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Path: utzoo!attcan!uunet!ginosko!gem.mps.ohio-state.edu!rpi!batcomputer!cornell!rochester!pt.cs.cmu.edu!MATHOM.GANDALF.CS.CMU.EDU!lindsay
From: lindsay@MATHOM.GANDALF.CS.CMU.EDU (Donald Lindsay)
Newsgroups: comp.lang.misc,comp.arch
Subject: Numbers (was Re: Fast conversions)
Keywords: fixed point integer
Message-ID: <6244@pt.cs.cmu.edu>
Date: 23 Sep 89 19:55:25 GMT
References: <832@dms.UUCP> <688@UALTAVM.BITNET> <9dAz02zs58y201@amdahl.uts.amdahl.com> <27935@winchester.mips.COM> <136@bbxsda.UUCP> <1989Sep22.201906.10618@utzoo.uucp>
Organization: Carnegie-Mellon University, CS/RI
Lines: 33

>In article <136@bbxsda.UUCP> scott@bbxsda.UUCP (Scott Amspoker) writes:
>..some of us need to have 100 pennies == 1 dollar.  Interest calculations
>carried out to 3 or 4 decimal places need to round correctly.

Well, it is possible to have decimal floating point. I myself
perpetrated a BCD floating-point package, back in the eight-bit days.

The military world has always been fond of putting decimal points in
the middle of binary integers, and calling it "fixed point". The
worst language I have ever used, the navy's CMS-2M, featured this
data type. The spec for Ada required it, so now Ada allows you to say

	type VOLT is delta 0.125;

This actually runs fine on normal machines, because it is the
compiler that maintains the scaling (in this case, by three bits).
The real problem is that the roundoff will be in eighths of a volt
(or 256ths of a penny, or 1024ths of a nautical mile, or whatever).
Also, the conversions are bad:

	voltage := 0.3;		-- choice is ,,0.25, 0.375,,,

In general, people would be better off if they could say something
like
	type MILLVOLT is integer;
	type VOLT is MILLIVOLT * 1000;

Normal floating point has a dynamic range of 10**76 (or whatever).  A
32 bit integer only has a range of 10**9, but IBM takes in more than
10**13 pennies per year. So, a 64 bit integer (10**19) is about right
when you want to compute in millicents, and this is what I recommend.
-- 
Don		D.C.Lindsay 	Carnegie Mellon Computer Science