上一周主要看了 Neural Networks and Deep Learning 网上在线课程的第二章的内容 和 斯坦福大学 《机器学习》的公开课,学习了BP( Back Propagation Algorithm, 反向传播算法)。现在总结如下:
只要使用神经网络就会用到BP算法,反向传播算法可以用来学习神经网络的权值,仍然采用梯度下降算法,以最小化网络的实际输出与目标输出之间的平方误差为目标。BP算法的目标是在神经网络中可以求任意权值w 和 偏置b 的成本函数(cost function)C 的两个偏导数——C对w的偏导数 和 C对b的偏导数
首先定义几个符号:
图1 的定义
同样的,再定义 和 :
,
由此,可以得到激励函数a的计算公式:
公式简写如下:
令 ,则 。
在正式介绍BP算法之前,先介绍一种运算规则——Hadamard product(阿达马乘积)
定义 :
根据公式 (23)、(29)可知,the error 是和C对w的偏导数 和 C对b的偏导数 有关。
现在给出BP算法使用的四个基本公式:
将(BP1)公式改写为矩阵形式的公式:
关于两层之间的delt计算公式:
关于偏置b 的BP公式:
上述公式简记为:
关于权值w 的BP公式:
上述公式简记为:
可以用下图来形象地表示上述关系式
有关上述四个公式的证明,这里就不再给出,如果感兴趣可以点击本文开头提供的网上在线课程的链接进行查看,主要就是运用了高数中的链式求导法则进行相关证明。
BP算法
BP(反向传播)算法提供了一种计算成本函数(cost function)梯度的方法。算法主要流程如下:
BP算法的代码实现
- class Network(object):
- ...
- def update_mini_batch(self, mini_batch, eta):
- """Update the network's weights and biases by applying
- gradient descent using backpropagation to a single mini batch.
- The "mini_batch" is a list of tuples "(x, y)", and "eta"
- is the learning rate."""
- nabla_b = [np.zeros(b.shape) for b in self.biases]
- nabla_w = [np.zeros(w.shape) for w in self.weights]
- for x, y in mini_batch:
- delta_nabla_b, delta_nabla_w = self.backprop(x, y) #使用BP算法求解两个偏导数,进而可求成本函数的梯度
- nabla_b = [nb+dnb for nb, dnb in zip(nabla_b, delta_nabla_b)]
- nabla_w = [nw+dnw for nw, dnw in zip(nabla_w, delta_nabla_w)]
- self.weights = [w-(eta/len(mini_batch))*nw
- for w, nw in zip(self.weights, nabla_w)]
- self.biases = [b-(eta/len(mini_batch))*nb
- for b, nb in zip(self.biases, nabla_b)]
反向传播算法的代码实现:
- class Network(object):
- ...
- def backprop(self, x, y):
- """Return a tuple "(nabla_b, nabla_w)" representing the
- gradient for the cost function C_x. "nabla_b" and
- "nabla_w" are layer-by-layer lists of numpy arrays, similar
- to "self.biases" and "self.weights"."""
- nabla_b = [np.zeros(b.shape) for b in self.biases]
- nabla_w = [np.zeros(w.shape) for w in self.weights]
- # feedforward
- activation = x
- activations = [x] # list to store all the activations, layer by layer
- zs = [] # list to store all the z vectors, layer by layer
- for b, w in zip(self.biases, self.weights):
- z = np.dot(w, activation)+b
- zs.append(z)
- activation = sigmoid(z)
- activations.append(activation)
- # backward pass
- delta = self.cost_derivative(activations[-1], y) * \
- sigmoid_prime(zs[-1])
- nabla_b[-1] = delta
- nabla_w[-1] = np.dot(delta, activations[-2].transpose())
- # Note that the variable l in the loop below is used a little
- # differently to the notation in Chapter 2 of the book. Here,
- # l = 1 means the last layer of neurons, l = 2 is the
- # second-last layer, and so on. It's a renumbering of the
- # scheme in the book, used here to take advantage of the fact
- # that Python can use negative indices in lists.
- for l in xrange(2, self.num_layers):
- z = zs[-l]
- sp = sigmoid_prime(z)
- delta = np.dot(self.weights[-l+1].transpose(), delta) * sp
- nabla_b[-l] = delta
- nabla_w[-l] = np.dot(delta, activations[-l-1].transpose())
- return (nabla_b, nabla_w)
-
- ...
-
- def cost_derivative(self, output_activations, y):
- """Return the vector of partial derivatives \partial C_x /
- \partial a for the output activations."""
- return (output_activations-y)
-
- def sigmoid(z):
- """The sigmoid function."""
- return 1.0/(1.0+np.exp(-z))
-
- def sigmoid_prime(z):
- """Derivative of the sigmoid function."""
- return sigmoid(z)*(1-sigmoid(z))