[PYTHON] Decision tree (load_iris)

■ Introduction

This time, we will summarize the implementation-plot of the decision tree.

[Target readers] ・ Those who want to learn the basic code in decision trees ・ I don't know the theory in detail, but those who want to see the implementation and give an image, etc.

1. Preparation of module

First, import the required modules.

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

from sklearn.datasets import load_iris
from sklearn.model_selection import train_test_split
from sklearn.tree import DecisionTreeClassifier
from sklearn.tree import plot_tree

iris = load_iris()
X, y = iris.data[:, [0, 2]], iris.target

print(X.shape)
print(y.shape)

# (150, 2)
# (150,)

Divide into train and test data.

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size = 0.3, random_state = 123)

print(X_train.shape)
print(y_train.shape)
print(X_test.shape)
print(y_train.shape)

# (105, 2)
# (105,)
# (45, 2)
# (45,)

In the decision tree, individual features are processed independently, and data division is scale-independent. No normalization or standardization is required.

3. Data visualization

Before modeling, let's plot the data.

fig, ax = plt.subplots()

ax.scatter(X_train[y_train == 0, 0], X_train[y_train == 0, 1], 
           marker = 'o', label = 'Setosa')

ax.scatter(X_train[y_train == 1, 0], X_train[y_train == 1, 1],
           marker = 'x', label = 'Versicolor')

ax.scatter(X_train[y_train == 2, 0], X_train[y_train == 2, 1],
           marker = 'x', label = 'Varginica')

ax.set_xlabel('Sepal Length')
ax.set_ylabel('Petal Length')
ax.legend(loc = 'best')

plt.show()

4. Creating a model

Create a model of the decision tree.

tree = DecisionTreeClassifier(max_depth = 3)
tree.fit(X_train, y_train)

'''
DecisionTreeClassifier(ccp_alpha=0.0, class_weight=None, criterion='gini',
                       max_depth=3, max_features=None, max_leaf_nodes=None,
                       min_impurity_decrease=0.0, min_impurity_split=None,
                       min_samples_leaf=1, min_samples_split=2,
                       min_weight_fraction_leaf=0.0, presort='deprecated',
                       random_state=None, splitter='best')

'''

At the same time, I will also visualize it.

fig, ax = plt.subplots(figsize=(10, 10))
plot_tree(tree, feature_names=iris.feature_names, filled=True)
plt.show()

5. Output of predicted value

Make predictions for test data.

y_pred = tree.predict(X_test)

print(y_pred[:10])
print(y_test[:10])

# [2 2 2 1 0 1 1 0 0 1]
# [1 2 2 1 0 2 1 0 0 1]

0：Setosa　1：Versicolor　2：Verginica

6. Performance evaluation

Find the correct answer rate in this classification prediction.

print('{:.3f}'.format(tree.score(X_test, y_test)))

# 0.956