[Numpy-discussion] Definitions of pv, fv, nper, pmt, and rate

David Goldsmith d_l_goldsmith@yahoo....
Wed Jun 10 00:00:14 CDT 2009


--- On Tue, 6/9/09, Skipper Seabold <jsseabold@gmail.com> wrote:

> I have implemented the ipmt and ppmt functions that were
> "not
> implemented" in numpy.lib.financial as well as written some
> tests.

Thanks!

> ipmt is one of the functions where there was a discrepancy
> between
> what OO and Excel report for the beginning of period
> payment
> assumptions and what Gnumeric and Kspread report as stated
> in the
> OpenFormula document referenced above (I discovered errors
> in this
> document as well as the openoffice documents referenced btw
> but they
> become obvious when you work these problems out). 

And the nightmare worsens (IMO).

> OpenFormula lists
> the Gnumeric/Kspread as the "result" in the document, but
> there is
> still a question to which is correct.  Well, I was
> able to derive both
> results, and as I suspected the Gnumeric/Kspread was based
> on an
> incorrect assumption (or a mistake in implementation) not a
> different
> formula.  My point with the derivations wasn't to
> include them in the
> documentation, but rather to find out what assumptions are
> being made
> and then deduce which results are correct.  In the
> cases of these
> simple spreadsheet functions I think it should be obvious
> if it's
> right or wrong.

OK, I concede defeat: if it is the wisdom of the PTB that numpy.financial be retained, I will stop messing w/ their help doc, 'cause I'm clearly in over my head.

DG

> If there is any interest in adding the ipmt and ppmt
> functions now, I
> can apply a patch.  If not, I can keep them separate
> as I work on the
> other functions.
> 
> Cheers,
> 
> Skipper
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