Monday, 13 September 2021

Get `bs` (splines) equivalent in Python

In the r programming language, the following

require(stats)
require(splines)
knots = quantile(women$height, seq(0.1,0.9,length.out = 5))
bs(women$height, knots=knots, degree=3)

returns

1   2   3   4   5   6   7   8
0.0000000000    0.000000000 0.000000000 0.000000000 0.000000000 0.000000000 0.0000000000    0.00000000
0.6284418529    0.323939099 0.024295432 0.000000000 0.000000000 0.000000000 0.0000000000    0.00000000
0.2155814707    0.599894720 0.182883868 0.001639942 0.000000000 0.000000000 0.0000000000    0.00000000
0.0349854227    0.495626822 0.438289602 0.031098154 0.000000000 0.000000000 0.0000000000    0.00000000
0.0001619695    0.245586330 0.620809038 0.133442663 0.000000000 0.000000000 0.0000000000    0.00000000
0.0000000000    0.072886297 0.584548105 0.338678328 0.003887269 0.000000000 0.0000000000    0.00000000
0.0000000000    0.009110787 0.384718173 0.561892614 0.044278426 0.000000000 0.0000000000    0.00000000
0.0000000000    0.000000000 0.166666667 0.666666667 0.166666667 0.000000000 0.0000000000    0.00000000
0.0000000000    0.000000000 0.044278426 0.561892614 0.384718173 0.009110787 0.0000000000    0.00000000
0.0000000000    0.000000000 0.003887269 0.338678328 0.584548105 0.072886297 0.0000000000    0.00000000
0.0000000000    0.000000000 0.000000000 0.133442663 0.620809038 0.245586330 0.0001619695    0.00000000
0.0000000000    0.000000000 0.000000000 0.031098154 0.438289602 0.495626822 0.0349854227    0.00000000
0.0000000000    0.000000000 0.000000000 0.001639942 0.182883868 0.599894720 0.2155814707    0.00000000
0.0000000000    0.000000000 0.000000000 0.000000000 0.024295432 0.323939099 0.6284418529    0.02332362
0.0000000000    0.000000000 0.000000000 0.000000000 0.000000000 0.000000000 0.0000000000    1.00000000

Is there a Python equivalent? I have tried BSpline from scipy, but it requires that the coefficients already be known and be passed in.

How can I just generate the B-spline basis matrix, having passed in an array, knots, and degree?


To reproduce the input Python, you can do:

import numpy as np

women_height = np.array([58,59,60,61,62,63,64,65,66,67,68,69,70,71,72])
knots = array([59.4, 62.2, 65. , 67.8, 70.6])


from Get `bs` (splines) equivalent in Python

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