GNOME Bugzilla – Bug 769746
Zero interest loan formula fails - An error occurred while processing pmt
Last modified: 2018-06-29 23:50:34 UTC
When setting up a loan using druid, and using 0 (zero) for interest rate, resultant formula includes 0.00001 Hence calculations produce errors. When attempting to change formula, ERROR returned: An error occurred while processing pmt( 0 : 60.00 : 50,000.00 : 0 : 0 ). formula lines by default: payment pmt( 0.00001 / 12.00 : 60.00 : 50,000.00 : 0 : 0 ) principal ppmt( 0.00001 / 12.00 : i : 60.00 : 50,000.00 : 0 : 0 ) interest ipmt( 0.00001 / 12.00 : i : 60.00 : 50,000.00 : 0 : 0 ) attempts to change all FAIL - examples: payment pmt( 0.00000 / 12.00 : 60.00 : 50,000.00 : 0 : 0 ) payment pmt( 0 / 12.00 : 60.00 : 50,000.00 : 0 : 0 ) payment pmt( 0 : 60.00 : 50,000.00 : 0 : 0 )
That's because the pmt functions divide by the interest rate, and as you probably know dividing by zero is an error. There's no reason to use the load druid for a zero-interest loan: ipmt = 0 and pmt = ppmt = principal/periods. I suppose we could add special-case code to the druid to do that, but it seems silly and a waste of time. Unless you can make a really good case to the contrary I'm inclined to close this as "won't fix".
We may not have to special case the calculation code but we could return a better error message, suggesting for a 0% rate loan you can just as well create a straightforward SX. That's less work, yet more user friendly.
We'd still need to add special case the code. The current minimum of .001% is imposed by the irate_spin control. Editing the payment formula on the third page doesn't actually do anything and while it is read the value is ignored so we should make it not editable. To do anything about this we'd need to change the minimum of irate_spin then check if it's zero and raise an error dialog -- and then set the rate to .001% anyway to prevent an SX crash later. Alternatively we could just leave it alone and put a tooltip on irate_spin saying that it's not able to handle rates below .001%. ISTM that any further discussion of why one can't have a 0 interest rate should go in the documentation. There's currently a very basic page about the loan assistant at http://www.gnucash.org/docs/v2.6/C/gnucash-help/trans-sched-loans.html; the Loans chapter in the Guide is about setting up the appropriate accounts and using the Loan Calculator.
Thanks for the analysis. I agree to all points.
I've disabled the editing of the amount_ent GtkEntry and added the following tooltip to irate_spin: "Enter the annual interest rate in percent. Accepts values from 0.001 - 100. The Mortgage Assistant does not support zero-interest loans." I've added similar documentation to the Mortgage Assistant's help page along with a brief explanation of how to create a zero-interest repayment scheduled transaction.
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