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From: "Jean-Pierre Rosen" <rosen.adalog@wanadoo.fr>
Subject: Re: Why C++ is successful
Date: 1998/08/17
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Date: 1998-08-17T00:00:00+00:00
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Robert Dewar a �crit dans le message ...
>JPR says
>
><<Certainly not. I just need to put a label Outer: on it, and a statement
>"exit outer" at any place where I discover that the array is sorted.
>>>
>
>Kindly then, please do so, we know the principles, the point is the
example.
>You posted a program that was syntactically incorrect, and also contained
>a fundamental semantic error. It is not so easy asyou think to avoid the
>flag here, please try -- don't just say you can do it without proving it!
OK, I confess I responded a bit hastilly (as proven by the obvious syntax
error). I focused on the case when the goto was executed, and forgot the
other one...

Now, just for fun, I'll give it a try anyway. Well, the original was:


<<Sort>> for J in 1 .. N - 1 loop
       if D (J) > D (J + 1) then
          Swap (D(J), D(J + 1));
          goto Sort;
       end if;

Obviously, we need to terminate the algorithm if we go to the end of the
loop. This woud translate as:
Outer: loop
   Inner: for J in 1 .. N - 1 loop
       if D (J) > D (J + 1) then
          Swap (D(J), D(J + 1));
          exit Inner;
       end if;
       if J = N-1 then
          exit Outer;
       end if;
   end loop Inner;
end loop Outer;

but of course, we add a spurious test in the inner loop, and I hate testing
for special values of the loop variable (sorry, constant!) in the body of
the loop. Actually, what we miss here is an exit from a *block*, but we can
use an extra loop to the same effect:

Outer: loop
   Restart : loop
      for J in 1 .. N - 1 loop
          if D (J) > D (J + 1) then
             Swap (D(J), D(J + 1));
             exit Restart;
          end if;
      end loop;
      exit Outer;
   end loop Restart;
end loop Outer;

OK, you asked me to prove it was feasible, I think I did (unless I missed
another obvious point, I'd better be careful now :-).
Now, don't tell me it's a lot of baggage for the sake of avoiding a goto, I
agree.