Decision Tree ID3 Algorithm in Python


Python Program to Implement Decision Tree ID3 Algorithm

Exp. No. 3. Write a program to demonstrate the working of the decision tree based ID3 algorithm. Use an appropriate data set for building the decision tree and apply this knowledge to classify a new sample. 

Decision Tree ID3 Algorithm Machine Learning

ID3(Examples, Target_attribute, Attributes)

Examples are the training examples. 
Target_attribute is the attribute whose value is to be predicted by the tree. 
Attributes is a list of other attributes that may be tested by the learned decision tree. 
Returns a decision tree that correctly classifies the given Examples.

Create a Root node for the tree
If all Examples are positive, Return the single-node tree Root, with label = +
If all Examples are negative, Return the single-node tree Root, with label = -
If Attributes is empty, Return the single-node tree Root, 
with label = most common value of Target_attribute in Examples

Otherwise Begin
   A ← the attribute from Attributes that best* classifies Examples
   The decision attribute for Root ← A
   For each possible value, vi, of A,
      Add a new tree branch below Root, corresponding to the test A = vi
      Let Examples vi, be the subset of Examples that have value vi for A
      If Examples vi , is empty
         Then below this new branch add a leaf node with 
         label = most common value of Target_attribute in Examples
         below this new branch add the subtree 
         ID3(Examples vi, Targe_tattribute, Attributes – {A}))
Return Root

The best attribute is the one with highest information gain


Entropy measures the impurity of a collection of examples.

Where,           p+ is the proportion of positive examples in S

p is the proportion of negative examples in S.


Information gain, is the expected reduction in entropy caused by partitioning the examples according to this attribute.

The information gain, Gain(S, A) of an attribute A, relative to a collection of examples S, is defined as


PlayTennis Dataset is saved as .csv (comma separated values) file in the current working directory otherwise use the complete path of the dataset set in the program:


Click here to download the dataset

Python Program to Implement and Demonstrate FIND-S Algorithm

Import libraries and read data using read_csv() function. Remove the target from the data and store attributes in the features variable.

import pandas as pd
import math
import numpy as np

data = pd.read_csv("Dataset/4-dataset.csv")
features = [feat for feat in data]

Create a class named Node with four members children, value, isLeaf and pred.

class Node:
    def __init__(self):
        self.children = []
        self.value = ""
        self.isLeaf = False
        self.pred = ""

Define a function called entropy to find the entropy oof the dataset.

def entropy(examples):
    pos = 0.0
    neg = 0.0
    for _, row in examples.iterrows():
        if row["answer"] == "yes":
            pos += 1
            neg += 1
    if pos == 0.0 or neg == 0.0:
        return 0.0
        p = pos / (pos + neg)
        n = neg / (pos + neg)
        return -(p * math.log(p, 2) + n * math.log(n, 2))

Define a function named info_gain to find the gain of the attribute

def info_gain(examples, attr):
    uniq = np.unique(examples[attr])
    #print ("\n",uniq)
    gain = entropy(examples)
    #print ("\n",gain)
    for u in uniq:
        subdata = examples[examples[attr] == u]
        #print ("\n",subdata)
        sub_e = entropy(subdata)
        gain -= (float(len(subdata)) / float(len(examples))) * sub_e
        #print ("\n",gain)
    return gain

Define a function named ID3 to get the decision tree for the given dataset

def ID3(examples, attrs):
    root = Node()

    max_gain = 0
    max_feat = ""
    for feature in attrs:
        #print ("\n",examples)
        gain = info_gain(examples, feature)
        if gain > max_gain:
            max_gain = gain
            max_feat = feature
    root.value = max_feat
    #print ("\nMax feature attr",max_feat)
    uniq = np.unique(examples[max_feat])
    #print ("\n",uniq)
    for u in uniq:
        #print ("\n",u)
        subdata = examples[examples[max_feat] == u]
        #print ("\n",subdata)
        if entropy(subdata) == 0.0:
            newNode = Node()
            newNode.isLeaf = True
            newNode.value = u
            newNode.pred = np.unique(subdata["answer"])
            dummyNode = Node()
            dummyNode.value = u
            new_attrs = attrs.copy()
            child = ID3(subdata, new_attrs)

    return root

Define a function named printTree to draw the decision tree

def printTree(root: Node, depth=0):
    for i in range(depth):
        print("\t", end="")
    print(root.value, end="")
    if root.isLeaf:
        print(" -> ", root.pred)
    for child in root.children:
        printTree(child, depth + 1)

Define a function named classify to classify the new example

def classify(root: Node, new):
    for child in root.children:
        if child.value == new[root.value]:
            if child.isLeaf:
                print ("Predicted Label for new example", new," is:", child.pred)
                classify (child.children[0], new)

Finally, call the ID3, printTree and classify functions

root = ID3(data, features)
print("Decision Tree is:")
print ("------------------")

new = {"outlook":"sunny", "temperature":"hot", "humidity":"normal", "wind":"strong"}
classify (root, new)


Decision Tree ID3 Algorithm in Python
Decision Tree is:
	overcast ->  ['yes']

			strong ->  ['no']

			weak ->  ['yes']

			high ->  ['no']

			normal ->  ['yes']

Predicted Label for new example {'outlook': 'sunny', 'temperature': 'hot', 'humidity': 'normal', 'wind': 'strong'}  is: ['yes']

Solved Numerical Examples and Tutorial on Decision Trees Machine Learning:

1. How to build a decision Tree for Boolean Function Machine Learning

2. How to build a decision Tree for Boolean Function Machine Learning

3. How to build Decision Tree using ID3 Algorithm – Solved Numerical Example – 1

4. How to build Decision Tree using ID3 Algorithm – Solved Numerical Example -2

5. How to build Decision Tree using ID3 Algorithm – Solved Numerical Example -3

6. Appropriate Problems for Decision Tree Learning Machine Learning Big Data Analytics

7. How to find the Entropy and Information Gain in Decision Tree Learning

8. Issues in Decision Tree Learning Machine Learning

9. How to Avoid Overfitting in Decision Tree Learning, Machine Learning, and Data Mining

10. How to handle Continuous Valued Attributes in Decision Tree Learning, Machine Learning 


This tutorial discusses how to Implement and demonstrate the Decision Tree ID3 Algorithm in Python. The training data is read from a .CSV file. If you like the tutorial share it with your friends. Like the Facebook page for regular updates and YouTube channel for video tutorials.

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