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prePEP: Decimal data type

Batista, Facundo FBatista at uniFON.com.ar
Thu Nov 6 13:49:03 EST 2003 
bokr at oz.net wrote:

#- I wonder how they deal with compound interest. E.g., may a 
#- contract for a loan
#- express interest as mathematically continuous between 
#- payments? In that case
#- does rounding from mathematically exact values occur only at 
#- payment times?
#- Is the rounding only applied to the payment amount, so that 
#- the princal is
#- decreased by the exact quantized payment, but the balance 
#- retains exactness?
#- Or is requantization of all values mandated at each 
#- transaction? Is each step in
#- a sum a separate transaction? What about the rules for 
#- pre-tax-calculation subtotals
#- vs accumulating actual receipt stated tax amounts? I 
#- remember as a kid bugging
#- store clerks to sell me two candies separately to stay under 
#- the minimum taxable
#- price threshold twice rather than exceed it once and pay the 
#- extra penny ;-)

The problem I see when you round is that

	d * (a + b) != d*a + d*b

i.e.:

	d = 1.23
	a = 5.15
	b = 3.66

half-up rounding in two decimals on *each* operation:

	1) 10.84
	2) 10.83

I think that is up to the human, knowing that he's rounding, to write the
expression in one or other way. Maybe following legislation, maybe at
random. But it seems to me pretty difficult to the class to predict each
behaviour and get the same result in both cases.

.	Facundo
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