吳恩達.深度學習系列-C1神經網路與深度學習-w3-(作業:一個隱藏層進行二維資料分類)
前言
**注意:coursera要求不要在網際網路公佈自己的作業。如果你在學習這個課程,建議你進入課程系統自行完成作業。使用邏輯迴歸作為一個最簡單的類似神經網路來進行影象判別。我覺得程式碼有參考和保留的意義。v
使用一個 2×4×1的網路來對資料進行二分類。**
比較麻煩的是什麼時候用點乘,什麼時候用矩陣乘法。見【4.3】小節的程式碼。雖然程式碼都提交通過稽核,但自己還需要梳理一下。
I.Cost的計算
用點乘:
logprobs = np.multiply(np.log(A2),Y)+np.multiply(np.log(1-A2),1-Y)
cost = - np.sum(logprobs)/m
II.gradient的計算
# Backward propagation: calculate dW1, db1, dW2, db2.
### START CODE HERE ### (≈ 6 lines of code, corresponding to 6 equations on slide above)
#按單樣本進行簡化考量,引數的梯度的形應該與引數的形一致,如:dZ2.shape==Z2.shape=(1,1);dW1.shape==W1=(4,2)
dZ2 = A2-Y # A2(1,1)-Y(1,1)=dZ2(1,1)
dW2 = np.dot(dZ2,A1.T)/m #dZ2(1,1)*A1.T(1,4)=dW2(1,4),==>使用矩陣乘!
db2 = np.sum(dZ2,axis=1,keepdims=True)/m #np.sum(dW2(1,4))/m=db2(1,1)
dZ1 = np.multiply(np.multiply (W2.T,dZ2),1 - np.power(A1, 2)) #輸出dZ1=(4,1)。W2.T(4,1)×dZ2(1,1) [1 - np.power(A1, 2)],所以內外兩層的乘號都是點乘。
dW1 = np.dot(dZ1,X.T)/m #dZ1(4,1)*X.T(1,2)=dW1(4,2)===>使用矩陣乘
db1 = np.sum(dZ1,axis=1,keepdims=True)/m
### END CODE HERE ###
是否使用點乘還是矩陣乘,有一個判別方法,把輸入的shape和應該輸出的shape提前標出來,那麼比較容易判別用點乘還是矩陣乘。輸入的形與輸出一致,那麼肯定是點乘,其他情況是用矩陣乘。如果標出的形還不好判別,可以進一步畫出是行還是列用來表達單個樣本中的n個特徵。或者只按一條樣本的shape來進行判斷。如上。
Planar data classification with one hidden layer
Welcome to your week 3 programming assignment. It’s time to build your first neural network, which will have a hidden layer. You will see a big difference between this model and the one you implemented using logistic regression.
You will learn how to:
- Implement a 2-class classification neural network with a single hidden layer
- Use units with a non-linear activation function, such as tanh
- Compute the cross entropy loss
- Implement forward and backward propagation
1 - Packages
Let’s first import all the packages that you will need during this assignment.
- numpy is the fundamental package for scientific computing with Python.
- sklearn provides simple and efficient tools for data mining and data analysis.
- matplotlib is a library for plotting graphs in Python.
- testCases provides some test examples to assess the correctness of your functions
- planar_utils provide various useful functions used in this assignment
# Package imports
import numpy as np
import matplotlib.pyplot as plt
from testCases_v2 import *
import sklearn
import sklearn.datasets
import sklearn.linear_model
from planar_utils import plot_decision_boundary, sigmoid, load_planar_dataset, load_extra_datasets
%matplotlib inline
np.random.seed(1) # set a seed so that the results are consistent
/opt/conda/lib/python3.5/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
warnings.warn('Matplotlib is building the font cache using fc-list. This may take a moment.')
/opt/conda/lib/python3.5/site-packages/matplotlib/font_manager.py:273: UserWarning: Matplotlib is building the font cache using fc-list. This may take a moment.
warnings.warn('Matplotlib is building the font cache using fc-list. This may take a moment.')
2 - Dataset
First, let’s get the dataset you will work on. The following code will load a “flower” 2-class dataset into variables X
and Y
.
X, Y = load_planar_dataset()
Visualize the dataset using matplotlib. The data looks like a “flower” with some red (label y=0) and some blue (y=1) points. Your goal is to build a model to fit this data.
# Visualize the data:
plt.scatter(X[0, :], X[1, :], c=Y, s=40, cmap=plt.cm.Spectral);
You have:
- a numpy-array (matrix) X that contains your features (x1, x2)
- a numpy-array (vector) Y that contains your labels (red:0, blue:1).
Lets first get a better sense of what our data is like.
Exercise: How many training examples do you have? In addition, what is the shape
of the variables X
and Y
?
Hint: How do you get the shape of a numpy array? (help)
### START CODE HERE ### (≈ 3 lines of code)
shape_X = X.shape
shape_Y = Y.shape
m = shape_X[1] # training set size
### END CODE HERE ###
print ('The shape of X is: ' + str(shape_X))
print ('The shape of Y is: ' + str(shape_Y))
print ('I have m = %d training examples!' % (m))
The shape of X is: (2, 400)
The shape of Y is: (1, 400)
I have m = 400 training examples!
Expected Output:
**shape of X** | (2, 400) |
**shape of Y** | (1, 400) |
**m** | 400 |
3 - Simple Logistic Regression
Before building a full neural network, lets first see how logistic regression performs on this problem. You can use sklearn’s built-in functions to do that. Run the code below to train a logistic regression classifier on the dataset.
# Train the logistic regression classifier
clf = sklearn.linear_model.LogisticRegressionCV();
clf.fit(X.T, Y.T);
/opt/conda/lib/python3.5/site-packages/sklearn/utils/validation.py:515: DataConversionWarning: A column-vector y was passed when a 1d array was expected. Please change the shape of y to (n_samples, ), for example using ravel().
y = column_or_1d(y, warn=True)
You can now plot the decision boundary of these models. Run the code below.
# Plot the decision boundary for logistic regression
plot_decision_boundary(lambda x: clf.predict(x), X, Y)
plt.title("Logistic Regression")
# Print accuracy
LR_predictions = clf.predict(X.T)
print ('Accuracy of logistic regression: %d ' % float((np.dot(Y,LR_predictions) + np.dot(1-Y,1-LR_predictions))/float(Y.size)*100) +
'% ' + "(percentage of correctly labelled datapoints)")
Accuracy of logistic regression: 47 % (percentage of correctly labelled datapoints)
Expected Output:
**Accuracy** | 47% |
Interpretation: The dataset is not linearly separable, so logistic regression doesn’t perform well. Hopefully a neural network will do better. Let’s try this now!
4 - Neural Network model
Logistic regression did not work well on the “flower dataset”. You are going to train a Neural Network with a single hidden layer.
Here is our model:
Mathematically:
For one example :