# Introduction of Neural Network

In this section, we introduce how to find the right linear equation by computer, that is, how to learn by computer.

Let's look at a simple example. As shown below, there are three blue dots and three red dots. Look for a straight line that distinguishes these dots. For a computer, it may randomly select a linear equation from a certain location (as shown in the right figure below). This line spatializes the whole sample into two regions - blue and red. It can be seen that the classification effect of this line is poor, so we need to move it, that is, to make the line closer to the two wrong points in the following figure (red dots in the blue area and blue dots in the red area), so as to make the classification results better and better.  Here's how to make a straight line closer to the target point. As shown in the following figure, assume that the equation of a straight line is The red dot in the picture is a misclassified point with coordinates of . In order to make the line close to the red point, we can do this: extract the coefficients of the linear equation, and add the bias unit 1 to the red point coordinates, instead of adding the bias unit 1 to the red point coordinates. Then the corresponding subtraction (as shown in the figure below) can be used as the coefficients of the linear equation.  But! In this way, the linear equation can move more, and the classification results may be worse when there are more data points, so we hope that the straight line will move to the red point slightly. So the concept of learning rate is introduced. From the above analysis, we can see that the learning rate should be a small number, assuming that the learning rate is 0.1, then multiply 0.1 by adding the red point coordinates of the bias unit, and subtract (as shown below). A new linear equation is obtained. At this point, you will be surprised to find that the straight line is closer to the red dot of the classification error!!! Yes, it's so simple and amazing. Similarly, if there is a blue dot Located in the red area, the line can also be made closer to the point in accordance with the above method. However, we should pay attention to the addition, not subtraction, when finding new parameters of linear equation. Keep this method in mind and it will be used frequently in the future.  Summarize the above knowledge points (pseudo-code as shown in the figure below). For n-dimensional data, the weights and biases are randomly allocated at first, and then the results of all points are calculated. For classification error points, we perform the following operations:

1. The point predicted to be zero is the blue point divided into the red area. We multiply the current weight plus the learning rate by the coordinates of the point as the new weight value, and the current bias plus the learning rate as the new bias.

2. The point predicted to be 1, that is, the red point divided into the blue area. We multiply the current weight minus the learning rate by the coordinates of the point as the new weight value, and subtract the learning rate from the current bias as the new bias. Let's do some exercises. Use the perceptron algorithm to classify the following data. ```0.78051,-0.063669,1
0.28774,0.29139,1
0.40714,0.17878,1
0.2923,0.4217,1
0.50922,0.35256,1
0.27785,0.10802,1
0.27527,0.33223,1
0.43999,0.31245,1
0.33557,0.42984,1
0.23448,0.24986,1
0.0084492,0.13658,1
0.12419,0.33595,1
0.25644,0.42624,1
0.4591,0.40426,1
0.44547,0.45117,1
0.42218,0.20118,1
0.49563,0.21445,1
0.30848,0.24306,1
0.39707,0.44438,1
0.32945,0.39217,1
0.40739,0.40271,1
0.3106,0.50702,1
0.49638,0.45384,1
0.10073,0.32053,1
0.69907,0.37307,1
0.29767,0.69648,1
0.15099,0.57341,1
0.16427,0.27759,1
0.33259,0.055964,1
0.53741,0.28637,1
0.19503,0.36879,1
0.40278,0.035148,1
0.21296,0.55169,1
0.48447,0.56991,1
0.25476,0.34596,1
0.21726,0.28641,1
0.67078,0.46538,1
0.3815,0.4622,1
0.53838,0.32774,1
0.4849,0.26071,1
0.37095,0.38809,1
0.54527,0.63911,1
0.32149,0.12007,1
0.42216,0.61666,1
0.10194,0.060408,1
0.15254,0.2168,1
0.45558,0.43769,1
0.28488,0.52142,1
0.27633,0.21264,1
0.39748,0.31902,1
0.5533,1,0
0.44274,0.59205,0
0.85176,0.6612,0
0.60436,0.86605,0
0.68243,0.48301,0
1,0.76815,0
0.72989,0.8107,0
0.67377,0.77975,0
0.78761,0.58177,0
0.71442,0.7668,0
0.49379,0.54226,0
0.78974,0.74233,0
0.67905,0.60921,0
0.6642,0.72519,0
0.79396,0.56789,0
0.70758,0.76022,0
0.59421,0.61857,0
0.49364,0.56224,0
0.77707,0.35025,0
0.79785,0.76921,0
0.70876,0.96764,0
0.69176,0.60865,0
0.66408,0.92075,0
0.65973,0.66666,0
0.64574,0.56845,0
0.89639,0.7085,0
0.85476,0.63167,0
0.62091,0.80424,0
0.79057,0.56108,0
0.58935,0.71582,0
0.56846,0.7406,0
0.65912,0.71548,0
0.70938,0.74041,0
0.59154,0.62927,0
0.45829,0.4641,0
0.79982,0.74847,0
0.60974,0.54757,0
0.68127,0.86985,0
0.76694,0.64736,0
0.69048,0.83058,0
0.68122,0.96541,0
0.73229,0.64245,0
0.76145,0.60138,0
0.58985,0.86955,0
0.73145,0.74516,0
0.77029,0.7014,0
0.73156,0.71782,0
0.44556,0.57991,0
0.85275,0.85987,0
0.51912,0.62359,0```

The program code is as follows:

```import numpy as np
# Setting the random seed, feel free to change it and see different solutions.
np.random.seed(42)

def stepFunction(t):
if t >= 0:
return 1
return 0

def prediction(X, W, b):
return stepFunction((np.matmul(X,W)+b))

# TODO: Fill in the code below to implement the perceptron trick.
# The function should receive as inputs the data X, the labels y,
# the weights W (as an array), and the bias b,
# update the weights and bias W, b, according to the perceptron algorithm,
# and return W and b.
def perceptronStep(X, y, W, b, learn_rate = 0.01):
# Fill in code
for i in range(len(X)):
y_hat = prediction(X[i],W,b)
if y[i]-y_hat == 1:
W += X[i]*learn_rate
W += X[i]*learn_rate
b += learn_rate
elif y[i]-y_hat == -1:
W -= X[i]*learn_rate
W -= X[i]*learn_rate
b -= learn_rate
return W, b

# This function runs the perceptron algorithm repeatedly on the dataset,
# and returns a few of the boundary lines obtained in the iterations,
# for plotting purposes.
# Feel free to play with the learning rate and the num_epochs,
# and see your results plotted below.
def trainPerceptronAlgorithm(X, y, learn_rate = 0.01, num_epochs = 25):
x_min, x_max = min(X.T), max(X.T)
y_min, y_max = min(X.T), max(X.T)
W = np.array(np.random.rand(2,1))
b = np.random.rand(1) + x_max
# These are the solution lines that get plotted below.
boundary_lines = []
for i in range(num_epochs):
# In each epoch, we apply the perceptron step.
W, b = perceptronStep(X, y, W, b, learn_rate)
boundary_lines.append((-W/W, -b/W))
return boundary_lines```

The experimental results are as follows: The green dashed line is the linear equation updated every time, and the black line is the final linear equation.

This paper mainly introduces the classification of linear data, then how to classify non-linear data? Let's meet next time.

Posted on Thu, 05 Sep 2019 22:59:41 -0700 by rokchik