[SciPy-User] help interpreting univariate spline
Elliot Hallmark
permafacture@gmail....
Fri Apr 23 18:53:13 CDT 2010
Hi there,
I am wanting to use a scipy interpolation routine to generate
polynomials approximating function I have sampled. these polynomials
will be solved in c (cython), so I need to extract the coefficencts
from the interpolation method and pass them to a c function for root
finding.
using UnivariateSpline I ran this code:
import numpy as np
from scipy.interpolate import splrep, UnivariateSpline
x = np.linspace(1, 10, 100)
y = 1/x #approximate a hyperbola
g = UnivariateSpline(x, y, w=None, k=3, s=.0005)
print g.get_knots()
print g.get_coeffs()
import matplotlib.pyplot as plt
plt.clf()
ax = plt.subplot(121)
l1, = ax.plot(x, y, "rs", ms=10)
l2, = ax.plot(x, g(x), "bo")
plt.show()
and the output was:
[ 1. 2.18181818 3.27272727 5.54545455 10. ]
[ 0.98462432 0.66575572 0.423 0.25461626 0.13929847 0.11763985
0.09929961]
That is 5 knots and 7 coefficients for a degree=3 spline. naively, I
was expecting 4 coefficents for each interval between knots.
how can I construct a piecewise polynomial from this output? or can I?
thx,
Elliot
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