How can I select only the real numbers from an array? (Python 3)

Question

I want to find the vertical asymptote for:

f=(3x^3 + 17x^2 + 6x + 1)/(2x^3 - x + 3)

So I want to find the roots for (2x^3 - x + 3) so I wrote:

 import sympy as sy
 x = sy.Symbol('x', real=True)
 asym1 = sy.solve(2*x**3-x+3,x)
 for i in range(len(asym1)):
     asym1[i] = asym1[i].evalf()
 print(asym1)

The output was:

[0.644811950742531 + 0.864492542166306*I, 0.644811950742531 - 
0.864492542166306*I, -1.28962390148506]

So right now the only number that makes sense in the output is -1.289 and the complex numbers don't have any meaning.

My question is: How can I only select the real numbers so the output says:

asym1 = -1.28962390148506

dilkash · Accepted Answer · 2018-07-28T20:08:28.070

4

you can do:

asym1 = [n for n in asym1 if n.is_real][0]

edited Jul 28 '18 at 20:08

answered Jul 28 '18 at 19:59

dilkash

562
3
15

What if `n` is positive? – taras Jul 28 '18 at 20:06

score 1 · Answer 2 · answered Jul 28 '18 at 20:03

1

Complex numbers are instances of complex class while real numbers are floats:

asym1 = [x for x in asym1 if isinstance(x, float)]

answered Jul 28 '18 at 20:03

taras

6,566
10
39
50

not required, various properties are provided to check real number – dilkash Jul 28 '18 at 20:10
1

@dilkash, this method is universal and not limited to sympy only. – taras Jul 28 '18 at 20:54

How can I select only the real numbers from an array? (Python 3)

2 Answers2