Decimal arithmatic, was Re: Python GUI app to impress the boss?

[Christian Tismer]

Paul Rubin wrote:
> "Chris Gonnerman" <chris.gonnerman at newcenturycomputers.net> writes:
> 
>>>What do you mean by "right"?  What do you mean by "all 
>>>cases"?
>>
>>The original subject is *business* arithmetic.  There is no
>>choice but to use decimal arithmetic if you want your math
>>results to match good-old-fashioned pen and paper math.
> 
> 
> No, I'm still not convinced.  Is business arithmetic well-defined
> enough that you can never do the same calculation two ways and get
> different results?
> 
> Example: bunches are three for a dollar.  How much do you pay if you
> buy nine bananas?
> 
>     Answer #1:
>       price_per_banana = 1.00 / 3
>       number_of_bananas = 9
>       total_price = number_of_bananas * price_per_banana
>       print total_price
> 
>     Answer #2:
>       price_per_bunch = 1.00   # three bananas
>       number_of_bunches = 3
>       total_price = number_of_bunches * price_per_banana
>       print total_price

Sorry, this is an argument I cannot accept.
A discussion about decimal or floating arithmetic
doesn't give the right to break basic rules of
numeric computation. It is a known fact that
division by a number that contains a prime factor
relatively prime to the number base produces
an error.
I would fire such a programmer in any case, whether the
error shows up in decimal math, or wether it hides
due to graceful rounding rules in floating point.

> With floating point and rounding on output, you get the same answer
> both ways.

Exercise: Find the extremes where this claim is proven wrong.

Finally I see a big advantage in decimal math:
Doing wrong numeric shows up earlier :-)

ciao - chris

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