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前言

介绍卷积神经网络的基本概念及具体实例

一、说明

1.如果一个网络由线性形式串联起来，那么就是一个全连接的网络。
2.全连接会丧失图像的一些空间信息，因为是按照一维结构保存。CNN是按照图像原始结构进行保存数据，不会丧失，可以保留原始空间信息。
3.图像卷积后仍是一个三维张量。
4.subsampling（下采样）后通道数不变，但是图像的高度和宽度变，减少数据数量，降低运算需求。
5.卷积运算示意图

6.padding参数：在输入外面再套圈，用0填充。
7.stride参数：做卷积操作时的步长。
8.下采样通常采用最大池化层，通道数量不变，图像宽和高改变。

二、具体实例

1.程序说明

输入尺寸为1*28*28，经过10个1*5*5的卷积操作变为10*24*24；经过2*2的最大池化变为10*12*12；经过20个10*5*5的卷积操作变为20*8*8；经过2*2的最大池化变为20*4*4；变为一维320个向量，再经过全连接层变为10个向量。

2.代码示例

代码如下（示例）：

import torch
from torchvision import transforms
from torchvision import datasets
from torch.utils.data import DataLoader
import torch.nn.functional as F
import torch.optim as optim
import pickle

# prepare dataset


# design model using class


class Net(torch.nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.conv1 = torch.nn.Conv2d(1, 10, kernel_size=5)
        self.conv2 = torch.nn.Conv2d(10, 20, kernel_size=5)
        self.pooling = torch.nn.MaxPool2d(2)
        self.fc = torch.nn.Linear(320, 10)

    def forward(self, x):
        # flatten data from (n,1,28,28) to (n, 784)
        batch_size = x.size(0)
        x = F.relu(self.pooling(self.conv1(x)))
        x = F.relu(self.pooling(self.conv2(x)))
        x = x.view(batch_size, -1)  # -1 此处自动算出的是320
        x = self.fc(x)

        return x





# training cycle forward, backward, update


def train(epoch):
    running_loss = 0.0
    loss_s = 0.0
    for batch_idx, data in enumerate(train_loader, 0):
        inputs, target = data
        optimizer.zero_grad()

        outputs = model(inputs)
        loss = criterion(outputs, target)
        loss.backward()
        optimizer.step()

        running_loss += loss.item()
        loss_s += loss.item()
        if batch_idx % 300 == 299:
            print('[%d, %5d] loss: %.3f' % (epoch + 1, batch_idx + 1, running_loss / 300))
            running_loss = 0.0
    return loss_s / len(train_loader)


def test():
    correct = 0
    total = 0
    with torch.no_grad():
        for data in test_loader:
            images, labels = data
            outputs = model(images)
            _, predicted = torch.max(outputs.data, dim=1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()
    print('accuracy on test set: %d %% ' % (100 * correct / total))
    return 100 * correct / total


if __name__ == '__main__':
    batch_size = 64
    transform = transforms.Compose([transforms.ToTensor(), transforms.Normalize((0.1307,), (0.3081,))])

    train_dataset = datasets.MNIST(root='../dataset/mnist/', train=True, download=True, transform=transform)
    train_loader = DataLoader(train_dataset, shuffle=True, batch_size=batch_size)
    test_dataset = datasets.MNIST(root='../dataset/mnist/', train=False, download=True, transform=transform)
    test_loader = DataLoader(test_dataset, shuffle=False, batch_size=batch_size)

    model = Net()

    # construct loss and optimizer
    criterion = torch.nn.CrossEntropyLoss()
    optimizer = optim.SGD(model.parameters(), lr=0.01, momentum=0.5)

    epoch_list = []
    loss_list = []
    accuracy_list = []
    for epoch in range(10):
        epoch_list.append(epoch)
        loss_lis=train(epoch)
        loss_list.append(loss_lis)
        tes=test()
        accuracy_list.append(tes)
        with open('9/epoch_list.pkl', 'wb') as f:
            pickle.dump(epoch_list, f)
        with open('9/loss_list.pkl', 'wb') as f:
            pickle.dump(loss_list, f)
        with open('9/accuracy_list.pkl', 'wb') as f:
            pickle.dump(accuracy_list, f)

画图程序如下：

import pickle
import matplotlib.pyplot as plt

with open('9/epoch_list.pkl', 'rb') as f:
    loaded_epoch_list = pickle.load(f)
with open('9/loss_list.pkl', 'rb') as f:
    loaded_loss_list = pickle.load(f)
with open('9/accuracy_list.pkl', 'rb') as f:
    loaded_acc_list = pickle.load(f)

plt.subplot(2, 1, 1)  # 创建子图，2行1列，第1个子图
plt.plot(loaded_epoch_list, loaded_loss_list)
plt.xlabel('epoch')
plt.ylabel('loss 1')


plt.subplot(2, 1, 2)  # 创建子图，2行1列，第2个子图
plt.plot(loaded_epoch_list, loaded_acc_list,'r')
plt.xlabel('epoch')
plt.ylabel('acc 1')
plt.show()

得到如下结果：

利用GPU运行的程序如下：

import torch
from torchvision import transforms
from torchvision import datasets
from torch.utils.data import DataLoader
import torch.nn.functional as F
import torch.optim as optim
import pickle
import time
# prepare dataset


# design model using class


class Net(torch.nn.Module):
    def __init__(self):
        super(Net, self).__init__()
        self.conv1 = torch.nn.Conv2d(1, 10, kernel_size=5)
        self.conv2 = torch.nn.Conv2d(10, 20, kernel_size=5)
        self.pooling = torch.nn.MaxPool2d(2)
        self.fc = torch.nn.Linear(320, 10)

    def forward(self, x):
        # flatten data from (n,1,28,28) to (n, 784)
        batch_size = x.size(0)
        x = F.relu(self.pooling(self.conv1(x)))
        x = F.relu(self.pooling(self.conv2(x)))
        x = x.view(batch_size, -1)  # -1 此处自动算出的是320
        x = self.fc(x)

        return x





# training cycle forward, backward, update


def train(epoch):
    running_loss = 0.0
    loss_s = 0.0
    for batch_idx, data in enumerate(train_loader, 0):
        inputs, target = data
        inputs, target = inputs.to(device), target.to(device)
        optimizer.zero_grad()

        outputs = model(inputs)
        loss = criterion(outputs, target)
        loss.backward()
        optimizer.step()

        running_loss += loss.item()
        loss_s += loss.item()
        if batch_idx % 300 == 299:
            print('[%d, %5d] loss: %.3f' % (epoch + 1, batch_idx + 1, running_loss / 300))
            running_loss = 0.0
    return loss_s / len(train_loader)


def test():
    correct = 0
    total = 0
    with torch.no_grad():
        for data in test_loader:
            images, labels = data
            images, labels = images.to(device), labels.to(device)
            outputs = model(images)
            _, predicted = torch.max(outputs.data, dim=1)
            total += labels.size(0)
            correct += (predicted == labels).sum().item()
    print('accuracy on test set: %d %% ' % (100 * correct / total))
    return 100 * correct / total


if __name__ == '__main__':
    start_time = time.time()
    batch_size = 64
    transform = transforms.Compose([transforms.ToTensor(), transforms.Normalize((0.1307,), (0.3081,))])

    train_dataset = datasets.MNIST(root='../dataset/mnist/', train=True, download=True, transform=transform)
    train_loader = DataLoader(train_dataset, shuffle=True, batch_size=batch_size)
    test_dataset = datasets.MNIST(root='../dataset/mnist/', train=False, download=True, transform=transform)
    test_loader = DataLoader(test_dataset, shuffle=False, batch_size=batch_size)

    model = Net()
    device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
    model.to(device)

    # construct loss and optimizer
    criterion = torch.nn.CrossEntropyLoss()
    optimizer = optim.SGD(model.parameters(), lr=0.01, momentum=0.5)

    epoch_list = []
    loss_list = []
    accuracy_list = []
    for epoch in range(10):
        epoch_list.append(epoch)
        loss_lis=train(epoch)
        loss_list.append(loss_lis)
        tes=test()
        accuracy_list.append(tes)
        with open('9/epoch_list.pkl', 'wb') as f:
            pickle.dump(epoch_list, f)
        with open('9/loss_list.pkl', 'wb') as f:
            pickle.dump(loss_list, f)
        with open('9/accuracy_list.pkl', 'wb') as f:
            pickle.dump(accuracy_list, f)
    end_time = time.time()

    print('training time: %.2f s' % (end_time - start_time))

得到如下结果：

总结

PyTorch学习9：卷积神经网络