使用python实现id3算法

好的，下面是使用Python实现ID3算法的代码：

import pandas as pd
import numpy as np
import math

class Node:
    def __init__(self, feature=None, threshold=None, left=None, right=None, value=None, predict=None):
        self.feature = feature
        self.threshold = threshold
        self.left = left
        self.right = right
        self.value = value
        self.predict = predict

class DecisionTree:
    def __init__(self):
        self.tree = None

    def fit(self, X, y):
        self.tree = build_tree(X, y)

    def predict(self, X):
        return [predict_one(x, self.tree) for x in X]

def build_tree(X, y):
    if same_class(y):
        return Node(predict=y[0])

    feature, threshold = select_feature(X, y)
    left_idx = X[:, feature] <= threshold
    right_idx = X[:, feature] > threshold

    left = build_tree(X[left_idx], y[left_idx])
    right = build_tree(X[right_idx], y[right_idx])

    return Node(feature=feature, threshold=threshold, left=left, right=right)

def predict_one(x, tree):
    if tree.predict is not None:
        return tree.predict
    if x[tree.feature] <= tree.threshold:
        return predict_one(x, tree.left)
    else:
        return predict_one(x, tree.right)

def same_class(y):
    return len(set(y)) == 1

def entropy(y):
    _, counts = np.unique(y, return_counts=True)
    p = counts / len(y)
    return -np.sum(p * np.log2(p))

def select_feature(X, y):
    best_feature, best_threshold, best_gain = None, None, 0

    for feature in range(X.shape[1]):
        thresholds = np.unique(X[:, feature])
        for threshold in thresholds:
            left_idx = X[:, feature] <= threshold
            left_y = y[left_idx]
            right_y = y[~left_idx]

            if len(left_y) == 0 or len(right_y) == 0:
                continue

            gain = entropy(y) - (len(left_y) / len(y)) * entropy(left_y) - (len(right_y) / len(y)) * entropy(right_y)

            if gain > best_gain:
                best_feature = feature
                best_threshold = threshold
                best_gain = gain

    return best_feature, best_threshold

以上是Python实现的ID3算法的代码。