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# How to get polynomials coefficients?

See original GitHub issue

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

### Issue Analytics

• State:
• Created 4 years ago

1reaction
cjekelcommented, Jan 23, 2020
1reaction
cjekelcommented, Aug 25, 2019

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.

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