Floris van Manen <v...@klankschap.nl> added the comment:
Well, the thing is that i pass two (apparent) identical values into the same
function, and get two different results.
Apparent as in one value generated via np.linspace() and one directly retrieved
from a list.
If i pass the np variable into to function, it will generate an NAN error.
If i pass the non np variable into the function, it will work.
.F
> On 12 Sep 2018, at 18:10, Steven D'Aprano <rep...@bugs.python.org> wrote:
>
>
> Steven D'Aprano <steve+pyt...@pearwood.info> added the comment:
>
> Your code gives runtime warnings of invalid values. You should fix that. If
> your values are invalid, there's probably a bug in your code.
>
> RuntimeWarning: invalid value encountered in double_scalars
>
> Your code is also very complex. You ought to simplify the example so that it
> is easier to understand, all the business with linspace and duplicated code
> just adds complexity and makes it hard to understand.
>
> I simplified your code to this:
>
> import numpy as np
> n=2.758
> n2 = 2.0 / n
> ct = np.cos(2 * np.pi * 2.0 / 5)
> print("numpy", ct, abs(ct ** n2) * 5.0)
>
>
> which gives this output:
>
> __main__:1: RuntimeWarning: invalid value encountered in double_scalars
> ('numpy', -0.80901699437494734, nan)
>
> So there's a problem. You're trying to raise a negative number to a positive
> value, and numpy doesn't like it and returns a NAN.
>
> But using the standard math library, raising a negative number to a positive
> value gives you a complex number:
>
> ct = math.cos(2 * math.pi * 2.0 / 5)
> print(ct**n2)
> print("math", ct, abs(ct ** n2) * 5.0)
>
>
> which gives this output:
>
> (-0.5572617094280153+0.6517928032447587j)
> math -0.8090169943749473 4.287698890886272
>
> So the behaviour is correct and this is not a bug in either math nor numpy.
> They're just doing different things.
>
> ----------
> nosy: +steven.daprano
> resolution: -> not a bug
> stage: -> resolved
> status: open -> closed
>
> _______________________________________
> Python tracker <rep...@bugs.python.org>
> <https://bugs.python.org/issue34645>
> _______________________________________
----------
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue34645>
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