[Numpy-discussion] runtime warning for where

[Matthieu Brucher]

Hi,

Don't forget that np.where is not smart. First np.sin(x)/x is computed
for the array, which is why you see the warning, and then np.where
selects the proper final results.

Cheers,

Matthieu

2013/11/16 David Pine <djpine at gmail.com>:
> The program at the bottom of this message returns the following runtime warning:
>
> python test.py
> test.py:5: RuntimeWarning: invalid value encountered in divide
>  return np.where(x==0., 1., np.sin(x)/x)
>
> The function works correctly returning
> x = np.array([  0.,   1.,   2.,   3.,   4.,   5.,   6.,   7.,   8.,   9.,  10.])
> y = np.array([ 1.        ,  0.84147098,  0.45464871,  0.04704   , -0.18920062,
>       -0.19178485, -0.04656925,  0.09385523,  0.12366978,  0.04579094,
>       -0.05440211])
>
> The runtime warning suggests that np.where evaluates np.sin(x)/x at all x, including x=0, even though the np.where function returns the correct value of 1. when x is 0.  This seems odd to me.  Why issue a runtime warning? Nothing is wrong.  Moreover, I don't recall numpy issuing such warnings in earlier versions.
>
> import numpy as np
> import matplotlib.pyplot as plt
>
> def sinc(x):
>    return np.where(x==0., 1., np.sin(x)/x)
>
> x = np.linspace(0., 10., 11)
> y = sinc(x)
>
> plt.plot(x, y)
> plt.show()
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