PyTorch LSTM的一个简单例子：实现MNIST图片分类

在上一篇博客中，我们实现了用LSTM对单词进行词性判断，本篇博客我们将实现用LSTM对MNIST图片分类。MNIST图片的大小为28*28，我们将其看成长度为28的序列，序列中的每个数据的维度是28，这样我们就可以把它变成一个序列数据了。代码如下，代码中的模型搭建参考了文末的参考资料[1]，其余部分参考了文末的参考资料[2]。

'''
本程序实现用LSTM对MNIST进行图片分类
'''

import torch
import numpy as np
import torch.nn as nn
import torch.utils.data as Data
import torchvision
import matplotlib.pyplot as plt


# Hyper parameter
EPOCH = 1
LR = 0.001    # learning rate
BATCH_SIZE = 50

# Mnist digit dataset
train_data = torchvision.datasets.MNIST(
    root='/Users/wangpeng/Desktop/all/CS/Courses/Deep Learning/mofan_PyTorch/mnist/',    # mnist has been downloaded before, use it directly
    train=True,    # this is training data
    transform=torchvision.transforms.ToTensor(),    # Converts a PIL.Image or numpy.ndarray to
                                                    # torch.FloatTensor of shape (C x H x W) and normalize in the range [0.0, 1.0]
    download=False,
)

# print(train_data.data.size())       # (60000, 28, 28)
# print(train_data.targets.size())    # (60000)
# plot one image
# plt.imshow(train_data.data[0].numpy(), cmap='gray')
# plt.title('{:d}'.format(train_data.targets[0]))
# plt.show()

# Data Loader for easy mini-batch return in training, the image batch shape will be (50, 1, 28, 28)
train_loader = Data.DataLoader(dataset=train_data, batch_size=BATCH_SIZE, shuffle=True)

test_data = torchvision.datasets.MNIST(
    root='/Users/wangpeng/Desktop/all/CS/Courses/Deep Learning/mofan_PyTorch/mnist/',
    train=False,  # this is training data
)
# print(test_data.data.size())       # (10000, 28, 28)
# print(test_data.targets.size())    # (10000)
# pick 2000 samples to speed up testing
test_x = test_data.data.type(torch.FloatTensor)[:2000]/255    # shape (2000, 28, 28), value in range(0,1)
test_y = test_data.targets[:2000]


class LSTMnet(nn.Module):
    def __init__(self, in_dim, hidden_dim, n_layer, n_class):
        super(LSTMnet, self).__init__()
        self.n_layer = n_layer
        self.hidden_dim = hidden_dim
        self.lstm = nn.LSTM(in_dim, hidden_dim, n_layer, batch_first=True)
        self.linear = nn.Linear(hidden_dim, n_class)

    def forward(self, x):                  # x's shape (batch_size, 序列长度, 序列中每个数据的长度)
        out, _ = self.lstm(x)              # out's shape (batch_size, 序列长度, hidden_dim)
        out = out[:, -1, :]                # 中间的序列长度取-1，表示取序列中的最后一个数据，这个数据长度为hidden_dim，
                                           # 得到的out的shape为(batch_size, hidden_dim)
        out = self.linear(out)             # 经过线性层后，out的shape为(batch_size, n_class)
        return out


model = LSTMnet(28, 64, 2, 10)             # 图片大小28*28，lstm的每个隐藏层64个节点，2层隐藏层
if torch.cuda.is_available():
    model = model.cuda()

optimizer = torch.optim.Adam(model.parameters(), lr=LR)
criterion = nn.CrossEntropyLoss()

# training and testing
for epoch in range(EPOCH):
    for iteration, (train_x, train_y) in enumerate(train_loader):    # train_x's shape (BATCH_SIZE,1,28,28)
        train_x = train_x.squeeze()        # after squeeze, train_x's shape (BATCH_SIZE,28,28),
                                           # 第一个28是序列长度，第二个28是序列中每个数据的长度。
        output = model(train_x)
        loss = criterion(output, train_y)  # cross entropy loss
        optimizer.zero_grad()              # clear gradients for this training step
        loss.backward()                    # backpropagation, compute gradients
        optimizer.step()                   # apply gradients

        if iteration % 100 == 0:
            test_output = model(test_x)
            predict_y = torch.max(test_output, 1)[1].numpy()
            accuracy = float((predict_y == test_y.numpy()).astype(int).sum()) / float(test_y.size(0))
            print('epoch:{:<2d} | iteration:{:<4d} | loss:{:<6.4f} | accuracy:{:<4.2f}'.format(epoch, iteration, loss, accuracy))


# print 10 predictions from test data
test_out = model(test_x[:10])
pred_y = torch.max(test_out, dim=1)[1].data.numpy()
print('The predict number is:')
print(pred_y)
print('The real number is:')
print(test_y[:10].numpy())

结果如下：

下图为本文的神经网络处理单张图片的过程：

参考资料：

[1] 10分钟快速入门PyTorch (6)

[2] 莫烦PyTorch教程系列：CNN卷积神经网络