[GNC] Loan Mortgage repayment assistant problem

[David Carlson]

I have created https://bugs.gnucash.org/show_bug.cgi?id=797032.

David C

On Wed, Jan 9, 2019 at 2:29 AM David Carlson <david.carlson.417 at gmail.com>
wrote:

> With those numbers payment number 2 has an error.
>
> David C
>
> On Wed, Jan 9, 2019 at 2:23 AM David Carlson <david.carlson.417 at gmail.com>
> wrote:
>
>> I am working on a test scenario.The interest formula is ipmt( 0.03500 /
>> 12.00 : i : 180.00 : 172,000.00 : 0 : 0 ), the others have the same numbers
>> and the 71st payment contains the error.  I am using these numbers because
>> they are exactly the same as the numbers that have already generated an
>> error.
>> i started the test loan in Jan 2000 with the first repayment in February
>> and I am creating one month at a time in case that makes a difference.  If
>> the test can repeat it I will submit the bug report.
>>
>> David C
>>
>> On Tue, Jan 8, 2019 at 10:52 PM John Ralls <jralls at ceridwen.us> 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
>>>
>>> > On Jan 8, 2019, at 5:46 PM, Christopher Lam <christopher.lck at gmail.com>
>>> wrote:
>>> >
>>> > Well the SCM has numerous references to "round" and "rounding errors"
>>> so it's to be expected.
>>> >
>>> > Please file a bug with example numbers, and hopefully these errors may
>>> be fixed.
>>> >
>>> > On 9/1/19 7:35 am, David Carlson wrote:
>>> >> John,
>>> >>
>>> >> Thanks for the reference.  Alas, I will need to do some homework to
>>> >> properly read SCM.  There must still be some sort of rounding issue
>>> >> creeping in if the Scheduled Transaction SLR sometimes generates an
>>> >> imbalance from ppmt = pmt - ipmt.
>>> >>
>>> >> David C
>>> >>
>>> >> On Tue, Jan 8, 2019 at 3:44 PM John Ralls <jralls at ceridwen.us> wrote:
>>> >>
>>> >>> The thee functions are defined in
>>> >>>
>>> https://github.com/Gnucash/gnucash/blob/maint/libgnucash/app-utils/fin.scm
>>> ,
>>> >>> prefixed with gnc: (so gnc:ipmt etc.).
>>> >>>
>>> >>> At line 41ff you'll see that ppmt = pmt - ipmt.
>>> >>>
>>> >>> Regards,
>>> >>> John Ralls
>>> >>>
>>> >>>
>>> >>>> On Jan 8, 2019, at 12:48 PM, David Carlson <
>>> david.carlson.417 at gmail.com>
>>> >>> wrote:
>>> >>>> My point is that the equations are defined by the assistant, and I
>>> think
>>> >>> that they can be expressed in a form that concentrates all the
>>> errors into
>>> >>> one line where a simple rounding would not generate a problem where
>>> the
>>> >>> parts do not add up to 100%.  Granted, the errors may sometimes cause
>>> >>> inaccurate results, but they would not add the imbalance line to the
>>> >>> transaction.
>>> >>>> The current equations are three  evaluations called pmt(numbers),
>>> >>> ppmt(numbers) and ipmt(numbers) with no clue how they are actually
>>> >>> evaluated.
>>> >>>> Apparently pmt is not always the sum of ppmt and ipmt. when rounding
>>> >>> happens.  If ipmt were expressed as pmt-ppmt, there would be no
>>> imbalance,
>>> >>> but one of the other two ways to express all three may be less
>>> likely to
>>> >>> generate inaccurate results.
>>> >>>> David C
>>> >>>>
>>> >>>> On Mon, Jan 7, 2019 at 10:19 PM John Ralls <jralls at ceridwen.us>
>>> wrote:
>>> >>>>
>>> >>>>
>>> >>>>> On Jan 7, 2019, at 3:14 PM, David Carlson <
>>> david.carlson.417 at gmail.com>
>>> >>> wrote:
>>> >>>>> I just had a scheduled transaction that was set up by the
>>> loan/mortgage
>>> >>>>> repayment assistant appear with 0.01 Imbalance-USD.
>>> >>>>>
>>> >>>>> I would think that GnuCash should have an accuracy method that
>>> would
>>> >>>>> prevent this from happening.  Is this a bug?
>>> >>>>>
>>> >>>>> I am using GnuCash 2.6.17 in Ubuntu 16.04 today.
>>> >>>> The accuracy method is what created the imbalance entry. ;-)
>>> >>>>
>>> >>>> There’s nothing in the SX system to pre-instantiate future scheduled
>>> >>> transactions and ensure that they’re balanced. What would you have
>>> the
>>> >>> check function do if it found an imbalanced case? How would you
>>> handle
>>> >>> variables whose values are set by the user in the SLR dialog?
>>> >>>> Regards,
>>> >>>> John Ralls
>>> >>>>
>>> >>>
>>> >> _______________________________________________
>>> >> gnucash-user mailing list
>>> >> gnucash-user at gnucash.org
>>> >> To update your subscription preferences or to unsubscribe:
>>> >> https://lists.gnucash.org/mailman/listinfo/gnucash-user
>>> >> If you are using Nabble or Gmane, please see
>>> https://wiki.gnucash.org/wiki/Mailing_Lists for more information.
>>> >> -----
>>> >> Please remember to CC this list on all your replies.
>>> >> You can do this by using Reply-To-List or Reply-All.
>>> > _______________________________________________
>>> > gnucash-user mailing list
>>> > gnucash-user at gnucash.org
>>> > To update your subscription preferences or to unsubscribe:
>>> > https://lists.gnucash.org/mailman/listinfo/gnucash-user
>>> > If you are using Nabble or Gmane, please see
>>> https://wiki.gnucash.org/wiki/Mailing_Lists for more information.
>>> > -----
>>> > Please remember to CC this list on all your replies.
>>> > You can do this by using Reply-To-List or Reply-All.
>>>
>>> _______________________________________________
>>> gnucash-user mailing list
>>> gnucash-user at gnucash.org
>>> To update your subscription preferences or to unsubscribe:
>>> https://lists.gnucash.org/mailman/listinfo/gnucash-user
>>> If you are using Nabble or Gmane, please see
>>> https://wiki.gnucash.org/wiki/Mailing_Lists for more information.
>>> -----
>>> Please remember to CC this list on all your replies.
>>> You can do this by using Reply-To-List or Reply-All.
>>
>>