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numpy / numpy-financial

Standalone package of the NumPy financial functions
BSD 3-Clause "New" or "Revised" License
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Financial function nper() incorrect for zero-rate annuities. #16

Closed adamrogoyski closed 4 years ago

adamrogoyski commented 7 years ago

When rate=0.0, the sign on present value is wrong, causing most degenerate cases to be incorrect.

As an example, making periodic payments of $10 for a $100 loan at progressively smaller interest rates shows the number of periods approaching 10:

numpy.nper(0.01, -10, 100, 0) 10.588644459423231

numpy.nper(0.001, -10, 100, 0) 10.055360184319873

numpy.nper(0.0001, -10, 100, 0) 10.005503577667028

numpy.nper(0.00001, -10, 100, 0) 10.000550035678859

numpy.nper(0.000001, -10, 100, 0) 10.000055001142128

If the interest rate is 0%, it should be exactly 10 payments, not -10:

numpy.nper(0, -10, 100, 0) -10.0

adamrogoyski commented 7 years ago

A fix for this: https://github.com/numpy/numpy/pull/8515