I am getting a warning <RuntimeWarning: invalid value encountered in sqrt>
Question:
I am trying to run a quadratic equation in python. However, it keeps on giving me a warning
RuntimeWarning: invalid value encountered in sqrt
Here’s my code:
import numpy as np
a = 0.75 + (1.25  0.75)*np.random.randn(10000)
print(a)
b = 8 + (12  8)*np.random.randn(10000)
print(b)
c = 12 + 2*np.random.randn(10000)
print(c)
x0 = (b  np.sqrt(b**2  (4*a*c)))/(2 * a)
print(x0)
Answers:
This is not 100% Python related. You can’t calculate the square root of a negative number (when dealing with real numbers that is).
You didn’t take any precautions for when b**2  (4*a*c)
is a negative number.
>>> import numpy as np
>>>
>>> np.sqrt(4)
2.0
>>> np.sqrt(4)
__main__:1: RuntimeWarning: invalid value encountered in sqrt
nan
Let’s test if you have negative values:
>>> import numpy as np
>>>
>>> a = 0.75 + (1.25  0.75) * np.random.randn(10000)
>>> b = 8 + (12  8) * np.random.randn(10000)
>>> c = 12 + 2 * np.random.randn(10000)
>>>
>>> z = b ** 2  (4 * a * c)
>>> print len([_ for _ in z if _ < 0])
71
If you’re hoping to do complex analysis (working with imaginary numbers as defined by sqrt(1)) you can import cmath and use cmath.sqrt(1) instead of numpy.sqrt(1).
For example, when I’m calculating the refractive index of materials from permittivity and permeability (by definition, j is involved), I’ll write functions in python as such:
def n(f):
y = cmath.sqrt(mu1f(f)  j*mu2f(f)) * (e1f(f)  j*e2f(f))
return y.real
Where e1f etc. are previously defined interpolating functions, all of which are a function of incident frequency f. The y resultant is, in it of itself, a complex value, the complex index of refraction, but I’m oftentimes only interested in the real portion (the refractive index) so that is what is returned.
Hope this helps
Square roots are not defined for strictly negative real numbers, and numpy will produce nan
for negative inputs of "real" dtype int
, float
, and it’s two special values np.inf
and nan
.
However, square roots are defined for all complex dtype:
dtype  e.g.x 
np.sqrt(x) 
RuntimeWarning 

Positive float  1. 
1. 

Positive int  1 
1 

Positive complex  1+0J 
1 

Negative float  1. 
nan 
⚠️ 
Negative int  1 
nan 
⚠️ 
Negative complex  1+0j 
1j 

Infinity  np.inf 
np.inf 

Negative infinity  np.inf 
nan 
⚠️ 
NaN  np.nan 
nan 
I would like to add an insight along with the above answers.
The cmath
function always returns complex numbers but if you want a unified treatment of complex and float object I would recommend using numpy.lib.scimath
.
Note: The sqrt()
function in this is slower that that of the versions from cmath
and math
module but it has flexibility to return a complex object in the case of a negative argument and a float object otherwise