Question

我正在为虹膜数据集实现一个简单的神经网络分类器。 NN具有3个输入节点，1个具有两个节点的隐藏层和3个输出节点。我已经实现了evrything但是偏导数的值没有正确计算。我已经筋疲力尽地寻找解决方案，但无法解决问题。这是我计算偏导数的代码。

def derivative_cost_function(self,X,Y,thetas):
    '''
        Computes the derivates of Cost function w.r.t input parameters (thetas)  
        for given input and labels.

        Input:
        ------
            X: can be either a single d X n-dimensional vector or d X n dimensional matrix of inputs
            theata: must  dk X 1-dimensional vector for representing vectors of k classes
            Y: Must be k X n-dimensional label vector
        Returns:
        ------
            partial_thetas: a dk X 1-dimensional vector of partial derivatives of cost function w.r.t parameters..
    '''

    #forward pass
    a2, a3=self.forward_pass(X,thetas)

    #now back-propogate 

    # unroll thetas
    l1theta, l2theta = self.unroll_thetas(thetas)


    nexamples=float(X.shape[1])

    # compute delta3, l2theta
    a3 = np.array(a3)
    a2 = np.array(a2)
    Y = np.array(Y)

    a3 = a3.T
    delta3 = (a3 * (1 - a3)) * (((a3 - Y)/((a3)*(1-a3)))) 
    l2Derivatives = np.dot(delta3, a2)
    #print "Layer 2 derivatives shape = ", l2Derivatives.shape
    #print "Layer 2 derivatives = ", l2Derivatives



    # compute delta2, l1 theta
    a2 = a2.T
    dotProduct = np.dot(l2theta.T,delta3)
    delta2 = dotProduct * (a2) * (1- a2)


    l1Derivatives = np.dot(delta2[1:], X.T)
    #print "Layer 1 derivatives shape = ", l1Derivatives.shape
    #print "Layer 1 derivatives = ", l1Derivatives


    #remember to exclude last element of delta2, representing the deltas of bias terms...
    # i.e. delta2=delta2[:-1]



    # roll thetas into a big vector
    thetas=(self.roll_thetas(l1Derivatives,l2Derivatives)).reshape(thetas.shape) # return the same shape as you received

    return thetas

Answer 1

为什么不在https://github.com/zizhaozhang/simple_neutral_network/blob/master/nn.py

中查看我的实施情况

衍生品实际上就在这里：

def dCostFunction(self, theta, in_dim, hidden_dim, num_labels, X, y):
        #compute gradient
        t1, t2 = self.uncat(theta, in_dim, hidden_dim)


        a1, z2, a2, z3, a3 = self._forward(X, t1, t2) # p x s matrix

        # t1 = t1[1:, :] # remove bias term
        # t2 = t2[1:, :]
        sigma3 = -(y - a3) * self.dactivation(z3) # do not apply dsigmode here? should I
        sigma2 = np.dot(t2, sigma3)
        term = np.ones((1,num_labels))
        sigma2 = sigma2 * np.concatenate((term, self.dactivation(z2)),axis=0)

        theta2_grad = np.dot(sigma3, a2.T)
        theta1_grad = np.dot(sigma2[1:,:], a1.T)

        theta1_grad = theta1_grad / num_labels
        theta2_grad = theta2_grad / num_labels

        return self.cat(theta1_grad.T, theta2_grad.T)

希望有所帮助

神经网络python中偏导数的错误值

1 个答案: