【机器学习】神经网络

Neural Networks

神经网络：一种计算模型，由大量的节点(或神经元)直接相互关联而构成。每个节点(除输入节点外)代表一种特定的输出函数(或者认为是运算),称为激励函数；每两个节点的连接都代表该信号在传输中所占的比重(即认为该信号对该节点的影响程度)

神经网络三要素：模型、策略、算法

概述

神经网络三层：

• 输入层：数据
• 隐藏层：模型
• 输出层：策略【reLU, sigmoid, tanh, softmax】

感知机

线性分类模型

\[f(x) = sign(w \cdot x + b) \]

其中

\[sign(X)=\left\{ \begin{matrix} +1, x \geqslant 0 \\ -1, x \lt 0 \end{matrix} \right. \]

BP算法

神经网络的三要素之算法——反向传播（关键是梯度下降）

activation

linear activation

\[f(x) = w \mathbf{x} + b \]

Sigmoid activation

\[f_{\mathbf{w},b}(x^{(i)}) = g(\mathbf{w}x^{(i)} + b) \]

\[g(x) = sigmoid(x) \]

ReLU Activation

\[a = max(0, z) \]

build the network layer

使用Numpy

代码

# 建立单层神经网络
def my_dense(a_in, W, b, g):
    """
    Computes dense layer
    Args:
      a_in (ndarray (n, )) : Data, 1 example 
      W    (ndarray (n,j)) : Weight matrix, n features per unit, j units
      b    (ndarray (j, )) : bias vector, j units  
      g    activation function (e.g. sigmoid, relu..)
    Returns
      a_out (ndarray (j,))  : j units|
    """
    units = W.shape[1]
    a_out = np.zeros(units)
    for j in range(units):               
        w = W[:,j]                                    
        z = np.dot(w, a_in) + b[j]         
        a_out[j] = g(z)               
    return(a_out)

# 建立两层神经网络
def my_sequential(x, W1, b1, W2, b2):
    a1 = my_dense(x,  W1, b1, sigmoid)
    a2 = my_dense(a1, W2, b2, sigmoid)
    return(a2)

# 预测
def my_predict(X, W1, b1, W2, b2):
    m = X.shape[0]
    p = np.zeros((m,1))
    for i in range(m):
        p[i,0] = my_sequential(X[i], W1, b1, W2, b2)
    return(p)

Multi-class Classification

preferred_model = Sequential(
    [
        # kernel_regularizer 正则化
        Dense(120, activation = 'relu', kernel_regularizer=tf.keras.regularizers.l2(0.1), name="L1"), 
        Dense(40, activation = 'relu', kernel_regularizer=tf.keras.regularizers.l2(0.1), name="L2"),         Dense(classes, activation = 'linear', name="L3")  
    ], name="ComplexRegularized"
)
preferred_model.compile(
    loss=tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True),  #<-- Note
    optimizer=tf.keras.optimizers.Adam(learning_rate=1e-3), # 优化Alpha
)

preferred_model.fit(
    X_train,y_train,
    epochs=10
)