這裏主要梳理一下作業的主要內容和思路，完整作業文件可參考:
https://github.com/pandenghuang/Andrew-Ng-Deep-Learning-notes/tree/master/assignments/C2W3
作業完整截圖，參考本文結尾：作業完整截圖。
TensorFlow Tutorial （Tensorflow教程）
Welcome to this week's programming assignment. Until now, you've always used numpy to build neural networks. Now we will step you through a deep learning framework that will allow you to build neural networks more easily. Machine learning frameworks like TensorFlow, PaddlePaddle, Torch, Caffe, Keras, and many others can speed up your machine learning development significantly. All of these frameworks also have a lot of documentation, which you should feel free to read. In this assignment, you will learn to do the following in TensorFlow:
- Initialize variables
- Start your own session
- Train algorithms
- Implement a Neural Network
Programing frameworks can not only shorten your coding time, but sometimes also perform optimizations that speed up your code.
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1 - Exploring the Tensorflow Library （導入Tensorflor庫）
To start, you will import the library:
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Writing and running programs in TensorFlow has the following steps:
- Create Tensors (variables) that are not yet executed/evaluated.
- Write operations between those Tensors.
- Initialize your Tensors.
- Create a Session.
- Run the Session. This will run the operations you'd written above.
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1.1 - Linear function（線性函數）
Lets start this programming exercise by computing the following equation: 𝑌=𝑊𝑋+𝑏Y=WX+b, where 𝑊W and 𝑋X are random matrices and b is a random vector.
Exercise: Compute 𝑊𝑋+𝑏WX+b where 𝑊,𝑋W,X, and 𝑏b are drawn from a random normal distribution. W is of shape (4, 3), X is (3,1) and b is (4,1). As an example, here is how you would define a constant X that has shape (3,1):
X = tf.constant(np.random.randn(3,1), name = "X")
You might find the following functions helpful:
- tf.matmul(..., ...) to do a matrix multiplication
- tf.add(..., ...) to do an addition
- np.random.randn(...) to initialize randomly
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1.2 - Computing the sigmoid（計算sigmoid函數值）
Great! You just implemented a linear function. Tensorflow offers a variety of commonly used neural network functions like tf.sigmoid and tf.softmax. For this exercise lets compute the sigmoid function of an input.
You will do this exercise using a placeholder variable x. When running the session, you should use the feed dictionary to pass in the input z. In this exercise, you will have to (i) create a placeholder x, (ii) define the operations needed to compute the sigmoid using tf.sigmoid, and then (iii) run the session.
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To summarize, you how know how to:
1. Create placeholders
2. Specify the computation graph corresponding to operations you want to compute
3. Create the session
4. Run the session, using a feed dictionary if necessary to specify placeholder variables' values.
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1.3 - Computing the Cost（計算成本函數值）
You can also use a built-in function to compute the cost of your neural network. So instead of needing to write code to compute this as a function of 𝑎[2](𝑖)a[2](i) and 𝑦(𝑖)y(i) for i=1...m:
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1.4 - Using One Hot encodings（使用one-hot（獨熱）編碼）
Many times in deep learning you will have a y vector with numbers ranging from 0 to C-1, where C is the number of classes. If C is for example 4, then you might have the following y vector which you will need to convert as follows:
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1.5 - Initialize with zeros and ones（初始化）
Now you will learn how to initialize a vector of zeros and ones. The function you will be calling is tf.ones(). To initialize with zeros you could use tf.zeros() instead. These functions take in a shape and return an array of dimension shape full of zeros and ones respectively.
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2 - Building your first neural network in tensorflow （使用Tensorflow構建第一個神經網絡）
In this part of the assignment you will build a neural network using tensorflow. Remember that there are two parts to implement a tensorflow model:
- Create the computation graph
- Run the graph
Let's delve into the problem you'd like to solve!
2.0 - Problem statement: SIGNS Dataset （問題描述）
One afternoon, with some friends we decided to teach our computers to decipher sign language. We spent a few hours taking pictures in front of a white wall and came up with the following dataset. It's now your job to build an algorithm that would facilitate communications from a speech-impaired person to someone who doesn't understand sign language.
- Training set: 1080 pictures (64 by 64 pixels) of signs representing numbers from 0 to 5 (180 pictures per number).
- Test set: 120 pictures (64 by 64 pixels) of signs representing numbers from 0 to 5 (20 pictures per number).
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2.1 - Create placeholders（創建佔位符）
Your first task is to create placeholders for X and Y. This will allow you to later pass your training data in when you run your session.
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2.2 - Initializing the parameters（初始化參數）
Your second task is to initialize the parameters in tensorflow.
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2.3 - Forward propagation in tensorflow（Tensorflow中的前向傳播）
You will now implement the forward propagation module in tensorflow. The function will take in a dictionary of parameters and it will complete the forward pass. The functions you will be using are:
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2.4 Compute cost（計算成本函數值）
As seen before, it is very easy to compute the cost using:
tf.reduce_mean(tf.nn.softmax_cross_entropy_with_logits(logits = ..., labels = ...))...
2.5 - Backward propagation & parameter updates（後向傳播和參數更新）
This is where you become grateful to programming frameworks. All the backpropagation and the parameters update is taken care of in 1 line of code. It is very easy to incorporate this line in the model.
After you compute the cost function. You will create an "optimizer" object. You have to call this object along with the cost when running the tf.session. When called, it will perform an optimization on the given cost with the chosen method and learning rate.
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2.6 - Building the model（構建模型）
Now, you will bring it all together!
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2.7 - Test with your own image (optional / ungraded exercise)（使用自己的照片進行測試）
Congratulations on finishing this assignment. You can now take a picture of your hand and see the output of your model. To do that:
1. Click on "File" in the upper bar of this notebook, then click "Open" to go on your Coursera Hub.
2. Add your image to this Jupyter Notebook's directory, in the "images" folder
3. Write your image's name in the following code
4. Run the code and check if the algorithm is right!...
What you should remember:
- Tensorflow is a programming framework used in deep learning
- The two main object classes in tensorflow are Tensors and Operators.
- When you code in tensorflow you have to take the following steps:
- Create a graph containing Tensors (Variables, Placeholders ...) and Operations (tf.matmul, tf.add, ...)
- Create a session
- Initialize the session
- Run the session to execute the graph
- You can execute the graph multiple times as you've seen in model()
- The backpropagation and optimization is automatically done when running the session on the "optimizer" object.
作業完整截圖：