" was an extremely small effort when writing the
generic.  Even if all Ada compilers had generalized garbage
collectors, the right choice here would be to build a procedure which
predictably cleaned up after itself rather than warning the user that
there would be lots of garbage generated which might affect global
performance.


					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...


----------------------------------------------------------------------------
-- generic
--   with procedure To_Do(File: in String);
with A_Strings;
with File_Names;
procedure Iterate_Files(Pattern: in String := "*";
                        Directory: in String := "") is
  Pat: A_Strings.A_String;
  Handle: File_Names.Find_File_Info;
  Next: A_Strings.A_String;
begin

  if Directory = "" 
  then Pat := A_Strings.To_A(Pattern);
  else Pat := A_Strings.To_A(Directory & "/" & Pattern);
  end if;

  Handle := File_Names.Init_Find_File(Pat);
  loop
     Next := File_Names.Find_File(Handle);
     if Directory = "" and then Next.s(1..2) = "./"
     then To_Do(Next.s(3..Next.len));
     else To_Do(Next.s);
     end if;
     A_Strings.Free(Next);
  end loop;
exception
  when File_Names.No_More_Files => A_Strings.Free(Pat);
end Iterate_Files;

-----------------------------------------------------------------------------
--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...