本次分類問題使用的數據集是MNIST，每個圖像的大小為\(28*28\)。

編寫代碼的步驟如下

1. 載入數據集，分別為訓練集和測試集
2. 讓數據集可以叠代
3. 定義模型，定義損失函數，訓練模型

代碼

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

'''下載訓練集和測試集'''
train_dataset = dsets.MNIST(root='./datasets',
                            train=True, 
                            transform=transforms.ToTensor(),
                            download=True)

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


'''讓數據集可以叠代'''
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)

'''定義模型'''
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

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

model = LogisticRegressionModel(input_dim, output_dim)

'''定義損失計算方式'''
criterion = nn.CrossEntropyLoss()


learning_rate = 0.001

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

'''訓練次數'''
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()
        
        #計算輸出
        outputs = model(images)
        
        #計算損失，內部會自動softmax然後進行Crossentropy
        loss = criterion(outputs, labels)
        
        #反向傳播
        loss.backward()
        
        #更新參數
        optimizer.step()
        
        iter += 1
        
        if iter % 500 == 0:
            #計算準確度
            correct = 0
            total = 0
            for images, labels in test_loader:
                images = Variable(images.view(-1, 28*28))
                
                #獲得輸出，輸出的大小為(batch_size,10)
                outputs = model(images)
                
                #獲得預測值，輸出的大小為(batch_size,1)
                _, predicted = torch.max(outputs.data, 1)
                
                #labels的size是(100,)
                total += labels.size(0)

                #返回的是預測值和標簽值相等的個數
                correct += (predicted == labels).sum()
            
            accuracy = 100 * correct / total
            
            # Print Loss
            print('Iteration: {}. Loss: {}. Accuracy: {}'.format(iter, loss.data[0], accuracy))
 

輸出如下

用Pytorch訓練分類模型