Print Real Roots Only In Numpy

I have something like this: coefs = [28, -36, 50, -22] print(numpy.roots(coefs)) Of course the result is: [ 0.35770550+1.11792657j 0.35770550-1.11792657j 0.57030329+0.j ] Howev

Solution 1:

Do NOT use .iscomplex() or .isreal(), because roots() is a numerical algorithm, and it returns the numerical approximation of the actual roots of the polynomial. This can lead to spurious imaginary parts, that are interpreted by the above methods as solutions.

Example:

# create a polynomial with these real-valued roots:
p = numpy.poly([2,3,4,5,56,6,5,4,2,3,8,0,10])
# calculate the roots from the polynomial:
r = numpy.roots(p)
print(r) # real-valued roots, with spurious imaginary partarray([ 56.00000000 +0.00000000e+00j,  10.00000000 +0.00000000e+00j,
         8.00000000 +0.00000000e+00j,   6.00000000 +0.00000000e+00j,
         5.00009796 +0.00000000e+00j,   4.99990203 +0.00000000e+00j,
         4.00008066 +0.00000000e+00j,   3.99991935 +0.00000000e+00j,
         3.00000598 +0.00000000e+00j,   2.99999403 +0.00000000e+00j,
         2.00000000 +3.77612207e-06j,   2.00000000 -3.77612207e-06j,
         0.00000000 +0.00000000e+00j])
# using isreal() fails: many correct solutions are discardedprint(r[numpy.isreal(r)])
[ 56.00000000+0.j  10.00000000+0.j   8.00000000+0.j   6.00000000+0.j
   5.00009796+0.j   4.99990203+0.j   4.00008066+0.j   3.99991935+0.j
   3.00000598+0.j   2.99999403+0.j   0.00000000+0.j]

Use some threshold depending on your problem at hand instead. Moreover, since you're interested in the real roots, keep only the real part:

real_valued = r.real[abs(r.imag)<1e-5] # where I chose 1-e5 as a thresholdprint(real_valued)

Solution 2:

You can do it, using iscomplex as follows:

r = numpy.roots(coefs)

In [15]: r[~numpy.iscomplex(r)]
Out[15]: array([ 0.57030329+0.j])

Also you can use isreal as pointed out in comments:

In [17]: r[numpy.isreal(r)]
Out[17]: array([ 0.57030329+0.j])

Solution 3:

I hope this help.

 roots = np.roots(coeff);
 for i in range(len(roots)):
     if np.isreal(roots[i]):
         print(np.real(roots[i]))