Re: "continue/next" for "loop"

["Robert I. Eachus" <rieachus@comcast.net>]

Jeffrey Carter wrote:

>> goto exit; -- exit innermost enclosing loop.
>> exit loop; -- alternative syntax. ;-)
> 
> 
> This already exists in Ada; it's syntax is
> 
> exit;
> 
> Without a loop name or when part, exit exits the innermost enclosing loop.

Correct.  I often use "exit when."  Actually for the type of cases 
discussed here, I often write the loop as an inlined procedure or 
function and use a return statement for premature loop termination. (As 
an example, walking an AVL tree looking for a particular key.  The 
return takes care of all the complex nesting issues that can occur.)

The goto exit was proposed as "syntactic sugar," since I often see 
programmers using gotos where exit would do the job. I thought "exit 
loop" was rubbing the sarcasm in even with the smiley. I have too many 
times been asked how can I exit a loop.  So I thought it was obvious I 
was being sarcastic.  (Maybe I should have said:

exit [loop_name]; -- alternative syntax. ;-)

Incidently, I the problem which results in exit statements being 
overlooked is classes that teach for loops, then while loops and maybe 
eventually get around to:

loop
   ...
   exit when Done;
   ...
end loop;

If you teach students about the general purpose loop first, then add for 
and while loops as special cases, maybe students will remember. I have 
no problem.  In the first Ada compiler I worked on, while loops were 
parsed exactly as if they said "loop exit when not <expression>; 
<sequence of statements> end loop;"  No reason to generate extra work 
for semantic analysis when we could eliminate some constructs in the 
parser by translating them into token streams with identical meanings.

As for the horrible example that started this thread, I have often had 
to deal with this type of twisted loop in parsing.  Often the best 
solution is instead of using two for loops use one plain loop, and make 
incrementing the loop variables explicit.  Sometimes that gets too heavy 
and you really do have to go to named states with gotos.

> procedure t is
> begin
>    for j in 2 .. 8 loop
>       for i in 2 .. 8 loop
>          if i mod j = 0 then  --  Assume complex statements here.
>             goto next;        --  In my real case, it's another loop,
>          end if;              --  which detects the end of current path inside.
>          Put(i'Img);
>          <<next>> null;
>       end loop;
>       New_Line;
>    end loop;
> end t;


procedure t is
begin
    for j in 2 .. 8 loop
       for i in 2 .. 8 loop
          if i mod j = 0 then  --  Assume complex statements here.
             exit;
          else
             Put(i'Img);
          end if;
       end loop;
       New_Line;
    end loop;
end t;

Which in part was why I thought this was another case of "Help! I'm 
stuck inside a loop and can't get out."

-- 

                                           Robert I. Eachus

100% Ada, no bugs--the only way to create software.