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High-Dimensional Fittings

Now we fit higher degree polynomials to the data and compare the results and errors.

Importing the libraries

In [1]:
import csv
import numpy as np
import matplotlib.pyplot as plt

Loading the data

We load the data and define the header and the respective vectors with the temperature and with the density values.

In [2]:
water_csv = list(csv.reader(open('water.csv',"r"), delimiter=","))
header = dict([(water_csv[0][i],water_csv[1][i]) for i in range(3)])
T, f = np.loadtxt(open('water.csv', "r"), delimiter=",", skiprows=2, usecols=(0,1), unpack=True)
N = len(T)
N_half = int(N/2)

The Vandermonde matrices

We build a number of Vandermonde matrices, up to the number of data points available.

In [3]:
A = list()
for j in range(N_half):
    A.append(np.vstack([T**i for i in range(j+1)]).T)

Solving the least-square problems

In [4]:
a = list()
for j in range(N_half):
    a.append(np.linalg.lstsq(A[j], f, rcond=None)[0])

Building the approximating polynomials

In [5]:
p = list()
for j in range(N_half):
    p.append(np.array(sum([a[j][i]*T**i for i in range(j+1)])))

Plotting the approximations

In [6]:
plt.figure(figsize=(10,5))
plt.plot(T, f, 'o', label='Data', color='tab:blue')
for j in range(N_half):
    plt.plot(T, p[j], label=f'degree {j}')
plt.title('Plot of the data and of the polynomial approximations', fontsize=14)
plt.xlabel(header['temp'], fontsize=12)
plt.ylabel(header['density'], fontsize=12) 
plt.legend()
plt.show()

Calculating the mean quadratic errors

In [7]:
Err = list()
for j in range(N_half):
    Err.append(np.linalg.lstsq(A[j], f, rcond=None)[1][0]/N)
    print(f'j={j}: Error={Err[j]:.2e}')
j=0: Error=1.75e-04
j=1: Error=9.22e-06
j=2: Error=1.33e-07
j=3: Error=3.16e-09
j=4: Error=3.27e-10
j=5: Error=2.64e-10
j=6: Error=2.64e-10

Plotting the mean quadratic errors

In [8]:
plt.figure(figsize=(10,5))

plt.plot(range(len(Err)), Err, 'o', color='tab:red', markersize=10)
plt.grid(True)
plt.yscale('log')
plt.ylim(10**(-10), 10**(-3))
plt.title('Mean quadratic error in terms of the degree of the approximating polynomial', fontsize=14)
plt.xlabel('degree', fontsize=12)
plt.ylabel('error', fontsize=12)
plt.show()

Notice how there is not much advantage going beyond degree four.