tensorflow 2.0 神经网络与全连接层 之 数据加载

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Outline

• keras.datasets
• tf.data.Dataset.from_tensor_slices
  • shuffle
  • map
  • batch
  • repeat
• we will talk Input Pipeline later

keras.dataset

• boston housing （波士顿房价数据集）
  • Boston housing price regression dataset.
• mnist/fashion mnist （Mnist数据集）
  • MNIST/Fashion-MNIST dataset.
• cifar10/100 （CV）
  • small images classification dataset.
• imdb（NLP）
  • sentiment classification dataset

值得注意的一点，是用 keras.dataset 加载的 数据 是 numpy array 类型。

MNIST 数据集

import tensorflow as tf
from tensorflow import keras
import os

os.environ['TF_CPP_MIN_LOG_LEVEL'] = '2'

(x, y), (x_test, y_test) = keras.datasets.mnist.load_data()
print(
    'train data shape:',
    x.shape,   # (60000, 28, 28) 
    y.shape,   # (60000,)
    '\ntest data shape:',
    x_test.shape,   # (10000, 28, 28)
    y_test.shape)   # (10000,)

print(
    'min:', x.min(),   # 0
    '\nmax:', x.max(),   # 255
    '\nmean:', x.mean())   # 33.318421449829934

print(y[:4])   # [5 0 4 1]

y_onehot = tf.one_hot(y, depth=10)
print(y_onehot[:2])
# tf.Tensor(
# [[0. 0. 0. 0. 0. 1. 0. 0. 0. 0.]
#  [1. 0. 0. 0. 0. 0. 0. 0. 0. 0.]], shape=(2, 10), dtype=float32)

注：

1. 由此加载的数据类型为 numpy.ndarray。

CIFAR10/100

(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
print(
    'train data shape:',
    x.shape,   # (50000, 32, 32, 3)
    y.shape,   # (50000, 1)
    '\ntest data shape:',
    x_test.shape,   # (10000, 32, 32, 3)
    y_test.shape)   # (10000, 1)

print(
    'min:', x.min(),   # 0
    '\nmax:', x.max(),   # 255
    '\nmean:', x.mean())   # 120.70756512369792

print(y[:4])
# [[6]
#  [9]
#  [9]
#  [4]]

print(
    type(x),   # <class 'numpy.ndarray'>
    type(y),   # <class 'numpy.ndarray'>
)

注：

1. 由此加载的数据类型为 numpy.ndarray。
2. 与 MINST 数据不同的一点是，CIFAR 数据集的 y 的有两个维度。

tf.data.Dataset

from_tensor_slices()

# from_tensor_slices()
(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
# x.shape: (50000, 32, 32, 3)
db = tf.data.Dataset.from_tensor_slices(x_test)
print(next(iter(db)).shape)   # (32, 32, 3)

db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
print(next(iter(db))[0].shape)   # (32, 32, 3)
print (type(next(iter(db))[0]))   # <class 'tensorflow.python.framework.ops.EagerTensor'>

.shuff

(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
db = db.shuffle(1000)
'''

shuffle(buffer_size, seed=None, reshuffle_each_iteration=None) method of tensorflow.python.data.ops.dataset_ops.ShuffleDataset instance
    Randomly shuffles the elements of this dataset.
	
	For instance, if your dataset contains 10,000 elements but `buffer_size` is
    set to 1,000, then `shuffle` will initially select a random element from
    only the first 1,000 elements in the buffer. Once an element is selected,
    its space in the buffer is replaced by the next (i.e. 1,001-st) element,
    maintaining the 1,000 element buffer.
'''

.map

def preprocess(x, y):
    x = tf.cast(x, dtype=tf.float32) / 255
    y = tf.cast(y, dtype=tf.int32)
    y = tf.one_hot(y, depth=10)
    return x, y


(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
db2 = db.map(preprocess)

res = next(iter(db2))

print(
    'x_shape:',
    res[0].shape,   # (32, 32, 3)
    '\ny_shape:',
    res[1].shape,   # (1, 10)  注意这和我们预想的不符， 由于载入的 y 有两个维度。
)
print(tf.squeeze(res[1]).shape)   # (10,)

注：

1. map 用于批量的数值转化 tf.float32, tf.int32。

.batch

(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
db2 = db.map(preprocess)

db3 = db2.batch(32)
res = next(iter(db3))

print(
    'x_shape:',
    res[0].shape,   # x_shape: (32, 32, 32, 3)
    '\ny_shape:',
    res[1].shape,   # y_shape: (32, 1, 10)  ==注意==
)

StopIteration

(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
db = tf.data.Dataset.from_tensor_slices((x_test, y_test))
db2 = db.map(preprocess)
db3 = db2.batch(32)

db_iter = iter(db3)

while True:
    next(db_iter)

	raise StopIteration
StopIteration

.repeat

(x, y), (x_test, y_test) = keras.datasets.cifar10.load_data()
db = tf.data.Dataset.from_tensor_slices((x_test, y_test))   # [10000, 32, 32, 4]
db2 = db.map(preprocess)
db3 = db2.batch(1000)
db4 = db3.repeat(2)

for i, (_, _) in enumerate(db4):
    print (i, )

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

注：

1. 测试集有 10000 张图片 batch 为 1000，重复了两次。因此迭代20次。
2. 重复迭代 i 累加。