pyspark mllib 二分类是以softmax作为输出的解释

直接上代码

我用的是GBDT，预测结果如下：

只给出关键代码

gbdt = GBTClassifier(featuresCol="features", labelCol="y", predictionCol="prediction",)

df_train_eval = model.transform(df_train)

df_train_eval.select(*['y', 'rawPrediction', 'probability', 'prediction']).show(truncate=False)

+---+----------------------------------------+-----------------------------------------+----------+
|y  |rawPrediction                           |probability                              |prediction|
+---+----------------------------------------+-----------------------------------------+----------+
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
|0  |[1.5435020027249835,-1.5435020027249835]|[0.9563534785727067,0.043646521427293306]|0.0       |
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
|1  |[-1.5435020027249835,1.5435020027249835]|[0.04364652142729318,0.9563534785727068] |1.0       |
+---+----------------------------------------+-----------------------------------------+----------+

y：原始标签rawPrediction：原始预测值probability：概率值prediction：预测结果

为什么说是softmax，而不是sigmoid？

原因一：

我们看概率值probability这一列，每一行加起来都是1，符合softmax的互斥原则。

原因二：

def softmax(x):
	e_x = np.exp(x)
	return e_x / e_x.sum()

def sigmoid(x):
	return 1. / (1 + np.exp(-x))

同过将原始预测值rawPrediction，带到softmax和sigmoid函数中，也可以证明，它的结果就是softmax的结果。