Re: Fixed Point Arithmetic

From: Marshall <marshall.spight_at_gmail.com>
Date: 29 Mar 2007 10:09:01 -0700
Message-ID: <1175188140.978189.245960@r56g2000hsd.googlegroups.com>

On Mar 29, 7:32 am, "David Cressey" <cresse..._at_verizon.net> wrote:

> These FP numbers do not have closure over multiplication, because the
> product may have up to four significant digits after the decimal point, and
> therefore have to be rounded off for storage purposes.

Yes, and they also have overflow.

> Anyway, I'm going to say that FP numbers with a scale factor of 2 save an
> enormous amount of head-scratching when working with bean counting
> applications. You won't spend time looking for the missing penny. OTOH,
> if you use these numbers for complicated arithmetic (example: amortization
> schedule) you have to think about roundoff errors.

At my company, we often use fixedpoint with a scale factor of 6 for money.

> When it comes to the stock market, it depends on the country. AFAIK, the
> NYSE still records stock "points" in units of dollars, with binary
> fractional points like 1/4, 1/32, and the like.

I flirted with the idea of stockbroker as career. Got the license, had some clients, etc. Watched the SUNW IPO; that was cool. Had a fancy real-time market data device with me on Black Monday '87, that was dramatic. How low can it go? Ultimately it wasn't for me. Back then (mid-eighties) the NYSE still used fractions, but it's decimal now:

http://finance.google.com/finance?q=ibm

(Gosh I love it when the URLs are nice and clean.)

> Are fixed point numbers a useful form for DBMSes to support?

I expect so. Wouldn't it be nice to be able to specify a fixed point type just by supplying the number of bits total and the number of bits to the right of the decimal?

Marshall Received on Thu Mar 29 2007 - 12:09:01 CDT