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Java开发实例:使用Java实现基本的决策树算法
2023-06-12 12:57:41 深夜i     --     --
Java编程 决策树算法 机器学习

在机器学习领域中,决策树是一种常用的算法,常用于分类和回归问题。在这里,我们将使用Java语言实现一个基本的决策树算法。

在这个实例中,我们将使用一个基于波士顿房价数据集的分类问题。我们首先需要获取数据集并解析其内容。这里我们用到了Java自带的CSV解析库,代码如下:

 java

Reader reader = Files.newBufferedReader(Paths.get("data.csv"));

CSVParser parser = new CSVParser(reader, CSVFormat.DEFAULT.withHeader());

List data = new ArrayList<>();

for (CSVRecord record : parser) {

  double[] instance = new double[record.size() - 1];

  for (int i = 0; i < instance.length; i++) {

    instance[i] = Double.parseDouble(record.get(i));

  }

  data.add(instance);

}

接下来,我们定义一个类来表示决策树的节点,并在其中实现建树的算法,代码如下:

 java

class DecisionTreeNode {

  int featureIndex = -1;

  double threshold = 0.0;

  DecisionTreeNode left = null;

  DecisionTreeNode right = null;

  int label = -1;

  public static DecisionTreeNode buildTree(List data, int depth) {

    DecisionTreeNode node = new DecisionTreeNode();

    double[] labelCounts = new double[NUM_LABELS];

    for (Double[] instance : data) {

      labelCounts[instance[instance.length - 1].intValue()]++;

    }

    int maxLabel = argmax(labelCounts);

    if (labelCounts[maxLabel] == data.size() || depth == MAX_DEPTH)

      node.label = maxLabel;

      return node;

    

    double bestScore = -1.0;

    int bestIndex = -1;

    double bestThreshold = 0.0;

    for (int i = 0; i < data.get(0).length - 1; i++) {

      double[] classCounts1 = new double[NUM_LABELS];

      double[] classCounts2 = new double[NUM_LABELS];

      int count1 = 0;

      int count2 = 0;

      for (Double[] instance : data) {

        if (instance[i] < threshold) {

          classCounts1[instance[instance.length - 1].intValue()]++;

          count1++;

        } else {

          classCounts2[instance[instance.length - 1].intValue()]++;

          count2++;

        }

      }

      double score = giniImpurity(classCounts1, count1) * count1 / (count1 + count2)

          + giniImpurity(classCounts2, count2) * count2 / (count1 + count2);

      if (score > bestScore)

        bestScore = score;

        bestIndex = i;

        bestThreshold = threshold;

      

    }

    node.featureIndex = bestIndex;

    node.threshold = bestThreshold;

    List data1 = new ArrayList<>();

    List data2 = new ArrayList<>();

    for (Double[] instance : data) {

      if (instance[bestIndex] < bestThreshold) {

        data1.add(instance);

      } else {

        data2.add(instance);

      }

    }

    node.left = buildTree(data1, depth + 1);

    node.right = buildTree(data2, depth + 1);

    return node;

  }

}

最后,我们使用上述代码来构建决策树,并对新实例进行分类,代码如下:

 java

DecisionTreeNode root = DecisionTreeNode.buildTree(data, 0);

double[] instance = new double[] 0.078;

int predictedLabel = classify(root, instance);

System.out.println(predictedLabel);

本次实例使用的数据集可以从以下链接中获取:https://archive.ics.uci.edu/ml/machine-learning-databases/housing/housing.data

  
  

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