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Date: 21 Jul 93 22:19:48 GMT
From: seas.gwu.edu!mfeldman@uunet.uu.net  (Michael Feldman)
Subject: Re: Can you help with fixed-point arithmetic?
Message-ID: <1993Jul21.221948.25724@seas.gwu.edu>
List-Id: <comp.lang.ada>

In article <1993Jul20.220309.1563@sei.cmu.edu> ae@sei.cmu.edu (Arthur Evans) wr
ites:
>mfeldman@seas.gwu.edu (Michael Feldman) asks (in effect) why his
>compiler reports that Constraint_Error will be raised from the statement
>       27    TimeThen := TimeNow + 10 * Duration'(86400.0);
>
>Consider in isolation
>	10 * Duration'(86400.0)
>This is a multiplication whose left operand is an integer and whose
>right operand is of type Duration, and we find its semantics in
>4.5.5(7), where we learn that its result type will be of type Duration.
>
>The probable problem is that the result is too big for that type.
>Although the LRM doesn't specify Duration's range (see C(19)), the usual
>value is not much more than enough to store 24 hours.  (A larger range
>is counter-productive, as it decreases the fineness.)  The range for
>Duration in Meridian Ada on the Mac is
>	-86400.0 .. 86400
>I suspect it's the same on the PC.
>
>Next question: how to get around the problem.  A way that at first seems
>plausible is to declare a fixed-point type with a large enough range and
>convert 86400.0 to that.  This won't work, though, since the '+' on line
>27 wouldn't be defined.  (It's now the '+' exported by package
>Calendar.)
>
Yep, I got stung by forgetting that the LRM does not require a Duration 
range of more than a day. In any event, thanks to all of you who pointed
this out by e-mail and on the net. I'll find another way.

Actually, I'd decided to represent a date as a record containing year
and Julian date within year (1..365 or 1..366) but thought I could get
away with simpler date arithmetic by using built-in operations> I guess
my original design was OK after all.

Thanks!

Mike Feldman
------------------------------------------------------------------------
Michael B. Feldman -  co-chair, SIGAda Education Committee
Professor, Dept. of Electrical Engineering and Computer Science
The George Washington University -  Washington, DC 20052 USA
202-994-5253 (voice) - 202-994-0227 (fax) - mfeldman@seas.gwu.edu (Internet)
"We just changed our CONFIG.SYS, then pressed CTRL-ALT-DEL. It was easy."
-- Alexandre Giglavyi, director Lyceum of Information Technologies, Moscow.
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