On Thu, May 23, 2013 at 3:12 AM, Guillaume VIRY <guillaume.v...@gmail.com>wrote:
> Good morning all > > I have been using the financial functions for a while now, and the npv > function has always bothered me. > All of them have always bothered me:) > Indeed neither the documentation nor the implementation seem to be > correct, and actually are not congruent with each other either. > > As stated in the bug report https://github.com/numpy/numpy/issues/649 : > > the documentation states that the summation is from 0 to M (should be M-1, > if the input data is of size M by the way) which is the correct definition, > but the implementation computes a summation from 1 to M, which is known to > be wrong, and a mimic of Excel's NPV behavior. > > So basically, we would have the following : > > RealNPV(data) = data[0] + CurrentNPV(data[1:]) > > Both behaviors are fine to me, as long as the documentation is clear about > it. Ideally the implementation should be corrected, but in any case there > should be a documentation fix at least. > > I'd be glad to have your opinion about it. > Looks like the code needs to be be changed. The current sum starting at 1 is different from what I see on wikipedia and other definitions scattered over the internet. Unless someone complains, I think just considering this a bug and changing the code is OK. Ralf
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