用Pytorch训练MNIST分类模型

本次分类问题使用的数据集是MNIST，每个图像的大小为\(28*28\)。

编写代码的步骤如下

1. 载入数据集，分别为训练集和测试集
2. 让数据集可以迭代
3. 定义模型，定义损失函数，训练模型

代码

import torch
import torch.nn as nn
import torchvision.transforms as transforms
import torchvision.datasets as dsets
from torch.autograd import Variable

'''下载训练集和测试集'''
train_dataset = dsets.MNIST(root='./datasets',
                            train=True,
                            transform=transforms.ToTensor(),
                            download=True)

test_dataset = dsets.MNIST(root='./datasets',
                           train=False,
                           transform=transforms.ToTensor())

'''让数据集可以迭代'''
batch_size = 100
n_iters = 3000
num_epochs = n_iters / (len(train_dataset) / batch_size)
num_epochs = int(num_epochs)

train_loader = torch.utils.data.DataLoader(dataset=train_dataset,
                                           batch_size=batch_size,
                                           shuffle=True)

test_loader = torch.utils.data.DataLoader(dataset=test_dataset,
                                          batch_size=batch_size,
                                          shuffle=False)

'''定义模型'''
class LogisticRegressionModel(nn.Module):
    def __init__(self, input_dim, output_dim):
        super(LogisticRegressionModel, self).__init__()
        self.linear = nn.Linear(input_dim, output_dim)

    def forward(self, x):
        out = self.linear(x)
        return out

'''实例化模型'''
input_dim = 28*28
output_dim = 10

model = LogisticRegressionModel(input_dim, output_dim)

'''定义损失计算方式'''
criterion = nn.CrossEntropyLoss()

learning_rate = 0.001

optimizer = torch.optim.SGD(model.parameters(), lr=learning_rate)

'''训练次数'''
iter = 0
for epoch in range(num_epochs):
    for i, (images, labels) in enumerate(train_loader):

        images = Variable(images.view(-1, 28*28))
        labels = Variable(labels)

        #梯度置零
        optimizer.zero_grad()

        #计算输出
        outputs = model(images)

        #计算损失，内部会自动softmax然后进行Crossentropy
        loss = criterion(outputs, labels)

        #反向传播
        loss.backward()

        #更新参数
        optimizer.step()

        iter += 1

        if iter % 500 == 0:
            #计算准确度
            correct = 0
            total = 0
            for images, labels in test_loader:
                images = Variable(images.view(-1, 28*28))

                #获得输出，输出的大小为(batch_size,10)
                outputs = model(images)

                #获得预测值，输出的大小为(batch_size,1)
                _, predicted = torch.max(outputs.data, 1)

                #labels的size是(100,)
                total += labels.size(0)

                #返回的是预测值和标签值相等的个数
                correct += (predicted == labels).sum()

            accuracy = 100 * correct / total

            # Print Loss
            print('Iteration: {}. Loss: {}. Accuracy: {}'.format(iter, loss.data[0], accuracy))

输出如下