Normal Equation Implementation From Scratch In Python

Part - 1 - Introduction

Normal equation is a more closed-form solution of figuring out the value of parameter or θ that minimizes the cost function. It's called closed-form solution in the sense that it gives the result directly though the equation.

Here, 

• The left hand side of the equation is the value of q  that's going to minimize the cost function
• y is a vector containing our target values or labels

Part - 2 - Data generation

Now, we need data on which we can apply the Normal Equation. Let's create some random data. We need to make sure that the data plot isn't a glorious line, so we're adding some noise to scatter them or create some deviation.

import numpy as np

def generate_data():
    X = 2 * np.random.rand(100, 1)
    y = 4 + 3 * X + np.random.randn(100, 1)

    # y = 4 + 3X + Gaussian noise
    # theta_0 or Bias Term = 4
    # theta_1 = 3

    return X, y

Now that our data is created, it is time to fetch our parameters from this dataset. Or we can say getting value of q that will minimize the cost function.

Part - 3 - Getting

So how do we find the best q? We will use the normal equation.

from numpy.linalg import inv

def get_best_param(X, y):
    X_transpose = X.T
    best_params = inv(X_transpose.dot(X)).dot(X_transpose).dot(y)
    # normal equation
    # theta_best = (X.T * X)^(-1) * X.T * y

    return best_params # returns a list

Part - 4 - Testing

We have our data, we have 𝛉, so let's test things!

X, y = generate_data()
%matplotlib inline
import matplotlib.pyplot as plt

plt.plot(X, y, "r.")

Had we not chosen to add some noise, this data would've been a dead straight line, and then we wouldn't need fancy machine learning. Simple math would do. [ Apparently we're not here for that ]

X_b = np.c_[np.ones((100, 1)), X] # set bias term to 1 for each sample
params = get_best_param(X_b, y)
print(params)
array([[ 3.94204397],
       [ 3.08256508]])

Well, the expected the values would've been  4 and 3 but we have some noise so deviation is acceptable. [ I'm not saying this is perfect! ]

So how do we get the prediction? Well another equation,

Here,

• Left hand side of the equation is the expected output,

# test prediction

test_X = np.array([[0], [2]])
test_X_b = np.c_[np.ones((2, 1)), test_X]

prediction = test_X_b.dot(params)

# y = h_Theta_X(Theta) = Theta.T * X
print(prediction)
array([[  3.94204397],
       [ 10.10717414]])

Okay numbers numbers all around. So let's plot the prediction and our dataset in a single graph and see how the model fits the dataset for a regression problem.

plt.plot(test_X, prediction, "r--")
plt.plot(X, y, "b.")
plt.axis([0, 2, 0, 15]) # x axis range 0 to 2, y axis range 0 to 15
plt.show()

So, we implemented Normal Equation and applied it to predict output for a Linear regression problem. So one curious question - what if we didn't add noise? How would the data plot look? let's find out!

Extra - What if no noise?

def generate_noiseless_data():
    X = 2 * np.random.rand(100, 1)
    y = 4 + 3 * X

    # y = 4 + 3X
    # theta_0 or Bias Term = 4
    # theta_1 = 3

    return X, y
X, y = generate_noiseless_data()
plt.plot(X, y, "r.")
[<matplotlib.lines.Line2D at 0x10f30f978>]

As you can see - a dead straight line (almost!) let's look at the param and prediction

X_b = np.c_[np.ones((100, 1)), X] # set bias term to 1 for each sample
param = get_best_param(X_b, y)
param
array([[ 4.],
       [ 3.]])

Well well, exactly 4 and 3. For such cases, you don't need machine learning. Just putting values in the line equation does the job.

test_X = np.array([[0], [2]])
test_X_b = np.c_[np.ones((2, 1)), test_X]

prediction = test_X_b.dot(params)
plt.plot(test_X, prediction, "r--")
plt.plot(X, y, "b.")
plt.axis([0, 2, 0, 15]) # x axis range 0 to 2, y axis range 0 to 15
plt.show()

Nothing to separate really!