Re: [boost] Re: Fixed-decimal partial review

From: Beman Dawes (bdawes_at_[hidden])
Date: 2003-07-20 13:10:52

At 08:44 PM 7/19/2003, Misha Bergal wrote:
 >"Beman Dawes" <bdawes_at_[hidden]> wrote in message
 >news:4.3.2.7.2.20030719112551.01ecfa40_at_mailhost.esva.net...
 >> Years ago I worked on some general business applications involving 
liquid
 >> inventories dealing in tank dimensions, gallons, coefficients of
 >expansion,
 >> and all sorts of other non-money related data. Fixed-point arithmetic 
was
 >> essential for those general business applications even though they 
aren't
 >> strictly speaking financial or money related. (The reasons floating 
point
 >> won't do is the legal and business need mandated others, including
 >> government regulators, to be able to repeat the exact computation, and
 >> produce the exact answer.)
 >
 >Beman,
 >
 >Could you elaborate a little more about the requirements to use fixed 
point
 >instead of floating point. Not being an expert in the area, I don't see
 >many use cases where fixed point would be preferable to floating point.
 >I really would appreciate if you described your ones.

Start by thinking of everyday financial calculations. Gasoline is priced 
(in the US) in terms of dollars, cents, and tenths of cents. Three decimal 
places. Gasoline quantities are measured in terms of gallons and tenths, or 
maybe gallons and hundredths. 1 or 2 decimal places. Note that consumer 
protections regulations may mandate the number of places.

Now if we multiply quantity to two decimal places of quantity times price 
to three decimal places, you might think the answer would have five decimal 
places. Wrong. Practical business considerations, and sometimes government 
regulations mandate the answer should be give to two decimal places 
(dollars and cents.) The rounding mode may also be mandated.

Likewise, even in a non-financial application like computing temperature 
compensated volume of propane, the requirement isn't to produce an answer 
accurate as many decimal places as possible. Rather, the requirement is to 
meet some mandate (from the government, auditors, or some other authority) 
which specifies how many decimal places are desired. Again, rounding mode 
and other details of the computation are sometimes specified, sometimes 
even at the level of intermediate calculation steps. There may also be a 
repeatability requirement - someone (using a different computer, or a hand 
calculator, or even a pencil and paper) may have to be able to repeat the 
calculation, and produce exactly the same answer.

If "as accurate as possible" is the requirement, floating point is probably 
best. But sometimes in business and industry a specific accuracy in terms 
of decimal places and rounding modes is the requirement. Fixed-point is 
better for those situations, IMO.

--Beman