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

Skipper Seabold jsseabold at gmail.com
Mon Jun 8 23:24:40 EDT 2009

On Mon, Jun 8, 2009 at 9:01 PM, Skipper Seabold<jsseabold at gmail.com> wrote:
> Just quickly comparing
> In [3]: np.lib.financial.fv(.1,10,-100,-350)
> Out[3]: 2501.5523211350032
> With OO Calc
> =fv(.1,10,-100,-350)
> =2501.55
> Both return the value of 350*1.1**10 + 100*1.1**9 + ... + 100*1.1
> which is what I would expect it to do.  I didn't look too closely at
> the docs though, so they might be a bit confusing and need some
> cleaning up.

I forgot the last payment (which doesn't earn any interest), so one more 100.

On Mon, Jun 8, 2009 at 11:18 PM, David Goldsmith<d_l_goldsmith at yahoo.com> wrote:
> --- On Mon, 6/8/09, Skipper Seabold <jsseabold at gmail.com> wrote:
>> There was a recent discussion about numpy.financial in this
>> thread
>> <http://mail.scipy.org/pipermail/numpy-discussion/2009-May/042709.html>.
>> Skipper
> Thanks, Skipper.  Having now read that thread (but not the arguments, provided elsewhere, for the existence of numpy.financial in the first place), and considering that the only references mentioned there are also electronic ones (which, for the purpose of referencing sources in the function docs, I believe we're wanting to shun as much as possible), I formally "move" that numpy.financial (or at least that subset of it consisting of functions which are commonly subject to multiple definitions) be moved out of numpy.  (Where _to_ exactly, I cannot say.)

I think fv is the expected behavior.  See my reply to the discussion
of fv to explain the notes section
and I think I can probably have a look at the consistency of the rest
pretty soon.  I don't have a more permanent reference for fv offhand,
but it should be in any corporate finance text etc.  Most of these
type of "formulas" use basic results of geometric series to simplify.


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