[Numpy-discussion] Arcos returns nan
Fri Mar 28 04:31:33 CDT 2008
David Cournapeau wrote:
> 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.
Note that the below program in C has the same behaviour than numpy if I
understand your situation right:
fprintf(stderr, "%f\n", acos(1.));
fprintf(stderr, "%f\n", acos(1. + DBL_EPSILON));
(EPSILON being by definition the smallest value such as 1. + EPSILON >
1.). So the problem in your case is likely to be caused by precision
problems when you normalized the scalar product. An easy solution would
be to clip all the values > 1. to 1 (same for values < -1); another
solution may be in a better way to do the normalization.
But I don't see how matlab or Fortran would be any different.
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