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

Skipper Seabold jsseabold at gmail.com
Mon Jun 8 21:01:25 EDT 2009

On Mon, Jun 8, 2009 at 5:44 PM, <d_l_goldsmith at yahoo.com> wrote:
> Hi, folks.  Unable to find a printed reference for the definition we use to compute the functions in the Subject line of this email, I posted a couple queries for help in this regard in the Discussion for fv
> (http://docs.scipy.org/numpy/docs/numpy.lib.financial.fv/#discussion-sec).  josef Pktd's reply (thanks!) just makes me even more doubtful that we're using the definition that most users from the financial community would be expecting.  At this point, I have to say, I'm very concerned that our implementation for these is "wrong" (or at least inconsistent with what's used in financial circles); if you know of a reference - less ephemeral than a solely electronic document - defining these functions as we've implemented them, please share.  Thanks!

Just quickly comparing

In [3]: np.lib.financial.fv(.1,10,-100,-350)
Out[3]: 2501.5523211350032

With OO Calc

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.

There was a recent discussion about numpy.financial in this thread
 The way that it was left is that they are there as teaching tools to
mimic *some* of the functionality of spreadsheets/ financials
calculators.  I'm currently working on implementing some other common
spreadsheet/ financial calculator on my own for possible inclusion
somewhere later, as I think was the original vision


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