Experiment 7
Implement Back Propagation Neural Network Algorithm for the given dataset using Python.
Backpropagation neural network is used to improve the accuracy of neural network and make them capable of self-learning. Backpropagation means “backward propagation of errors”. Here error is spread into the reverse direction in order to achieve better performance. Backpropagation is an algorithm for supervised learning of artificial neural networks that uses the gradient descent method to minimize the cost function. It searches for optimal weights that optimize the mean-squared distance between the predicted and actual labels.
BPN learns in an iterative manner. In each iteration, it compares training examples with the actual target label. Target label can be a class label or continuous value. The backpropagation algorithm works in the following three steps:
load_iris() function of scikit-learn library and separate them into features and target labels.get_dummies() function.train_test_split(). You need to pass basically 3 parameters features, target, and test_set size. Additionally, you can use random_state in order to get the same kind of train and test set.sigmoid, mean_squared_error, and accuracy. We will create a for loop for given number of iterations that execute the three steps (feedforward propagation, error calculation, and backpropagation phase) and update the weights in each iteration.plot() function.
# Import libraries
import numpy as np
import pandas as pd
from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
import matplotlib.pyplot as plt
# Loading dataset
data = load_iris()
X = data.data
y = data.target
# Split dataset into training and test sets
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=20, random_state=4)
# Hyperparameters
learning_rate = 0.1
iterations = 5000
N = y_train.size
input_size = 4
hidden_size = 2
output_size = 3
np.random.seed(10)
W1 = np.random.normal(scale=0.5, size=(input_size, hidden_size))
W2 = np.random.normal(scale=0.5, size=(hidden_size, output_size))
# Helper functions
def sigmoid(x):
return 1 / (1 + np.exp(-x))
def mean_squared_error(y_pred, y_true):
# One-hot encode y_true (i.e., convert [0, 1, 2] into [[1, 0, 0], [0, 1, 0], [0, 0, 1]])
y_true_one_hot = np.eye(output_size)[y_true]
# Reshape y_true_one_hot to match y_pred shape
y_true_reshaped = y_true_one_hot.reshape(y_pred.shape)
# Compute the mean squared error between y_pred and y_true_reshaped
error = ((y_pred - y_true_reshaped)**2).sum() / (2*y_pred.size)
return error
def accuracy(y_pred, y_true):
acc = y_pred.argmax(axis=1) == y_true.argmax(axis=1)
return acc.mean()
results = pd.DataFrame(columns=["mse", "accuracy"])
# Training loop
for itr in range(iterations):
# Feedforward propagation
Z1 = np.dot(X_train, W1)
A1 = sigmoid(Z1)
Z2 = np.dot(A1, W2)
A2 = sigmoid(Z2)
# Calculate error
mse = mean_squared_error(A2, y_train)
acc = accuracy(np.eye(output_size)[y_train], A2)
new_row = pd.DataFrame({"mse": [mse], "accuracy": [acc]})
results = pd.concat([results, new_row], ignore_index=True)
# Backpropagation
E1 = A2 - np.eye(output_size)[y_train]
dW1 = E1 * A2 * (1 - A2)
E2 = np.dot(dW1, W2.T)
dW2 = E2 * A1 * (1 - A1)
# Update weights
W2_update = np.dot(A1.T, dW1) / N
W1_update = np.dot(X_train.T, dW2) / N
W2 = W2 - learning_rate * W2_update
W1 = W1 - learning_rate * W1_update
# Visualizing the results
results.mse.plot(title="Mean Squared Error")
plt.show()
results.accuracy.plot(title="Accuracy")
plt.show()
# Test the model
Z1 = np.dot(X_test, W1)
A1 = sigmoid(Z1)
Z2 = np.dot(A1, W2)
A2 = sigmoid(Z2)
test_acc = accuracy(np.eye(output_size)[y_test], A2)
print("Test accuracy: {}".format(test_acc))
Implementation of BPN for Iris dataset is carried out here. BPN is a method to optimize neural networks by propagating the error or loss into a backward direction. It finds loss for each node and updates its weights accordingly in order to minimize the loss using gradient descent.