3：用tensorflow搭个神经网络出来

        为什么用tensorflow呢，应为谷歌是亲爹啊，虽然有些人说caffe更适合图像啊mxnet效率更高等等，但爸爸就是爸爸，Android都能那么火，一个道理嘛。其实这些个框架一通百通，就是语法不一样了些。从tensorflow开始吧。

        关于tf的安装详见另一篇博文，此处tensorflow的学习基本来自Udacity中google的深度学习课程。

1:tensorflow的计算图

在tensorflow中编写代码可以分成两个部分，首先是要定义一个计算的流程，或者叫计算图，然后再建立一个任务，让tensorflow调用系统资源去运算这个东西，举个栗子：

import tensorflow as tf #导入tensorflow库
matrix1=tf.constant([[3.,3.]])#创建常量节点
matrix2=tf.constant([2.],[2.])
product=tf.matmul(matrix1,matrix2)#创建矩阵乘法节点

上边并没有运算具体的值，而只是一个运算图。

真正的计算要用到session：

sess=tf.Session()#启动默认图
#运行这里会有一堆运行信息出来
result = sess.run(product)#调用sess的run方法来执行矩阵乘法节点的操作，product代表了矩阵乘法这个节点的输出
print result
sess.close()#完成任务后关闭会话

这就是tf的基础运行方式，对于变量，使用Variable方法定义：

W1=tf.Variable(tf.zeros((2,2)), name=”weights”)
sess.run(tf.initialize_all_variables())#变量需要预先初始化
print sess.run(W1)

另一个栗子：

state = tf.Variable(0,name=”counter”)
new_value=tf.add(state, tf.constant(1))#对state加1
update=tf.assign(state,new_value)#将自增后的值重新赋值给state
with tf.Session() as sess: #使用with可以省去close()操作，还可以处理一些操作出现的异常(也可以用try)
    sess.run(tf.initialize_all_variables())
    print(sess.run(state))#输出计数器值
    for _ in range(3):
        sess.run(update)
        print(sess.run(state))

为毛谷歌爸爸这么蛋疼呢，直接算不好吗？其实一点也不蛋疼，这么设计，同样一套计算图，就可以扔给不同的设备或者分布式的设备去运算了：

with tf.Session() as sess:
    with tf.device(“/gpu:1”):
    …

另一个好处就是python的运算效率较低，所以设计成使用python编写运算图，之后再使用python之外的运算器(比如底层的C++)去计算。

2:使用tensorflow搭建一个卷积神经网络

这里会详解Google发布在udacity中使用CNN分类not_MINIST数据代码，这些代码包含在了tensorflow源代码中的examples中

https://github.com/tensorflow/tensorflow/tree/master/tensorflow/examples/udacity

<1>:准备数据(notMINIST)

代码的第一部分是载入数据：

# These are all the modules we'll be using later. Make sure you can import them
# before proceeding further.
from __future__ import print_function
import numpy as np
import matplotlib.pyplot as plt
import os
import sys
import tarfile
from IPython.display import display, Image
from scipy import ndimage
from sklearn.linear_model import LogisticRegression
from six.moves.urllib.request import urlretrieve
import tensorflow as tf
from six.moves import cPickle as pickle
from six.moves import range
# Config the matplotlib backend as plotting inline in IPython
%matplotlib inline

url = 'http://commondatastorage.googleapis.com/books1000/'
last_percent_reported = None

def download_progress_hook(count, blockSize, totalSize):
  """A hook to report the progress of a download. This is mostly intended for users with
  slow internet connections. Reports every 5% change in download progress.
  """
  global last_percent_reported
  percent = int(count * blockSize * 100 / totalSize)

  if last_percent_reported != percent:
    if percent % 5 == 0:
      sys.stdout.write("%s%%" % percent)
      sys.stdout.flush()
    else:
      sys.stdout.write(".")
      sys.stdout.flush()
      
    last_percent_reported = percent
        
def maybe_download(filename, expected_bytes, force=False):
  """Download a file if not present, and make sure it's the right size."""
  if force or not os.path.exists(filename):
    print('Attempting to download:', filename) 
    filename, _ = urlretrieve(url + filename, filename, reporthook=download_progress_hook)
    print('\nDownload Complete!')
  statinfo = os.stat(filename)
  if statinfo.st_size == expected_bytes:
    print('Found and verified', filename)
  else:
    raise Exception(
      'Failed to verify ' + filename + '. Can you get to it with a browser?')
  return filename

train_filename = maybe_download('notMNIST_large.tar.gz', 247336696)
test_filename = maybe_download('notMNIST_small.tar.gz', 8458043)

上边的代码是下载所需要的数据集压缩包，下一步是解压

num_classes = 10
np.random.seed(133)

def maybe_extract(filename, force=False):
  root = os.path.splitext(os.path.splitext(filename)[0])[0]  # remove .tar.gz
  if os.path.isdir(root) and not force:
    # You may override by setting force=True.
    print('%s already present - Skipping extraction of %s.' % (root, filename))
  else:
    print('Extracting data for %s. This may take a while. Please wait.' % root)
    tar = tarfile.open(filename)
    sys.stdout.flush()
    tar.extractall()
    tar.close()
  data_folders = [
    os.path.join(root, d) for d in sorted(os.listdir(root))
    if os.path.isdir(os.path.join(root, d))]
  if len(data_folders) != num_classes:
    raise Exception(
      'Expected %d folders, one per class. Found %d instead.' % (
        num_classes, len(data_folders)))
  print(data_folders)
  return data_folders
  
train_folders = maybe_extract(train_filename)
test_folders = maybe_extract(test_filename)

解压后可以查看一下代码文件所在的文件夹中会有两个文件夹not_MNIST_large和not_MNIST_small，large用来训练，small用来验证，每个文件夹中都有10个文件夹，分别保存了A到J的图像(28*28)，这些图像就是数据集，标签就是A到J，当然之前下载的压缩文件也在。下一步是将这些数据转换成python中更容易处理的pickle格式，为了确保内存装得下，我们把每一个类别分别转换成一个独立的pickle文件，同时也对数据进行去均值和归一化，在这个过程中可能会有一些文件是不可读的，跳过即可，无所谓：

image_size = 28  # Pixel width and height.
pixel_depth = 255.0  # Number of levels per pixel.

def load_letter(folder, min_num_images):
  """Load the data for a single letter label."""
  image_files = os.listdir(folder)
  dataset = np.ndarray(shape=(len(image_files), image_size, image_size),
                         dtype=np.float32)
  print(folder)
  num_images = 0
  for image in image_files:
    image_file = os.path.join(folder, image)#文件路径拼接
    try:
      image_data = (ndimage.imread(image_file).astype(float) - 
                    pixel_depth / 2) / pixel_depth #去均值和归一化
      if image_data.shape != (image_size, image_size):
        raise Exception('Unexpected image shape: %s' % str(image_data.shape))
      dataset[num_images, :, :] = image_data
      num_images = num_images + 1
    except IOError as e:
      print('Could not read:', image_file, ':', e, '- it\'s ok, skipping.')
    
  dataset = dataset[0:num_images, :, :]
  if num_images < min_num_images:
    raise Exception('Many fewer images than expected: %d < %d' %
                    (num_images, min_num_images))
    
  print('Full dataset tensor:', dataset.shape)
  print('Mean:', np.mean(dataset))
  print('Standard deviation:', np.std(dataset))
  return dataset
        
def maybe_pickle(data_folders, min_num_images_per_class, force=False):
  dataset_names = []
  for folder in data_folders:#本例中就是not_MNIST_large/A, not_MNIST_large/B等等
    set_filename = folder + '.pickle'#folders是A到J，设定文件名
    dataset_names.append(set_filename)#往dataset_names后边添加set_filename
    if os.path.exists(set_filename) and not force:
      # You may override by setting force=True.
      print('%s already present - Skipping pickling.' % set_filename)
    else:
      print('Pickling %s.' % set_filename)
      dataset = load_letter(folder, min_num_images_per_class)
      try:
        with open(set_filename, 'wb') as f:
          pickle.dump(dataset, f, pickle.HIGHEST_PROTOCOL)
      except Exception as e:
        print('Unable to save data to', set_filename, ':', e)
  
  return dataset_names

train_datasets = maybe_pickle(train_folders, 45000)
test_datasets = maybe_pickle(test_folders, 1800)

上边的代码就是把数据压缩到了一个pickle文件中去了，这样生成的数据文件可以在后续的程序中继续使用，这也就是没有直接采集图像数据的原因之一，下一步是将这些pickle文件中的数据进行合并和分类，生成一个拥有训练集、测试集合验证集的文件，训练数据的量取决于内存，如果非要使用超出内存的量的数据必须就分开运算了。

def make_arrays(nb_rows, img_size):#在merge_dagasets方法中把数据转换成图片个数*imgsize*imgsize(28)，同时建一个标签向量，大小为nb_rows
  if nb_rows:
    dataset = np.ndarray((nb_rows, img_size, img_size), dtype=np.float32)
    labels = np.ndarray(nb_rows, dtype=np.int32)
  else:
    dataset, labels = None, None
  return dataset, labels

def merge_datasets(pickle_files, train_size, valid_size=0):
  num_classes = len(pickle_files)
  valid_dataset, valid_labels = make_arrays(valid_size, image_size)
  train_dataset, train_labels = make_arrays(train_size, image_size)
  vsize_per_class = valid_size // num_classes
  tsize_per_class = train_size // num_classes
    
  start_v, start_t = 0, 0
  end_v, end_t = vsize_per_class, tsize_per_class
  end_l = vsize_per_class+tsize_per_class
  for label, pickle_file in enumerate(pickle_files):#将分布在10个pickle文件中的数据合并成一个张量     
    try:
      with open(pickle_file, 'rb') as f:
        letter_set = pickle.load(f)
        # 将读取到的pickle文件中的数据打乱
        np.random.shuffle(letter_set)
        if valid_dataset is not None:
          valid_letter = letter_set[:vsize_per_class, :, :]
          valid_dataset[start_v:end_v, :, :] = valid_letter
          valid_labels[start_v:end_v] = label
          start_v += vsize_per_class
          end_v += vsize_per_class
                    
        train_letter = letter_set[vsize_per_class:end_l, :, :]
        train_dataset[start_t:end_t, :, :] = train_letter
        train_labels[start_t:end_t] = label
        start_t += tsize_per_class
        end_t += tsize_per_class
    except Exception as e:
      print('Unable to process data from', pickle_file, ':', e)
      raise
    
  return valid_dataset, valid_labels, train_dataset, train_labels
            
            
train_size = 200000
valid_size = 10000
test_size = 10000

valid_dataset, valid_labels, train_dataset, train_labels = merge_datasets(
  train_datasets, train_size, valid_size)
_, _, test_dataset, test_labels = merge_datasets(test_datasets, test_size)

print('Training:', train_dataset.shape, train_labels.shape)
print('Validation:', valid_dataset.shape, valid_labels.shape)
print('Testing:', test_dataset.shape, test_labels.shape)

最后将数据再次打乱保存后，就得到了最后的pickle文件。

def randomize(dataset, labels):
  permutation = np.random.permutation(labels.shape[0])
  shuffled_dataset = dataset[permutation,:,:]
  shuffled_labels = labels[permutation]
  return shuffled_dataset, shuffled_labels
train_dataset, train_labels = randomize(train_dataset, train_labels)
test_dataset, test_labels = randomize(test_dataset, test_labels)
valid_dataset, valid_labels = randomize(valid_dataset, valid_labels)

pickle_file = 'notMNIST.pickle'
try:
  f = open(pickle_file, 'wb')
  save = {#存到一个dictionary中去
    'train_dataset': train_dataset,#num*28*28
    'train_labels': train_labels,#num*10
    'valid_dataset': valid_dataset,#…
    'valid_labels': valid_labels,
    'test_dataset': test_dataset,
    'test_labels': test_labels,
    }
  pickle.dump(save, f, pickle.HIGHEST_PROTOCOL)
  f.close()
except Exception as e:
  print('Unable to save data to', pickle_file, ':', e)
  raise
statinfo = os.stat(pickle_file)
print('Compressed pickle size:', statinfo.st_size)

上面是一些预操作，然后我们读取这个pickle文件，得到卷积神经网络要使用的数据文件：

pickle_file = 'notMNIST.pickle'

with open(pickle_file, 'rb') as f:
  save = pickle.load(f)
  train_dataset = save['train_dataset']
  train_labels = save['train_labels']
  valid_dataset = save['valid_dataset']
  valid_labels = save['valid_labels']
  test_dataset = save['test_dataset']
  test_labels = save['test_labels']
  del save  # hint to help gc free up memory
  print('Training set', train_dataset.shape, train_labels.shape)
  print('Validation set', valid_dataset.shape, valid_labels.shape)
  print('Test set', test_dataset.shape, test_labels.shape)

运行后：

Training set (200000, 28, 28) (200000,)

Validation set (10000, 28, 28) (10000,)

Test set (18724, 28, 28) (18724,)

可见训练集、验证集和测试集的原始格式。如果要将数据用到一个人工神经网络中，就要把每个图像数据都转换成一个长×宽维的向量，而在卷积神经网络中我们需要将图片数据转换成长×宽×深度的样子，同时将labels转换成one-hot encodings格式，于是：

image_size = 28
num_labels = 10
num_channels = 1 # grayscale，如果要使用RGB格式数据就是3了

import numpy as np

def reformat(dataset, labels):
  dataset = dataset.reshape(
    (-1, image_size, image_size, num_channels)).astype(np.float32)
#-1表示我懒得计算该填什么数字，由python通过a和其他的值3推测出来(这句话来自知乎，感觉好精辟啊)
  labels = (np.arange(num_labels) == labels[:,None]).astype(np.float32) 
#这句写的很迷，肿么解释。。。
#labels[:,None]相当于把shape为(20w,)的labels转换成shape为(20w,1)的数组，从[1,2,3,4...]到[[1],[2],[3],…]
#np.arange是生成了一个[0,1,2,3...]的(10,)的数组
#判断一个(10,)是否等于一个(10,1)的数组，或者说判断一个列向量是否等于一个行向量，可理解为矩阵乘法了，定义乘法规则为一样就是true，不一样就是false，那么这个判断式的结果就是一个20w*10的数组。
  return dataset, labels
train_dataset, train_labels = reformat(train_dataset, train_labels)
valid_dataset, valid_labels = reformat(valid_dataset, valid_labels)
test_dataset, test_labels = reformat(test_dataset, test_labels)
print('Training set', train_dataset.shape, train_labels.shape)
print('Validation set', valid_dataset.shape, valid_labels.shape)
print('Test set', test_dataset.shape, test_labels.shape)

运行结果为：

Training set (200000, 28, 28, 1) (200000, 10)

Validation set (10000, 28, 28, 1) (10000, 10)

Test set (10000, 28, 28, 1) (10000, 10)

下一步我们先定义一个用来检测预测精度的方法：

def accuracy(predictions, labels):
  return (100.0 * np.sum(np.argmax(predictions, 1) == np.argmax(labels, 1))
          / predictions.shape[0])#注意这里的argmax方法返回的是数组的索引值。

<2>:Draw a graph

前边说过了，tensorflow中进行运算，首先需要构建一个运算图，在这里将建立一个拥有两个卷积层和一个全连接层的卷积神经网络，算这个东西需要很土豪的显卡，所以限制了一下深度和全卷积层的节点。

batch_size = 16 #SGD每次选取的图片个数
patch_size = 5 #卷积窗口大小
depth = 16 #卷积深度，就是特征图的个数
num_hidden = 64 #全连接层隐层大小

graph = tf.Graph()

with graph.as_default():

  # Input data.4
  tf_train_dataset = tf.placeholder(
    tf.float32, shape=(batch_size, image_size, image_size, num_channels))#每次选出batch_size个图片参与运算
  tf_train_labels = tf.placeholder(tf.float32, shape=(batch_size, num_labels))
  tf_valid_dataset = tf.constant(valid_dataset)
  tf_test_dataset = tf.constant(test_dataset)
  
  # Variables.
  layer1_weights = tf.Variable(tf.truncated_normal(
      [patch_size, patch_size, num_channels, depth], stddev=0.1))#随机初始化第一卷积层权重参数，depth*num_channels张特征图，滑动窗口大小为5*5
  layer1_biases = tf.Variable(tf.zeros([depth]))#第一卷积层bias项初始化为0
  layer2_weights = tf.Variable(tf.truncated_normal(#随机初始化第二卷积层权重参数，depth*depth张特征图，滑动窗口5*5
      [patch_size, patch_size, depth, depth], stddev=0.1))
  layer2_biases = tf.Variable(tf.constant(1.0, shape=[depth]))#第二卷积层bias项初始化为0
  layer3_weights = tf.Variable(tf.truncated_normal(
      [image_size // 4 * image_size // 4 * depth, num_hidden], stddev=0.1))#全连接层第一层，//4是因为后边定义模型的时候定义stride为2，
    #所以两次卷积后的数据就是7*7*16*16了？？？？？？？
  layer3_biases = tf.Variable(tf.constant(1.0, shape=[num_hidden]))
  layer4_weights = tf.Variable(tf.truncated_normal(#全连接层第二层
      [num_hidden, num_labels], stddev=0.1))
  layer4_biases = tf.Variable(tf.constant(1.0, shape=[num_labels]))
  
  # Model.
  def model(data):
    conv = tf.nn.conv2d(data, layer1_weights, [1, 2, 2, 1], padding='SAME')#这里1,2,2,1是stride，依次对应到data的格式中去
    #same padding是补0的那种padding模式，比较便于运算，所以基本上都用这种的。
    hidden = tf.nn.relu(conv + layer1_biases)
    conv = tf.nn.conv2d(hidden, layer2_weights, [1, 2, 2, 1], padding='SAME')
    hidden = tf.nn.relu(conv + layer2_biases)
    shape = hidden.get_shape().as_list()
    reshape = tf.reshape(hidden, [shape[0], shape[1] * shape[2] * shape[3]])
    hidden = tf.nn.relu(tf.matmul(reshape, layer3_weights) + layer3_biases)
    return tf.matmul(hidden, layer4_weights) + layer4_biases
  
  # Training computation.
  logits = model(tf_train_dataset)
  loss = tf.reduce_mean(
    tf.nn.softmax_cross_entropy_with_logits(logits, tf_train_labels))
    
  # Optimizer.
  optimizer = tf.train.GradientDescentOptimizer(0.05).minimize(loss)#梯度下降
  
  # Predictions for the training, validation, and test data.
  train_prediction = tf.nn.softmax(logits)
  valid_prediction = tf.nn.softmax(model(tf_valid_dataset))
  test_prediction = tf.nn.softmax(model(tf_test_dataset))

这里看程序的话感觉好像是计算了，尤其是最后几句话，其实并没有计算的，下一步才是使用session来计算。

num_steps = 1001
#batch_size=16
with tf.Session(graph=graph) as session:
  #tf.global_variables_initializer().run()#for old version of tf0
  session.run(tf.initialize_all_variables())
  print('Initialized')
  for step in range(num_steps):
    offset = (step * batch_size) % (train_labels.shape[0] - batch_size)#这句是防止迭代次数过多超出数据集范围，就通过取余数改变取batch的偏置
    batch_data = train_dataset[offset:(offset + batch_size), :, :, :]
    batch_labels = train_labels[offset:(offset + batch_size), :]
    feed_dict = {tf_train_dataset : batch_data, tf_train_labels : batch_labels}
    
    _, l, predictions = session.run(
      [optimizer, loss, train_prediction], feed_dict=feed_dict)
    if (step % 50 == 0):
      print('Minibatch loss at step %d: %f' % (step, l))
      print('Minibatch accuracy: %.1f%%' % accuracy(predictions, batch_labels))
      print('Validation accuracy: %.1f%%' % accuracy(
        valid_prediction.eval(), valid_labels))
  print('Test accuracy: %.1f%%' % accuracy(test_prediction.eval(), test_labels))

以上就是实用卷积神经网络简单的区分not_MNIST数据的程序，池化层我还没加上，还有dropout防止过拟合也没有添加，待续。先看rfcn吧。