机器学习算法(5):卷积神经网络原理及其keras实现

时间：2021-12-15 16:14:38

1.原理

CNN的资料特别多，这里不再赘述，仅收集相关的资料供大家参考：

a.Deep learning：五十一(CNN的反向求导及练习)

b.Deep Learning

2.实现

我们使用keras实现CNN，Keras的使用文档请参考

a.Keras中文文档

b.Keras英文文档。

参考keras官方的例子，我们使用keras，对数据集mnist训练一个cnn模型，实现的代码如下：

'''Trains a simple convnet on the MNIST dataset.

Gets to 99.25% test accuracy after 12 epochs

(there is still a lot of margin for parameter tuning).

16 seconds per epoch on a GRID K520 GPU.

'''

from __future__ import print_function

import keras

from keras.datasets import mnist

from keras.models import Sequential

from keras.layers import Dense, Dropout, Flatten

from keras.layers import Conv2D, MaxPooling2D

from keras import backend as K

batch_size = 128

num_classes = 10

epochs = 12

# input image dimensions

img_rows, img_cols = 28, 28

# the data, shuffled and split between train and test sets

(x_train, y_train), (x_test, y_test) = mnist.load_data()

if K.image_data_format() == 'channels_first':

    x_train = x_train.reshape(x_train.shape[0], 1, img_rows, img_cols)

    x_test = x_test.reshape(x_test.shape[0], 1, img_rows, img_cols)

    input_shape = (1, img_rows, img_cols)

else:

    x_train = x_train.reshape(x_train.shape[0], img_rows, img_cols, 1)

    x_test = x_test.reshape(x_test.shape[0], img_rows, img_cols, 1)

    input_shape = (img_rows, img_cols, 1)

x_train = x_train.astype('float32')

x_test = x_test.astype('float32')

x_train /= 255

x_test /= 255

print('x_train shape:', x_train.shape)

print(x_train.shape[0], 'train samples')

print(x_test.shape[0], 'test samples')

# convert class vectors to binary class matrices

y_train = keras.utils.to_categorical(y_train, num_classes)

y_test = keras.utils.to_categorical(y_test, num_classes)

model = Sequential()

model.add(Conv2D(32, kernel_size=(3, 3),

                 activation='relu',

                 input_shape=input_shape))

model.add(Conv2D(64, (3, 3), activation='relu'))

model.add(MaxPooling2D(pool_size=(2, 2)))

model.add(Dropout(0.25))

model.add(Flatten())

model.add(Dense(128, activation='relu'))

model.add(Dropout(0.5))

model.add(Dense(num_classes, activation='softmax'))

model.compile(loss=keras.losses.categorical_crossentropy,

              optimizer=keras.optimizers.Adadelta(),

              metrics=['accuracy'])

model.fit(x_train, y_train,

          batch_size=batch_size,

          epochs=epochs,

          verbose=1,

          validation_data=(x_test, y_test))

score = model.evaluate(x_test, y_test, verbose=0)

print('Test loss:', score[0])

print('Test accuracy:', score[1])

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