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From: ags@j.cc.purdue.edu (Dave Seaman)
Newsgroups: sci.math,comp.lang.modula2
Subject: Re: perpetual calendar (also day of week)
Message-ID: <6413@j.cc.purdue.edu>
Date: 11 Feb 88 16:54:42 GMT
References: <8802101656.AA08424@decwrl.dec.com>
Reply-To: ags@j.cc.purdue.edu.UUCP (Dave Seaman)
Organization: Purdue University
Lines: 35

In article <8802101656.AA08424@decwrl.dec.com> fulton@navion.dec.com (10-Feb-1988 0957) writes:

 [ description of perpetual calendar formula omitted ]

>        I have been intrigued by the above formula, and I coded it up in
>        Logitech Modula-2 to try it out.  It appears that there are some
>        cases where it doesn't work quite right.  For instance:

 [ example leading to negative numbers omitted ]

>        Then you have -1 MOD 7  =>  -1,  which does not equate to any of
>        the numbers assigned to Sunday thru Saturday, namely 0 thru 6. 

 [ illustration of adding 7 to make result positive ]
 
>        I guess that  this  is  an  after-the-fact  kludge;  perhaps the
>        original formula could be  changed  so  that  the result doesn't
>        require "fixing" if it comes out negative.

I don't have my copy of the Modula-2 standard available at the moment, but
I believe that the definition of the MOD operator is similar to Pascal,
where the standard says:

	A term of the form i mod j shall be an error if j is zero or
	negative, otherwise the value of i mod j shall be that value of
	(i-(k*j)) for integral k such that 0 <= i mod j < j.

Therefore the correct value of (-1 MOD 7) is 6, not -1.  In fact, I believe
that in Modula-2 the result of the MOD operator is type CARDINAL and
therefore cannot be negative.  It sounds as if you may be stuck with a
flawed implementation of Modula-2.

-- 
Dave Seaman	  					
ags@j.cc.purdue.edu