# Re: Fixed Point Arithmetic

Date: 29 Mar 2007 10:09:01 -0700

Message-ID: <1175188140.978189.245960_at_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 - 19:09:01 CEST