The goal of this notebook is to code a decision tree classifier that can be used with the following API:
df = pd.read_csv("data.csv")
train_df, test_df = train_test_split(df, test_size=0.2)
tree = decision_tree_algorithm(train_df)
accuracy = calculate_accuracy(test_df, tree)
The algorithm that is going to be implemented looks like this:
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns
import random
from pprint import pprint
%matplotlib inline
sns.set_style("darkgrid")
df = pd.read_csv("../data/Iris.csv")
df = df.drop("Id", axis=1)
df = df.rename(columns={"species": "label"})
df.head()
def train_test_split(df, test_size):
if isinstance(test_size, float):
test_size = round(test_size * len(df))
indices = df.index.tolist()
test_indices = random.sample(population=indices, k=test_size)
test_df = df.loc[test_indices]
train_df = df.drop(test_indices)
return train_df, test_df
random.seed(0)
train_df, test_df = train_test_split(df, test_size=20)
The helper functions operate on a NumPy 2d-array. Therefore, let’s create a variable called “data” to see what we will be working with.
data = train_df.values
data[:5]
def check_purity(data):
label_column = data[:, -1]
unique_classes = np.unique(label_column)
if len(unique_classes) == 1:
return True
else:
return False
def classify_data(data):
label_column = data[:, -1]
unique_classes, counts_unique_classes = np.unique(label_column, return_counts=True)
index = counts_unique_classes.argmax()
classification = unique_classes[index]
return classification
def get_potential_splits(data):
potential_splits = {}
_, n_columns = data.shape
for column_index in range(n_columns - 1): # excluding the last column which is the label
potential_splits[column_index] = []
values = data[:, column_index]
unique_values = np.unique(values)
for index in range(len(unique_values)):
if index != 0:
current_value = unique_values[index]
previous_value = unique_values[index - 1]
potential_split = (current_value + previous_value) / 2
potential_splits[column_index].append(potential_split)
return potential_splits
def split_data(data, split_column, split_value):
split_column_values = data[:, split_column]
data_below = data[split_column_values <= split_value]
data_above = data[split_column_values > split_value]
return data_below, data_above
def calculate_entropy(data):
label_column = data[:, -1]
_, counts = np.unique(label_column, return_counts=True)
probabilities = counts / counts.sum()
entropy = sum(probabilities * -np.log2(probabilities))
return entropy
def calculate_overall_entropy(data_below, data_above):
n = len(data_below) + len(data_above)
p_data_below = len(data_below) / n
p_data_above = len(data_above) / n
overall_entropy = (p_data_below * calculate_entropy(data_below)
+ p_data_above * calculate_entropy(data_above))
return overall_entropy
def determine_best_split(data, potential_splits):
overall_entropy = 9999
for column_index in potential_splits:
for value in potential_splits[column_index]:
data_below, data_above = split_data(data, split_column=column_index, split_value=value)
current_overall_entropy = calculate_overall_entropy(data_below, data_above)
if current_overall_entropy <= overall_entropy:
overall_entropy = current_overall_entropy
best_split_column = column_index
best_split_value = value
return best_split_column, best_split_value
sub_tree = {"question": ["yes_answer",
"no_answer"]}
example_tree = {"petal_width <= 0.8": ["Iris-setosa",
{"petal_width <= 1.65": [{"petal_length <= 4.9": ["Iris-versicolor",
"Iris-virginica"]},
"Iris-virginica"]}]}
def decision_tree_algorithm(df, counter=0, min_samples=2, max_depth=5):
# data preparations
if counter == 0:
global COLUMN_HEADERS
COLUMN_HEADERS = df.columns
data = df.values
else:
data = df
# base cases
if (check_purity(data)) or (len(data) < min_samples) or (counter == max_depth):
classification = classify_data(data)
return classification
# recursive part
else:
counter += 1
# helper functions
potential_splits = get_potential_splits(data)
split_column, split_value = determine_best_split(data, potential_splits)
data_below, data_above = split_data(data, split_column, split_value)
# instantiate sub-tree
feature_name = COLUMN_HEADERS[split_column]
question = "{} <= {}".format(feature_name, split_value)
sub_tree = {question: []}
# find answers (recursion)
yes_answer = decision_tree_algorithm(data_below, counter, min_samples, max_depth)
no_answer = decision_tree_algorithm(data_above, counter, min_samples, max_depth)
# If the answers are the same, then there is no point in asking the qestion.
# This could happen when the data is classified even though it is not pure
# yet (min_samples or max_depth base cases).
if yes_answer == no_answer:
sub_tree = yes_answer
else:
sub_tree[question].append(yes_answer)
sub_tree[question].append(no_answer)
return sub_tree
tree = decision_tree_algorithm(train_df, max_depth=3)
pprint(tree)
sub_tree
example = test_df.iloc[0]
example
def classify_example(example, tree):
question = list(tree.keys())[0]
feature_name, comparison_operator, value = question.split()
# ask question
if example[feature_name] <= float(value):
answer = tree[question][0]
else:
answer = tree[question][1]
# base case
if not isinstance(answer, dict):
return answer
# recursive part
else:
residual_tree = answer
return classify_example(example, residual_tree)
classify_example(example, tree)
def calculate_accuracy(df, tree):
df["classification"] = df.apply(classify_example, axis=1, args=(tree,))
df["classification_correct"] = df["classification"] == df["label"]
accuracy = df["classification_correct"].mean()
return accuracy
accuracy = calculate_accuracy(test_df, tree)
accuracy