如何使用tensorflow快速搭建起一个深度学习项目

在上一讲中，我们学习了如何利用numpy手动搭建卷积神经网络。但在实际的图像识别中，使用numpy去手写 CNN 未免有些吃力不讨好。在 DNN 的学习中，我们也是在手动搭建之后利用Tensorflow去重新实现一遍，一来为了能够对神经网络的传播机制能够理解更加透彻，二来也是为了更加高效使用开源框架快速搭建起深度学习项目。本节就继续和大家一起学习如何利用Tensorflow搭建一个卷积神经网络。

我们继续以 NG 课题组提供的 sign 手势数据集为例，学习如何通过Tensorflow快速搭建起一个深度学习项目。数据集标签共有零到五总共 6 类标签，示例如下：

先对数据进行简单的预处理并查看训练集和测试集维度：

X_train = X_train_orig/255.X_test = X_test_orig/255.Y_train = convert_to_one_hot(Y_train_orig, 6).T Y_test = convert_to_one_hot(Y_test_orig, 6).Tprint ("number of training examples = " + str(X_train.shape[0]))print ("number of test examples = " + str(X_test.shape[0]))print ("X_train shape: " + str(X_train.shape))print ("Y_train shape: " + str(Y_train.shape))print ("X_test shape: " + str(X_test.shape))print ("Y_test shape: " + str(Y_test.shape))

可见我们总共有 1080 张 64643 训练集图像，120 张 64643 的测试集图像，共有 6 类标签。下面我们开始搭建过程。

创建placeholder

首先需要为训练集预测变量和目标变量创建占位符变量placeholder，定义创建占位符变量函数：

def create_placeholders(n_H0, n_W0, n_C0, n_y):        """    Creates the placeholders for the tensorflow session.    Arguments:    n_H0 -- scalar, height of an input image    n_W0 -- scalar, width of an input image    n_C0 -- scalar, number of channels of the input    n_y -- scalar, number of classes    Returns:    X -- placeholder for the data input, of shape [None, n_H0, n_W0, n_C0] and dtype "float"    Y -- placeholder for the input labels, of shape [None, n_y] and dtype "float"    """    X = tf.placeholder(tf.float32, shape=(None, n_H0, n_W0, n_C0), name='X')    Y = tf.placeholder(tf.float32, shape=(None, n_y), name='Y')        return X, Y

参数初始化

然后需要对滤波器权值参数进行初始化：

def initialize_parameters():        """    Initializes weight parameters to build a neural network with tensorflow.    Returns:    parameters -- a dictionary of tensors containing W1, W2    """    tf.set_random_seed(1)                                W1 = tf.get_variable("W1", [4,4,3,8], initializer = tf.contrib.layers.xavier_initializer(seed = 0))    W2 = tf.get_variable("W2", [2,2,8,16], initializer = tf.contrib.layers.xavier_initializer(seed = 0))    parameters = {"W1": W1,                                    "W2": W2}        return parameters

执行卷积网络的前向传播过程

前向传播过程如下所示：CONV2D -> RELU -> MAXPOOL -> CONV2D -> RELU -> MAXPOOL -> FLATTEN -> FULLYCONNECTED

可见我们要搭建的是一个典型的 CNN 过程，经过两次的卷积-relu激活-最大池化，然后展开接上一个全连接层。利用Tensorflow 搭建上述传播过程如下：

def forward_propagation(X, parameters):        """    Implements the forward propagation for the model    Arguments:    X -- input dataset placeholder, of shape (input size, number of examples)    parameters -- python dictionary containing your parameters "W1", "W2"                  the shapes are given in initialize_parameters    Returns:    Z3 -- the output of the last LINEAR unit    """    # Retrieve the parameters from the dictionary "parameters"    W1 = parameters['W1']    W2 = parameters['W2']        # CONV2D: stride of 1, padding 'SAME'    Z1 = tf.nn.conv2d(X,W1, strides = [1,1,1,1], padding = 'SAME')        # RELU    A1 = tf.nn.relu(Z1)        # MAXPOOL: window 8x8, sride 8, padding 'SAME'    P1 = tf.nn.max_pool(A1, ksize = [1,8,8,1], strides = [1,8,8,1], padding = 'SAME')        # CONV2D: filters W2, stride 1, padding 'SAME'    Z2 = tf.nn.conv2d(P1,W2, strides = [1,1,1,1], padding = 'SAME')        # RELU    A2 = tf.nn.relu(Z2)      # MAXPOOL: window 4x4, stride 4, padding 'SAME'    P2 = tf.nn.max_pool(A2, ksize = [1,4,4,1], strides = [1,4,4,1], padding = 'SAME')        # FLATTEN    P2 = tf.contrib.layers.flatten(P2)    Z3 = tf.contrib.layers.fully_connected(P2, 6, activation_fn = None)        return Z3

计算当前损失

在Tensorflow 中计算损失函数非常简单，一行代码即可：

def compute_cost(Z3, Y):        """    Computes the cost    Arguments:    Z3 -- output of forward propagation (output of the last LINEAR unit), of shape (6, number of examples)    Y -- "true" labels vector placeholder, same shape as Z3    Returns:    cost - Tensor of the cost function    """    cost = tf.reduce_mean(tf.nn.softmax_cross_entropy_with_logits(logits=Z3, labels=Y))        return cost

定义好上述过程之后，就可以封装整体的训练过程模型。可能你会问为什么没有反向传播，这里需要注意的是Tensorflow帮助我们自动封装好了反向传播过程，无需我们再次定义，在实际搭建过程中我们只需将前向传播的网络结构定义清楚即可。

封装模型

def model(X_train, Y_train, X_test, Y_test, learning_rate = 0.009,          num_epochs = 100, minibatch_size = 64, print_cost = True):        """    Implements a three-layer ConvNet in Tensorflow:    CONV2D -> RELU -> MAXPOOL -> CONV2D -> RELU -> MAXPOOL -> FLATTEN -> FULLYCONNECTED    Arguments:    X_train -- training set, of shape (None, 64, 64, 3)    Y_train -- test set, of shape (None, n_y = 6)    X_test -- training set, of shape (None, 64, 64, 3)    Y_test -- test set, of shape (None, n_y = 6)    learning_rate -- learning rate of the optimization    num_epochs -- number of epochs of the optimization loop    minibatch_size -- size of a minibatch    print_cost -- True to print the cost every 100 epochs    Returns:    train_accuracy -- real number, accuracy on the train set (X_train)    test_accuracy -- real number, testing accuracy on the test set (X_test)    parameters -- parameters learnt by the model. They can then be used to predict.    """    ops.reset_default_graph()                            tf.set_random_seed(1)                                seed = 3                                            (m, n_H0, n_W0, n_C0) = X_train.shape                n_y = Y_train.shape[1]                                costs = []                                          # Create Placeholders of the correct shape    X, Y = create_placeholders(n_H0, n_W0, n_C0, n_y)      # Initialize parameters    parameters = initialize_parameters()        # Forward propagation    Z3 = forward_propagation(X, parameters)        # Cost function    cost = compute_cost(Z3, Y)        # Backpropagation    optimizer = tf.train.AdamOptimizer(learning_rate = learning_rate).minimize(cost)    # Initialize all the variables globally    init = tf.global_variables_initializer()        # Start the session to compute the tensorflow graph    with tf.Session() as sess:                # Run the initialization        sess.run(init)                # Do the training loop        for epoch in range(num_epochs):            minibatch_cost = 0.            num_minibatches = int(m / minibatch_size)            seed = seed + 1            minibatches = random_mini_batches(X_train, Y_train, minibatch_size, seed)                        for minibatch in minibatches:                                # Select a minibatch                (minibatch_X, minibatch_Y) = minibatch                _ , temp_cost = sess.run([optimizer, cost], feed_dict={X: minibatch_X, Y: minibatch_Y})                minibatch_cost += temp_cost / num_minibatches                            # Print the cost every epoch            if print_cost == True and epoch % 5 == 0:                              print ("Cost after epoch %i: %f" % (epoch, minibatch_cost))                        if print_cost == True and epoch % 1 == 0:                costs.append(minibatch_cost)                # plot the cost        plt.plot(np.squeeze(costs))        plt.ylabel('cost')        plt.xlabel('iterations (per tens)')        plt.title("Learning rate =" + str(learning_rate))        plt.show()        # Calculate the correct predictions        predict_op = tf.argmax(Z3, 1)        correct_prediction = tf.equal(predict_op, tf.argmax(Y, 1))                # Calculate accuracy on the test set        accuracy = tf.reduce_mean(tf.cast(correct_prediction, "float"))        print(accuracy)        train_accuracy = accuracy.eval({X: X_train, Y: Y_train})        test_accuracy = accuracy.eval({X: X_test, Y: Y_test})        print("Train Accuracy:", train_accuracy)        print("Test Accuracy:", test_accuracy)                       return train_accuracy, test_accuracy, parameters

对训练集执行模型训练：

_, _, parameters = model(X_train, Y_train, X_test, Y_test)

训练迭代过程如下：

我们在训练集上取得了 0.67 的准确率，在测试集上的预测准确率为 0.58 ，虽然效果并不显著，模型也有待深度调优，但我们已经学会了如何用Tensorflow 快速搭建起一个深度学习系统了。

注：本深度学习笔记系作者学习 Andrew NG 的 deeplearningai 五门课程所记笔记，其中代码为每门课的课后assignments作业整理而成。