NetNews Usenet Archive 1992 #19/NN_1992_19.iso/spool/comp/lang/misc/2925

Text File | 1992-09-03 | 3.5 KB | 91 lines

Newsgroups: comp.lang.misc
Path: sparky!uunet!mcsun!news.funet.fi!funic!news.cs.hut.fi!news!Ari.Huttunen
From: [e-mail] (Ari Juhani Huttunen)
Subject: Sather loops (was Re: Scientists as ...
In-Reply-To: [e-mail]'s message of 30 Aug 92 17:25:05 GMT
Message-ID: <[e-mail]>
Sender: [e-mail] (Uutis Ankka)
Organization: Helsinki University of Technology, Finland
References: <[e-mail]> <[e-mail]>
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Date: Fri, 4 Sep 1992 00:18:32 GMT
Lines: 76

! In article <[e-mail]> [e-mail] (Ari Juhani Huttunen) writes:
!    Yes, I would myself like to see these loop-constructs in Sather:

!   loop       until <xxxx> loop   while <xxxx> loop  for <xxxx> to <yyyy> loop
!     <xxxx>     <yyyy>              <yyyy>             <zzzz>
!   end        end                 end                end

                   ^
                   |
                   | Forget that one.

! IMHO the above are quite poor syntax for expressing the kinds of loops
! that I see in numerical code.  Quite often it is possible to work in
! parallel, and all the above do is make it harder to extract the
! parallelism (of course so does the FORTRAN notation, but there is not
! reason to make the same mistake :-).

I can't know what kinds of loops you see in numerical code, so I can't
comment on that. I got the impression from the previous article that
a for-loop is wanted. Now it is not wanted?

As for the parallelism... You *do not* extract parallelism from
a sequential program. You design a parallel program. That is not
the same thing. Extracting parallelism from a sequantial loop does
not help 0.01 cents if the computer has 64000 processors and now
it uses 3 of them instead of 1.

!  In the cases where it is not
! numerical, syntax like the above is often useless as it is usually
! restricted so that you can only iterate over integer indices (cf
! Oberon-2).  Note some languages get this right and let you iterate
! over whatever you want e.g. Eiffel and C (given Sather's parentage I
! guess it does to)

Sather has the same loop construct that Eiffel has, and it is written
on my list. The reason why I put the for-loop in the list is that it
is so common. Take a look at this piece of code from the Sather library:

      -- Clear out the array.
      i:INT; until i=asize1 loop
     j:INT; until j=asize2 loop
        k:INT; until k=asize3 loop
           [i,j,k]:=void;
           k:=k+1;
        end; -- loop
        j:=j+1;
     end; -- loop
     i:=i+1;
      end; -- loop

With the for-loop it becomes shorter and much easier to read. To see
what I mean, note that at the first sight, you look at the beginning
of the rows and see 'for ...'. You immediately notice a keyword. In
contrast the above code has 'k:INT; ...' at the beginning.

      -- Clear out the array.
      for i:INT to asize1 loop
     for j:INT to asize2 loop
        for k:INT to asize3 loop
           [i,j,k]:=void;
        end; -- loop
     end; -- loop
      end; -- loop

You wouldn't need the for-loop for iterating over the elements of a list
(for example), so it need not be overly general. The existing loop is
perfect for that. You could, however, have something like:

    for r=0.0 to 5.3 step 0.1 loop
        ...
    end
--
...............................................................................
Ari Huttunen                      Any similarity to other alien life forms
                          is purely coincidental.
                          <Alien 3 misquote>