July 2018
Intermediate to advanced
310 pages
8h 21m
Chinese
Content preview from 深度学习入门 : 基于Python的理论与实现
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5.5 激活函数层的实现
141
如图 5
-
18 所示，如果正向传播时的输入值小于等于 0，则反向传播的值为 0。
因此，反向传播中会使用正向传播时保存的
mask
，将从上游传来的
dout
的
mask
中的元素为
True
的地方设为0。
ReLU层的作用就像电路中的开关一样。正向传播时，有电流通过
的话，就将开关设为ON；没有电流通过的话，就将开关设为OFF。
反向传播时，开关为ON的话，电流会直接通过；开关为OFF的话，
则不会有电流通过。
5.5.2
Sigmoid层
接下来，我们来实现
sigmoid
函数。
sigmoid
函数由式（5.9）表示。
（5.9）
用计算图表示式（5.9）的话，则如图 5
-
19 所示。
图5-19 sigmoid 层的计算图（仅正向传播）
1
−
1
y
1
× + /
exp
x
−x exp(−x) 1+exp(−x) 1+exp(−x)
图5
-
19 中 ， 除 了“
×
”和“
+
”节点外，还出现了新的“
exp
”和“ /”节 点 。
“
exp
”节点会进行y
= exp(
x
)
的计算，“/”节点会进行
的计算。
如图5
-
19 所示，式（5.9）的计算由局部计算的传播构成。下面我们就来
进行图5
-
19 的计算图的反向传播。这里，作为总结，我们来依次看一下反向
传播的流程。
第 5章 误差反向传播法
142
步骤 ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.