ml-model-training（机器学习模型训练）

ML Model Training

Training machine learning models involves selecting appropriate algorithms, preparing data, and optimizing model parameters to achieve strong predictive performance.

Training Phases

• Data Preparation: Cleaning, encoding, normalization
• Feature Engineering: Creating meaningful features
• Model Selection: Choosing appropriate algorithms
• Hyperparameter Tuning: Optimizing model settings
• Validation: Cross-validation and evaluation metrics
• Deployment: Preparing models for production

Common Algorithms

• Regression: Linear, Ridge, Lasso, Random Forest
• Classification: Logistic, SVM, Random Forest, Gradient Boosting
• Clustering: K-Means, DBSCAN, Hierarchical
• Neural Networks: MLPs, CNNs, RNNs, Transformers

Python Implementation

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
from sklearn.model_selection import train_test_split, cross_val_score
from sklearn.preprocessing import StandardScaler
from sklearn.ensemble import RandomForestClassifier, GradientBoostingClassifier
from sklearn.linear_model import LogisticRegression
from sklearn.metrics import (accuracy_score, precision_score, recall_score,
                            f1_score, confusion_matrix, roc_auc_score)
import torch
import torch.nn as nn
from torch.utils.data import DataLoader, TensorDataset
import tensorflow as tf
from tensorflow import keras

# 1. Generate synthetic dataset
np.random.seed(42)
n_samples = 1000
n_features = 20

X = np.random.randn(n_samples, n_features)
y = (X[:, 0] + X[:, 1] - X[:, 2] + np.random.randn(n_samples) * 0.5 > 0).astype(int)

# Split data
X_train, X_test, y_train, y_test = train_test_split(
    X, y, test_size=0.2, random_state=42
)

# Normalize features
scaler = StandardScaler()
X_train_scaled = scaler.fit_transform(X_train)
X_test_scaled = scaler.transform(X_test)

print("Dataset shapes:")
print(f"Training: {X_train_scaled.shape}, Testing: {X_test_scaled.shape}")
print(f"Class distribution: {np.bincount(y_train)}")

# 2. Scikit-learn models
print("\n=== Scikit-learn Models ===")

models = {
    'Logistic Regression': LogisticRegression(max_iter=1000),
    'Random Forest': RandomForestClassifier(n_estimators=100, random_state=42),
    'Gradient Boosting': GradientBoostingClassifier(n_estimators=100, random_state=42),
}

sklearn_results = {}
for name, model in models.items():
    model.fit(X_train_scaled, y_train)
    y_pred = model.predict(X_test_scaled)
    y_pred_proba = model.predict_proba(X_test_scaled)[:, 1]

    sklearn_results[name] = {
        'accuracy': accuracy_score(y_test, y_pred),
        'precision': precision_score(y_test, y_pred),
        'recall': recall_score(y_test, y_pred),
        'f1': f1_score(y_test, y_pred),
        'roc_auc': roc_auc_score(y_test, y_pred_proba)
    }

    print(f"\n{name}:")
    for metric, value in sklearn_results[name].items():
        print(f"  {metric}: {value:.4f}")

# 3. PyTorch neural network
print("\n=== PyTorch Model ===")

class NeuralNetPyTorch(nn.Module):
    def __init__(self, input_size):
        super().__init__()
        self.fc1 = nn.Linear(input_size, 64)
        self.fc2 = nn.Linear(64, 32)
        self.fc3 = nn.Linear(32, 1)
        self.relu = nn.ReLU()
        self.dropout = nn.Dropout(0.3)

    def forward(self, x):
        x = self.relu(self.fc1(x))
        x = self.dropout(x)
        x = self.relu(self.fc2(x))
        x = self.dropout(x)
        x = torch.sigmoid(self.fc3(x))
        return x

device = torch.device('cuda' if torch.cuda.is_available() else 'cpu')
pytorch_model = NeuralNetPyTorch(n_features).to(device)
criterion = nn.BCELoss()
optimizer = torch.optim.Adam(pytorch_model.parameters(), lr=0.001)

# Create data loaders
train_dataset = TensorDataset(torch.FloatTensor(X_train_scaled),
                             torch.FloatTensor(y_train).unsqueeze(1))
train_loader = DataLoader(train_dataset, batch_size=32, shuffle=True)

# Train PyTorch model
epochs = 50
pytorch_losses = []
for epoch in range(epochs):
    total_loss = 0
    for batch_X, batch_y in train_loader:
        batch_X, batch_y = batch_X.to(device), batch_y.to(device)

        optimizer.zero_grad()
        outputs = pytorch_model(batch_X)
        loss = criterion(outputs, batch_y)
        loss.backward()
        optimizer.step()
        total_loss += loss.item()

    pytorch_losses.append(total_loss / len(train_loader))
    if (epoch + 1) % 10 == 0:
        print(f"Epoch {epoch + 1}/{epochs}, Loss: {pytorch_losses[-1]:.4f}")

# Evaluate PyTorch
pytorch_model.eval()
with torch.no_grad():
    y_pred_pytorch = pytorch_model(torch.FloatTensor(X_test_scaled).to(device))
    y_pred_pytorch = (y_pred_pytorch.cpu().numpy() > 0.5).astype(int).flatten()
    print(f"\nPyTorch Accuracy: {accuracy_score(y_test, y_pred_pytorch):.4f}")

# 4. TensorFlow/Keras model
print("\n=== TensorFlow/Keras Model ===")

tf_model = keras.Sequential([
    keras.layers.Dense(64, activation='relu', input_shape=(n_features,)),
    keras.layers.Dropout(0.3),
    keras.layers.Dense(32, activation='relu'),
    keras.layers.Dropout(0.3),
    keras.layers.Dense(1, activation='sigmoid')
])

tf_model.compile(
    optimizer='adam',
    loss='binary_crossentropy',
    metrics=['accuracy']
)

history = tf_model.fit(
    X_train_scaled, y_train,
    batch_size=32,
    epochs=50,
    validation_split=0.2,
    verbose=0
)

y_pred_tf = (tf_model.predict(X_test_scaled) > 0.5).astype(int).flatten()
print(f"TensorFlow Accuracy: {accuracy_score(y_test, y_pred_tf):.4f}")

# 5. Visualization
fig, axes = plt.subplots(2, 2, figsize=(12, 10))

# Model comparison
models_names = list(sklearn_results.keys()) + ['PyTorch', 'TensorFlow']
accuracies = [sklearn_results[m]['accuracy'] for m in sklearn_results.keys()] + \
             [accuracy_score(y_test, y_pred_pytorch),
              accuracy_score(y_test, y_pred_tf)]

axes[0, 0].bar(range(len(models_names)), accuracies, color='steelblue')
axes[0, 0].set_xticks(range(len(models_names)))
axes[0, 0].set_xticklabels(models_names, rotation=45)
axes[0, 0].set_ylabel('Accuracy')
axes[0, 0].set_title('Model Comparison')
axes[0, 0].set_ylim([0, 1])

# Training loss curves
axes[0, 1].plot(pytorch_losses, label='PyTorch', linewidth=2)
axes[0, 1].plot(history.history['loss'], label='TensorFlow', linewidth=2)
axes[0, 1].set_xlabel('Epoch')
axes[0, 1].set_ylabel('Loss')
axes[0, 1].set_title('Training Loss Comparison')
axes[0, 1].legend()
axes[0, 1].grid(True, alpha=0.3)

# Scikit-learn metrics
metrics = ['accuracy', 'precision', 'recall', 'f1']
rf_metrics = [sklearn_results['Random Forest'][m] for m in metrics]
axes[1, 0].bar(metrics, rf_metrics, color='coral')
axes[1, 0].set_ylabel('Score')
axes[1, 0].set_title('Random Forest Metrics')
axes[1, 0].set_ylim([0, 1])

# Validation accuracy over epochs
axes[1, 1].plot(history.history['accuracy'], label='Training', linewidth=2)
axes[1, 1].plot(history.history['val_accuracy'], label='Validation', linewidth=2)
axes[1, 1].set_xlabel('Epoch')
axes[1, 1].set_ylabel('Accuracy')
axes[1, 1].set_title('TensorFlow Training History')
axes[1, 1].legend()
axes[1, 1].grid(True, alpha=0.3)

plt.tight_layout()
plt.savefig('model_training_comparison.png', dpi=100, bbox_inches='tight')
print("\nVisualization saved as 'model_training_comparison.png'")

print("\nModel training completed!")

Training Best Practices

• Data Split: 70/15/15 for train/validation/test
• Scaling: Normalize features before training
• Cross-validation: Use K-fold for robust evaluation
• Early Stopping: Prevent overfitting
• Class Balancing: Handle imbalanced datasets

Key Metrics

• Accuracy: Overall correctness
• Precision: Positive prediction accuracy
• Recall: True positive detection rate
• F1 Score: Harmonic mean of precision/recall
• ROC-AUC: Threshold-independent metric

Deliverables

• Trained model checkpoint
• Performance metrics on test set
• Feature importance analysis
• Learning curves
• Hyperparameter configuration
• Model evaluation report