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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Software landmines (loops)
Date: 1998/08/30
Message-ID: <m3ogt3qgca.fsf@mheaney.ni.net>#1/1
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NNTP-Posting-Date: Sun, 30 Aug 1998 03:44:04 PDT
Newsgroups: comp.lang.eiffel,comp.object,comp.software-eng,comp.lang.ada
Date: 1998-08-30T00:00:00+00:00
List-Id: <comp.lang.ada>

Nick Leaton <nickle@calfp.co.uk> writes:

> I have just done a quick check through some Eiffel code. 
> 
> Out of 633 loops, just 1 needed a flag for early termination of the
> loop.

My own experience is quite different from yours.  Although I haven't
counted, I use early loop termination ALL THE TIME.

For example, I routinely implement an equality test (say, for a stack)
as

function "=" ... return Boolean is
begin
...

  for Index in Positive range 1 .. L.Depth loop
     if L.Items (Index) /= R.Items (Index) then
        return False;
     end if;
  end loop;

  return True;

end "=";


If my loop termination depends on a value I just read, then exit from
the middle is the simplest solution:

loop
   Get (N);

   exit when N = 0;

   <process N>
end loop;

For example, this is a simplest way to tokenize a lexeme.  You exit the
loop in the middle, after determining that the character just consumed
isn't part of the current token.

Exiting is how you terminate passive iteration early:

procedure For_Every_Item (Stack : in Stack_Type) is
 
   Done : Boolean := False;

begin

   for Index in reverse Positive range 1 .. Stack.Top loop
      Process (Stack.Items (Index), Done);
      exit when Done;
   end loop;

end For_Every_Item;


Searching is also implemented using early loop termination:

function Get_Position
  (C : Character; 
   S : String) return Natural is
begin
   for Index in S'Range loop
      if S (Index) = C then
         return Index;
      end if;
   end loop;

   return 0;
end Get_Position;
   

No auxilliary variables or flags are required.

Below is a portion of my response to another thread on comp.lang.ada re
the goto statement.

Matt

(start of quote)
Any theory about using (or not using) a programming construct needs to
be tested against observation, by performing an empirical study that
measures the defect rates of programmers using the construct.

If theory says use a construct, but observation reveals that programmers
who use the construct produce more errors, then the theory needs to be
thrown out.

For example, there's a pernicious myth that exiting (or returning) from
the middle of a loop is bad, and that the only proper way to write a
loop is to state the termination condition explicitly, as a predicate
appearing at the top of the loop.

This theory was indeed put to the test, and guess what?  Programmers
using a test-and-exit from the middle of the loop produced fewer errors
than those programmers who tried to put the test at the top of the loop.
The researchers found that the exit-from-the-middle construct had a
better "cognitive fit" than the other constructs.

If you want to read the gory details, the article is

Cognitive Strategies and Looping Constructs: An Empirical Study
Soloway, Bonar, Ehrlich
CACM, Nov 83, Vol 26, No 11, p 853-860

The goodness of a language construct should not be determined by
doctrinaire computer scientists or mathematicians.  The only thing that
matters is whether working programmers think it's easier to understand,
and whether by using the construct programmers inject fewer errors into
the code.

I think it was Stroustrup who said, "Programming is a human activity.
Forget that, and all is lost."
(end of quote)