[Numpy-discussion] Overloading sqrt(5.5)*myvector

Bruce Sherwood Bruce_Sherwood@ncsu....
Wed Dec 26 00:11:52 CST 2007

Sorry to repeat myself and be insistent, but could someone please at 
least comment on whether I'm doing anything obviously wrong, even if you 
don't immediately have a solution to my serious problem? There was no 
response to my question (see copy below) which I sent to both the numpy 
and Boost mailing lists.

To the numpy experts: Is there something wrong, or something I 
could/should change in how I'm trying to overload multiplication of a 
numpy square root (or other numpy function) times my own "vector" 
object? I'm seeing a huge performance hit in going from Numeric to numpy 
because Numeric sqrt returned float whereas numpy sqrt returns 
numpy.float64, so that the result is not one of my vector objects. I 
don't have a problem with myvector*sqrt(5.5).


Bruce Sherwood

I'm not sure whether this is a Numpy problem or a Boost problem, so I'm 
posting to both communities. (I'm subscribed to both lists, but an 
attempt to post yesterday to this Boost list seems never have gotten to 
the archives, so I'm trying again. My apologies if this shows up twice 

In old Numeric, type(sqrt(5.5)) was float, but in numpy, type(sqrt(5.5)) 
is numpy.float64. This leads to a big performance hit in calculations in 
a beta version of VPython, using the VPython 3D "vector" class, compared 
with the old version that used Numeric (VPython is a 3D graphics module 
for Python; see vpython.org).

Operator overloading of the VPython vector class works fine for 
vector*sqrt(5.5) but not for sqrt(5.5)*vector. The following free 
function catches 5.5*vector but fails to catch sqrt(5.5)*vector, whose 
type ends up as numpy.ndarray instead of the desired vector, with 
concomitant slow conversions in later vector calculations:

inline vector
operator*( const double& s, const vector& v)
{ return vector( s*v.x, s*v.y, s*v.z); }

I've thrashed around on this, including trying to add this:

inline vector
operator*( const npy_float64& s, const vector& v)
{ return vector( s*v.x, s*v.y, s*v.z); }

But the compiler correctly complains that this is in conflict with the 
version of double*vector, since in fact npy_float64 is actually double.

It's interesting and presumably meaningful to the knowledgeable (not me) 
that vector*sqrt(5.5) yields a vector, even though the overloading 
speaks of double, not a specifically numpy name:

inline vector
operator*( const double s) const throw()
{ return vector( s*x, s*y, s*z); }

VPython uses Boost, and the glue concerning vectors includes the following:

py::class_<vector>("vector", py::init< py::optional<double, double, 
double> >())
     .def( self * double())
     .def( double() * self)

As far as I can understand from the Boost Python documentation, this is 
the proper way to specify the left-hand and right-hand overloadings. But 
do I have to add something like .def( npy_float64() * self)? Help would 
be much appreciated.

Bruce Sherwood

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