How to get polynomials coefficients?

Hi,

Is there a way to get the polynomial coefficients in a similar way it was provided in numpy.polyfit?

I did the following as a simple example to illustrate what I was thinking:

import numpy as np
import matplotlib.pyplot as plt
import pwlf

# create the data
x = np.arange(10)
y = np.zeros(10)
y[:5] = 0 + 2 * x[:5]
y[5:] = 20 - 3 * x[5:]

degree = 1
segments = 2

my_pwlf = pwlf.PiecewiseLinFit(x, y, degree=degree, disp_res=True)
res = my_pwlf.fit(segments)

# predict
xHat = np.linspace(min(x), max(x), num=10000)
yHat = my_pwlf.predict(xHat)

# get polynomial coefficients
def get_coeffs(pwlf) -> np.ndarray:
    coefficients = np.array(list(zip(pwlf.slopes, pwlf.intercepts)))
    return coefficients

coeffs = get_coeffs(my_pwlf)
p1 = np.poly1d(coeffs[0])
p2 = np.poly1d(coeffs[1])

breaks = my_pwlf.fit_breaks[1]

# test polynomial
ytest = np.zeros(len(x))
for i in range(len(x)):
    if x[i] <= breaks:
        ytest[i] = p1(x[i])
    else:
        ytest[i] = p2(x[i])

plt.figure()
plt.plot(x, y, 'o', label='data')
plt.plot(xHat, yHat, '-', label='predict')
plt.plot(x, ytest, '--', label='fit test')
plt.legend()
plt.show()

Thank you

[cjekel]

Added example here https://github.com/cjekel/piecewise_linear_fit_py/commit/e57d1060b576f357a7cdbb9a60a5c554bff73d3e

[cjekel]

I don’t think (right now) I want to add sympy as a requirement to this library, but maybe I’ll change my mind in the future.

At the very least, I should add this as an example, since people have asked about equations before.

You bring up a good point, let me reopen this until something is added to the library.