手写BP神经网络识别MNIST数据集（pytorch读入数据）

我就是我 2022-11-27 15:51 191阅读 0赞

BP神经网络原理可以参考：[反向传播算法(Backpropagation)----Gradient Descent的推导过程][Backpropagation_----Gradient Descent]  
主要步骤：

1.  input layer有784（28X28）个节点；3层hidden layer，每一层20个节点；output layer有10个节点，分别表示输出为0-9的概率
2.  初始化所有w为(-1, 1)的随机值
3.  前向传播，计算每一层输入输出的关系
4.  反向传播计算损失函数的导数
5.  更新权值

代码：

""" @Time ： 2020/8/15 10:19 @Author ：KI @File ：bp_mnist.py @Motto：Hungry And Humble """
    import numpy as np
    import torch
    import torchvision
    import torchvision.transforms as transforms
    from torch.utils.data import DataLoader
    import cv2
    
    #input layer：784 nodes; hidden layer：three hidden layers with 20 nodes in each layer
    #output layer：10 nodes
    class BP:
        def __init__(self):
            self.input = np.zeros((100, 784))   #100 samples per round
            self.hidden_layer_1 = np.zeros((100, 20))
            self.hidden_layer_2 = np.zeros((100, 20))
            self.hidden_layer_3 = np.zeros((100, 20))
            self.output_layer = np.zeros((100, 10))
            self.w1 = 2 * np.random.random((784, 20)) - 1   #limit to (-1, 1)
            self.w2 = 2 * np.random.random((20, 20)) - 1
            self.w3 = 2 * np.random.random((20, 20)) - 1
            self.w4 = 2 * np.random.random((20, 10)) - 1
            self.error = np.zeros(10)
            self.learning_rate = 0.1
    
        def sigmoid(self, x):
            return 1 / (1 + np.exp(-x))
    
        def sigmoid_deri(self, x):
            return x * (1 - x)
    
        def forward_prop(self, data, label):   #label:100 X 10,data: 100 X 784
            self.input = data
            self.hidden_layer_1 = self.sigmoid(np.dot(self.input, self.w1))
            self.hidden_layer_2 = self.sigmoid(np.dot(self.hidden_layer_1, self.w2))
            self.hidden_layer_3 = self.sigmoid(np.dot(self.hidden_layer_2, self.w3))
            self.output_layer = self.sigmoid(np.dot(self.hidden_layer_3, self.w4))
            #error
            self.error = label - self.output_layer
            return self.output_layer
    
        def backward_prop(self):
            output_diff = self.error * self.sigmoid_deri(self.output_layer)
            hidden_diff_3 = np.dot(output_diff, self.w4.T) * self.sigmoid_deri(self.hidden_layer_3)
            hidden_diff_2 = np.dot(hidden_diff_3, self.w3.T) * self.sigmoid_deri(self.hidden_layer_2)
            hidden_diff_1 = np.dot(hidden_diff_2, self.w2.T) * self.sigmoid_deri(self.hidden_layer_1)
            #update
            self.w4 += self.learning_rate * np.dot(self.hidden_layer_3.T, output_diff)
            self.w3 += self.learning_rate * np.dot(self.hidden_layer_2.T, hidden_diff_3)
            self.w2 += self.learning_rate * np.dot(self.hidden_layer_1.T, hidden_diff_2)
            self.w1 += self.learning_rate * np.dot(self.input.T, hidden_diff_1)
    
    #from torchvision load data
    def load_data():
        datasets_train = torchvision.datasets.MNIST(root='./data/', train=True, transform=transforms.ToTensor())
        datasets_test = torchvision.datasets.MNIST(root='./data/', train=False, transform=transforms.ToTensor())
        data_train = datasets_train.data
        X_train = data_train.numpy()
        X_test = datasets_test.data.numpy()
        X_train = np.reshape(X_train, (60000, 784))
        X_test = np.reshape(X_test, (10000, 784))
        Y_train = datasets_train.targets.numpy()
        Y_test = datasets_test.targets.numpy()
    
        real_train_y = np.zeros((60000, 10))
        real_test_y = np.zeros((10000, 10))
        #each y has ten dimensions
        for i in range(60000):
            real_train_y[i, Y_train[i]] = 1
        for i in range(10000):
            real_test_y[i, Y_test[i]] = 1
        index = np.arange(60000)
        np.random.shuffle(index)
        #shuffle train_data
        X_train = X_train[index]
        real_train_y = real_train_y[index]
    
        X_train = np.int64(X_train > 0)
        X_test = np.int64(X_test > 0)
    
    
        return X_train, real_train_y, X_test, real_test_y
    
    
    
    def bp_network():
        nn = BP()
        X_train, Y_train, X_test, Y_test = load_data()
        batch_size = 100
        epochs = 6000
        for epoch in range(epochs):
            start = (epoch % 600) * batch_size
            end = start + batch_size
            print(start, end)
            nn.forward_prop(X_train[start: end], Y_train[start: end])
            nn.backward_prop()
    
        return nn
    
    
    def bp_test():
        nn = bp_network()
        sum = 0
        X_train, Y_train, X_test, Y_test = load_data()
        #test：
        for i in range(len(X_test)):
            res = nn.forward_prop(X_test[i], Y_test[i])
            res = res.tolist()
            index = res.index(max(res))
            if Y_test[i, index] == 1:
                sum += 1
    
        print('accuracy：', sum / len(Y_test))
    
    
    if __name__ == '__main__':
        bp_test()

结果：  
![在这里插入图片描述][20200816112603329.png_pic_center]  
注意第一次运行时，导入数据的第一条语句：

datasets_train = torchvision.datasets.MNIST(root='./data/', train=True, transform=transforms.ToTensor())

应该在后面加上

download = True

[Backpropagation_----Gradient Descent]: https://blog.csdn.net/Cyril_KI/article/details/107030538
[20200816112603329.png_pic_center]: /images/20221124/05986a2697604292b604f85f28fbf536.png