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

[Paul Rubin]

"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

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

With decimal arithmetic, you get $2.97 the first way and $3.00 the
second way.

For "business arithmetic" to always give the same answer, it has to
forbid at least one of those methods of doing the problem.