C45决策树python代码

C4.5决策树是一种经典的机器学习算法，用于分类和回归任务。下面是一个简单的C4.5决策树的Python代码示例：

import numpy as np
from collections import Counter

class Node:
    def __init__(self, feature=None, threshold=None, label=None):
        self.feature = feature  # 分裂特征的索引
        self.threshold = threshold  # 分裂特征的阈值
        self.label = label  # 叶节点的类别
        self.left = None  # 左子树
        self.right = None  # 右子树

class C45DecisionTree:
    def __init__(self, min_samples_split=2, max_depth=float('inf'), min_info_gain=1e-7):
        self.min_samples_split = min_samples_split  # 分裂所需的最小样本数
        self.max_depth = max_depth  # 树的最大深度
        self.min_info_gain = min_info_gain  # 分裂所需的最小信息增益

    def _calculate_entropy(self, y):
        counter = Counter(y)
        probs = [count / len(y) for count in counter.values()]
        entropy = -sum(p * np.log2(p) for p in probs)
        return entropy

    def _calculate_info_gain(self, X, y, feature, threshold):
        left_mask = X[:, feature] <= threshold
        right_mask = X[:, feature] > threshold
        left_entropy = self._calculate_entropy(y[left_mask])
        right_entropy = self._calculate_entropy(y[right_mask])
        info_gain = self._calculate_entropy(y) - (left_entropy * sum(left_mask) / len(y) + right_entropy * sum(right_mask) / len(y))
        return info_gain

    def _split(self, X, y):
        m, n = X.shape
        best_info_gain = 0
        best_feature = None
        best_threshold = None

        for feature in range(n):
            thresholds = np.unique(X[:, feature])
            for threshold in thresholds:
                info_gain = self._calculate_info_gain(X, y, feature, threshold)
                if info_gain > best_info_gain:
                    best_info_gain = info_gain
                    best_feature = feature
                    best_threshold = threshold

        return best_feature, best_threshold

    def _build_tree(self, X, y, depth):
        if len(set(y)) == 1:
            return Node(label=y[0])

        if depth >= self.max_depth or len(X) < self.min_samples_split:
            counter = Counter(y)
            most_common_label = counter.most_common(1)[0][0]
            return Node(label=most_common_label)

        feature, threshold = self._split(X, y)
        if feature is None or threshold is None:
            counter = Counter(y)
            most_common_label = counter.most_common(1)[0][0]
            return Node(label=most_common_label)

        left_mask = X[:, feature] <= threshold
        right_mask = X[:, feature] > threshold
        left_subtree = self._build_tree(X[left_mask], y[left_mask], depth + 1)
        right_subtree = self._build_tree(X[right_mask], y[right_mask], depth + 1)

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

    def fit(self, X, y):
        self.tree = self._build_tree(X, y, 0)

    def _predict_single(self, x, node):
        if node.label is not None:
            return node.label

        if x[node.feature] <= node.threshold:
            return self._predict_single(x, node.left)
        else:
            return self._predict_single(x, node.right)

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

这段代码实现了一个基本的C4.5决策树算法，包括计算熵、计算信息增益、选择最佳分裂特征和阈值、构建决策树等功能。你可以使用`fit`方法来训练模型，使用`predict`方法来进行预测。