Re: [Numpy-discussion] Unexpected RuntimeWarning

2012-11-23 Thread Charles R Harris 
On Fri, Nov 23, 2012 at 4:38 AM, Bob Dowling  wrote:

> I have a simple function defined in the following snippet:
>
>   --- start ---
> import numpy
>
> def chebyshev(x, m):
>  '''Calculates Chebyshev functions of the first kind using the
> trigonometric identities.'''
>
>  theta = numpy.where(
>  numpy.abs(x)<=1.0,
>  numpy.arccos(x),
>  numpy.arccosh(numpy.abs(x))
>  )
>
>  y = numpy.where(
>  numpy.abs(x)<=1.0,
>  numpy.cos(m*theta),
>  numpy.cosh(m*theta) * numpy.where(
>  x > 0.0,
>  1.0,
>  -1.0
>  )**m
>  )
>
>  return y
>
>
> if __name__ == '__main__':
>  x = numpy.linspace(-2.0, 2.0, 21)
>  y = chebyshev(x,3)
>  print(y)
>
>   --- end ---
>
>
> I'm using the numpy.where() call to extract only legal values for the
> circular and hyperbolic trigonometric functions.  But I still get
> warnings that I'm passing invalid anrguments:
>
> --- start---
>
> $ python3 demo.py
> demo.py:8: RuntimeWarning: invalid value encountered in arccos
>numpy.arccos(x),
> demo.py:9: RuntimeWarning: invalid value encountered in arccosh
>numpy.arccosh(numpy.abs(x))
> [ -2.6000e+01  -1.7928e+01  -1.1584e+01  -6.7760e+00
>-3.3120e+00  -1.e+00   3.5200e-01   9.3600e-01
> 9.4400e-01   5.6800e-01  -1.83697020e-16  -5.6800e-01
>-9.4400e-01  -9.3600e-01  -3.5200e-01   1.e+00
> 3.3120e+00   6.7760e+00   1.1584e+01   1.7928e+01
> 2.6000e+01]
>
>   --- end ---
>
> (I get the same with Python 2 so don't get excited about that.)
>
>
> I'm guessing that numpy.where() is evaluating the complete arccos and
> arccosh arrays and therefore getting invalid arguments.
>
> Now, I can turn off the warnings with numpy.seterr(invalid='ignore') but
> that's not what I would regard as good practice.
>
> Is there a "numpythonic" way to address the issue?
>

Are you aware of the Chebyshev series in numpy ?

In [1]: from numpy.polynomial import Chebyshev as T

In [2]: p = T([1,2,3])

In [3]: p(linspace(-2,2, 10))
Out[3]:
array([ 18.,   9.40740741,   3.18518519,  -0.6667,
-2.14814815,  -1.25925926,   2.,   7.62962963,
15.62962963,  26.])




> ___
> NumPy-Discussion mailing list
> NumPy-Discussion@scipy.org
> http://mail.scipy.org/mailman/listinfo/numpy-discussion
>
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Unexpected RuntimeWarning

2012-11-23 Thread Bob Dowling 
> You may want to use this:
> http://docs.scipy.org/doc/numpy/reference/generated/numpy.piecewise.html

Thank you.  That's just what I needed.

Works a treat:
  --- start ---

import numpy

def chebyshev(x, m):
 '''Calculates Chebyshev functions of the first kind using the 
trigonometric identities.'''

 zone_a = x < -1.0
 zone_b = numpy.logical_and(-1.0 <= x, x <= 1.0)
 zone_c = 1.0 < x

 theta = numpy.piecewise(
 x,
 [zone_a, zone_b, zone_c],
 [lambda z: numpy.arccosh(numpy.abs(z)), numpy.arccos, 
numpy.arccosh]
 )

 y = numpy.piecewise(
 theta,
 [zone_a, zone_b, zone_c],
 [lambda z: (-1.0)**m*numpy.cosh(m*z), lambda z: numpy.cos(m*z), 
lambda z: numpy.cosh(m*z)]
 )

 return y


if __name__ == '__main__':
 x = numpy.linspace(-2.0, 2.0, 21)
 y = chebyshev(x,3)
 print(y)

  --- end---
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion

Re: [Numpy-discussion] Unexpected RuntimeWarning

2012-11-23 Thread Pauli Virtanen 
Bob Dowling  cam.ac.uk> writes:
[clip]
> I'm guessing that numpy.where() is evaluating the complete arccos and 
> arccosh arrays and therefore getting invalid arguments.
> 
> Now, I can turn off the warnings with numpy.seterr(invalid='ignore') but 
> that's not what I would regard as good practice.
> 
> Is there a "numpythonic" way to address the issue?

Correct, the arguments are evaluated first, due to how Python semantics work.

You may want to use this:
http://docs.scipy.org/doc/numpy/reference/generated/numpy.piecewise.html

___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion

[Numpy-discussion] Unexpected RuntimeWarning

2012-11-23 Thread Bob Dowling 
I have a simple function defined in the following snippet:

  --- start ---
import numpy

def chebyshev(x, m):
 '''Calculates Chebyshev functions of the first kind using the 
trigonometric identities.'''

 theta = numpy.where(
 numpy.abs(x)<=1.0,
 numpy.arccos(x),
 numpy.arccosh(numpy.abs(x))
 )

 y = numpy.where(
 numpy.abs(x)<=1.0,
 numpy.cos(m*theta),
 numpy.cosh(m*theta) * numpy.where(
 x > 0.0,
 1.0,
 -1.0
 )**m
 )

 return y


if __name__ == '__main__':
 x = numpy.linspace(-2.0, 2.0, 21)
 y = chebyshev(x,3)
 print(y)

  --- end ---


I'm using the numpy.where() call to extract only legal values for the 
circular and hyperbolic trigonometric functions.  But I still get 
warnings that I'm passing invalid anrguments:

--- start---

$ python3 demo.py
demo.py:8: RuntimeWarning: invalid value encountered in arccos
   numpy.arccos(x),
demo.py:9: RuntimeWarning: invalid value encountered in arccosh
   numpy.arccosh(numpy.abs(x))
[ -2.6000e+01  -1.7928e+01  -1.1584e+01  -6.7760e+00
   -3.3120e+00  -1.e+00   3.5200e-01   9.3600e-01
9.4400e-01   5.6800e-01  -1.83697020e-16  -5.6800e-01
   -9.4400e-01  -9.3600e-01  -3.5200e-01   1.e+00
3.3120e+00   6.7760e+00   1.1584e+01   1.7928e+01
2.6000e+01]

  --- end ---

(I get the same with Python 2 so don't get excited about that.)


I'm guessing that numpy.where() is evaluating the complete arccos and 
arccosh arrays and therefore getting invalid arguments.

Now, I can turn off the warnings with numpy.seterr(invalid='ignore') but 
that's not what I would regard as good practice.

Is there a "numpythonic" way to address the issue?
___
NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion