How to extract coefficients from a line equation in python without using numpy?

Question

I have 2 lists RED and BLUE in the form of (x,y) tuples and a list of line equations in the form of ax+by+c. My requirement is to extract the coefficients from each line equation and determine based on the plots for the 2 set of points whether the points are clearly separated on both sides of the line. Challenge is that I cannot use numpy.

My approach has been to zip the 2 lists RED and BLUE points using pyplot. Now I am trying to extract the coefficients using regular expression as below.

lines = ["1x+1y+0","1x-1y+0","1x+0y-3","0x+1y-0.5"]

for i in lines:
    z = re.match('(\d+)?(x)?\+(\d+)?(y)?\+(\d)?', i)

However, Im not able to use 'z' as it is of 'NoneType'. Even if I am able to use it somehow, I am not sure how to use the intercept and slope to determine that the RED and BLUE points are on either side of the line.

Any pointers are hugely appreciated.

Tried plotting the points using matplotlib

Red_x = [(x,y) for x,y in Red]
Blue_x = [(x,y) for x,y in Blue]

plt.plot(*zip(*Red_x),'or')
plt.scatter(*zip(*Blue_x))

If you don't insist on regex there is an answer here using Sympy https://stackoverflow.com/q/22955888/4711754 — Andrew Allen, Jul 09 '19 at 08:47

score 7 · Accepted Answer · answered Jul 09 '19 at 08:58

I believe what you want to use is findall.

You can start with a simple pattern of [\d\.\-\+]+. This will catch all coefficients assuming the coefficients are properly formatted (e.g. no double periods in digits).

>>> lines = ["1x+1y+0", "1x-1y+0", "1x+0y-3", "0x+1y-0.5"]
>>> for i in lines:
...     z = re.findall(r'[\d\.\-\+]+', i)
...     print(z)
... 
['1', '+1', '+0']
['1', '-1', '+0']
['1', '+0', '-3']
['0', '+1', '-0.5']

Obviously, you'll have to do some additional parsing of the resulting list of strings to convert them to numbers, but that will be an exercise for you :)

this regular expression will fail when ever the position of the intercept is changed e.g. this equation: "7y-50+2X" — Adarsh Namdev, May 01 '20 at 12:33

score 1 · Answer 2 · answered Jul 03 '20 at 10:40

1

import re
s = "1x-2.1y-0.5"
s = [float(i) for i in re.split('[xy]', s)]
print(s)

[ 1.0 , -2.1 , -0.5 ]

answered Jul 03 '20 at 10:40

Tanmoy Chowdhury

127
1
2

score 0 · Answer 3 · answered Jan 02 '20 at 05:49

0

import re
str = "112x-12y+0"
res = re.findall(r'[0-9\-\+]+', str)
print(res)

output: ['112', '-12', '+0']

answered Jan 02 '20 at 05:49

Sreenath Nukala

27
1
2
9

score 0 · Answer 4 · answered May 01 '20 at 17:20

This solution will take care of the order in which the equation's terms x,y,c are placed.

import re

all_equations = ["1x+1y+2", "-1x+12Y-6", "2-5y-3x", "7y-50+2X", "3.14x-1.5y+9", "11.0x-1.5y+9.8"]

def CoefficientIntercept(equation):
    coef_x = re.findall('-?[0-9.]*[Xx]', equation)[0][:-1]
    coef_y = re.findall('-?[0-9.]*[Yy]', equation)[0][:-1]
    intercept = re.sub("[+-]?\d+[XxYy]|[+-]?\d+\.\d+[XxYy]","", equation)    

    return float(coef_x), float(coef_y), float(intercept)

for equation in all_equations:
    print(CoefficientIntercept(equation))

Output:

(1.0, 1.0, 2.0)
(-1.0, 12.0, -6.0)
(-3.0, -5.0, 2.0)
(2.0, 7.0, -50.0)
(3.14, -1.5, 9.0)
(11.0, -1.5, 9.8)

Can you explain, what you did in the intercept part – Gupta Dec 14 '20 at 02:41 — Gupta, Dec 14 '20 at 02:41

How to extract coefficients from a line equation in python without using numpy?

4 Answers4