Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!seismo!sundc!pitstop!sun!decwrl!decvax!necntc!culdev1!drw
From: drw@culdev1.UUCP (Dale Worley)
Newsgroups: comp.lang.misc,comp.software-eng
Subject: Software Technology is NOT Primitive
Message-ID: <1720@culdev1.UUCP>
Date: Thu, 29-Oct-87 10:56:06 EST
Article-I.D.: culdev1.1720
Posted: Thu Oct 29 10:56:06 1987
Date-Received: Tue, 3-Nov-87 04:23:01 EST
Organization: Cullinet Software, Westwood, MA, USA
Lines: 33
Xref: mnetor comp.lang.misc:816 comp.software-eng:25

crowl@cs.rochester.edu (Lawrence Crowl) writes:
| For instance, the arithmetic right shift
| provided by many machines provides division by two except when the number is
| negative and odd.  Should languages be designed around this quirk?  I do not
| think so.

This is not so simple...  What do you mean by "division by 2"?  There
are actually at least two ways of doing integer division:  one way is
to always round the mathematical quotient towards 0.  This is the
common, or "Fortran", way to do it, but to a certain extent it is
historical accident that most programming languages do it this way.
Of course, this is *not* what arithmetic right shift does.

But this method is not necessarily the most natural way to do
division.  In many cases, this is more natural:  round the
mathematical quotient *down*, that is, more negative.  This is
equivalent to the common way for positive quotients, but is different
for negative quotients:  (-1)/2 = -1.

(If you think this method is silly, consider the problem:  I give you
a time expressed in minutes before-or-after midnight, 1 Jan 1980.
Find the hour it occurs in, expressed in hours before-or-after
midnight, 1 Jan 1980.  The advantage of the second division here is
that the quotient is computed so the remainder is always positive.)

I have had to write functions in several programming languages to
perform this second form of division.

Dale
-- 
Dale Worley    Cullinet Software      ARPA: culdev1!drw@eddie.mit.edu
UUCP: ...!seismo!harvard!mit-eddie!culdev1!drw
If you get fed twice a day, how bad can life be?