I define the piecewise function
def Li(x):
return piecewise(x, [x < 0, x >= 0], [lambda t: sin(t), lambda t: cos(t)])
And when I evaluate Li(1.0)
The answer is correct
Li(1.0)=array(0.5403023058681398),
But if I write Li(1) the answer is array(0).
I don't understand this behaviour.
This function runs correctly.
def Li(x):
return piecewise(float(x),
[x < 0, x >= 0],
[lambda t: sin(t), lambda t: cos(t)])
x.astype(float) instead of float(x).It seems that piecewise() converts the return values to the same type as the input so, when an integer is input an integer conversion is performed on the result, which is then returned. Because sine and cosine always return values between −1 and 1 all integer conversions will result in 0, 1 or -1 only - with the vast majority being 0.
>>> x=np.array([0.9])
>>> np.piecewise(x, [True], [float(x)])
array([ 0.9])
>>> x=np.array([1.0])
>>> np.piecewise(x, [True], [float(x)])
array([ 1.])
>>> x=np.array([1])
>>> np.piecewise(x, [True], [float(x)])
array([1])
>>> x=np.array([-1])
>>> np.piecewise(x, [True], [float(x)])
array([-1])
In the above I have explicitly cast the result to float, however, an integer input results in an integer output regardless of the explicit cast. I'd say that this is unexpected and I don't know why piecewise() should do this.
I don't know if you have something more elaborate in mind, however, you don't need piecewise() for this simple case; an if/else will suffice instead:
from math import sin, cos
def Li(t):
return sin(t) if t < 0 else cos(t)
>>> Li(0)
1.0
>>> Li(1)
0.5403023058681398
>>> Li(1.0)
0.5403023058681398
>>> Li(-1.0)
-0.8414709848078965
>>> Li(-1)
-0.8414709848078965
You can wrap the return value in an numpy.array if required.
I am sorry, but this example is taken and modified from
http://docs.scipy.org/doc/numpy/reference/generated/numpy.piecewise.html
But, in fact, using ipython with numpy 1.9
""" Python 2.7.8 |Anaconda 2.1.0 (64-bit)| (default, Aug 21 2014, 18:22:21) Type "copyright", "credits" or "license" for more information. IPython 2.2.0 -- An enhanced Interactive Python. """
I have no errors, but "ValueError: too many boolean indices" error appears if I use python 2.7.3 with numpy 1.6
""" Python 2.7.3 (default, Feb 27 2014, 19:58:35) """ I test this function under Linux and Windows and the same error occurs.
Obviously, It is very easy to overcome this situation, but I think that this behaviour is a mistake in the numpy library.
ValueError: too many boolean indiceswhen running either example. You need to pass anndarraytoLi(). Please update your question with working code.