[Numpy-discussion] Arcos returns nan
Fri Mar 28 04:22:44 CDT 2008
João Quinta da Fonseca wrote:
> I have a function that returns the dot product of two unit vectors.
> When I try to use arcos on the values returned I sometimes get the
> "Warning: invalid value encountered in arccos", and the angle
> returned is nan. I found out that this happens for essentially co-
> linear vectors, for which the dot product function returns 1.0. This
> looks like 1.0 no matter how I print it but if I do: >>N.dot(a,b)>1,
> I get: >>True.
> Now I guess this arises because of the way computers store floats but
> shouldn't numpy take care of this somehow? Is it a bug? I don't seem
> to have this problem with Matlab or Fortran.
I am not sure I understand which behaviour you would expect. In matlab:
>> a = 1.; b = 1 + eps;
0 + 2.1073e-08i
So do you expect acos to handle values outside the [1;-1] range (cos
considered as a 'generalized' complex function) ?
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