[GNC] Loan Mortgage repayment assistant problem

Stephen M. Butler kg7je at arrl.net
Wed Jan 9 13:12:18 EST 2019


On 1/9/19 6:26 AM, Michael or Penny Novack wrote:
> On 1/8/2019 11:50 PM, John Ralls wrote:
>> The answer is probably in app-utils/calculation. All of that was
>> probably written before Guile grew a rational number class and I
>> think that GncNumerics get converted to doubles before being passed
>> to the interest functions. That would pretty much guarantee rounding
>> issues.
>>
>> Regards,
>> John Ralls
>  I will repeat, this sort of "rounding error" is not a bug.
>
> If two persons set out to perform a calculation which can be done by a
> number of methods with rounding involved at numerous places and no
> agreement as to either the method to be used or the precision
> maintained at each step THERE IS NO REASONABLE EXPECTATION THAT THEIR
> ANSWERS WILL MATCH EXACTLY.
>
> Treated as a "bug", it might* be possible to change the code so as to
> match the answers of a particular lending institution, but then could
> be wrong for another lending institution that used different methods
> or rounding than the first.
>
> Michael D Novack    << who in my working days often had to compute the
> "fuzz" for comparisons >>
>
> * Might, because you have no reason to suppose that a lending
> institution necessarily used the same methods/rounding for car loans
> as for house mortgages.
And some use generic 30 day months while others use the actual number of
days in each month.  Some compound daily while others use simple interest.

-- 
Stephen M Butler, PMP, PSM
Stephen.M.Butler51 at gmail.com
kg7je at arrl.net
253-350-0166
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