本文记录了在学习BP的过程中，借由吴恩达Deep Learning第一课第二周为模板写的猫识别算法。（其实相当于是课后作业hh）

　　1.加载库，用matplot作图，里面的lr_utils是吴恩达打包好的一个用来加载数据的包，如果是直接运行的话可能会报错，最后面给出一个别的大佬写的代码也可以用。

import numpy as np
import matplotlib.pyplot as plt
import h5py
import scipy
from PIL import Image
from scipy import ndimage
from lr_utils import load_dataset

%matplotlib inline

　　2.加载训练集和测试集

train_set_x_orig, train_set_y, test_set_x_orig, test_set_y, classes = load_dataset()

　　3.记录样本数m和图像的长宽（长宽相等，只用了一个num_px），打印一下看看长什么样

m_train=train_set_x_orig.shape[0]
m_test=test_set_x_orig.shape[0]
num_px=train_set_x_orig.shape[1]

print ("Number of training examples: m_train = " + str(m_train))
print ("Number of testing examples: m_test = " + str(m_test))
print ("Height/Width of each image: num_px = " + str(num_px))
print ("Each image is of size: (" + str(num_px) + ", " + str(num_px) + ", 3)")
print ("train_set_x shape: " + str(train_set_x_orig.shape))
print ("train_set_y shape: " + str(train_set_y.shape))
print ("test_set_x shape: " + str(test_set_x_orig.shape))
print ("test_set_y shape: " + str(test_set_y.shape))

　　4.把获得的数据转换成一个二维数组的形式，打印看看转换之后的样子

train_set_x_flatten=train_set_x_orig.reshape(train_set_x_orig.shape[0],-1).T
test_set_x_flatten=test_set_x_orig.reshape(test_set_x_orig.shape[0],-1).T

print ("train_set_x_flatten shape: " + str(train_set_x_flatten.shape))
print ("train_set_y shape: " + str(train_set_y.shape))
print ("test_set_x_flatten shape: " + str(test_set_x_flatten.shape))
print ("test_set_y shape: " + str(test_set_y.shape))
print ("sanity check after reshaping: " + str(train_set_x_flatten[0:5,0]))

　　5.彩色图像像素值实际上是一个由三个数字组成的向量，范围从0到255。这里给他标准化一下，不是图像需要从每个示例中减去整个数组的平均值，然后将每个样本除以整个数组的标准偏差，图像的话直接除以255就完事儿了

train_set_x = train_set_x_flatten/255.
test_set_x = test_set_x_flatten/255.

　　6.使用np.exp写激活函数sigmoid

def sigmoid(z):

    s=1/(1+np.exp(-z))

    return s

　　7.由于是简单的单层神经网络，直接初始化W,B为0就可以了，assert负责检查一下w、b的格式（形状），看看和预期的一不一致，这里最好注意一下，不然深层的网络查错会很麻烦

def initialize_with_zeros(dim):

###      
    w -- initialized vector of shape (dim, 1)
    b -- initialized scalar (corresponds to the bias)
###
    w=np.zeros(shape=(dim,1))
    b=0

    assert(w.shape == (dim, 1))
    assert(isinstance(b, float) or isinstance(b, int))
    
    return w, b 

　　8.前向传播的模块 np.squeeze()负责删掉维度为1的，防止后面cost出现一些奇奇怪怪的形状

def propagate(w, b, X, Y):
   
    m = X.shape[1]
    
    A=sigmoid(np.dot(w.T,X)+b)
    cost = -1/m * np.sum(Y*np.log(A)+(1-Y)*np.log(1-A))
    
    dw = 1/m *np.dot(X ,(A-Y).T)
    db=1/m*np.sum(A-Y)
    

    assert(dw.shape == w.shape)
    assert(db.dtype == float)
    cost = np.squeeze(cost)
    assert(cost.shape == ())
    
    grads = {"dw": dw,"db": db}
    
    return grads, cost

　　9.优化optimize的模块,每迭代100次记录一下他的代价，并保存到costs里面，方便后面作图

def optimize(w, b, X, Y, num_iterations, learning_rate, print_cost = False):
   
    costs = []
    
    for i in range(num_iterations):

        grads, cost = propagate(w,b,X,Y)

        dw = grads["dw"]
        db = grads["db"]

        w=w-learning_rate*dw
        b=b-learning_rate*db
        
        if i % 100 == 0:
            costs.append(cost)

        if print_cost and i % 100 == 0:
            print ("Cost after iteration %i: %f" %(i, cost))
    
    params = {"w": w,
              "b": b}
    
    grads = {"dw": dw,
             "db": db}
    
    return params, grads, costs

　　10.对图片进行预测的模块

def predict(w, b, X):

    m = X.shape[1]
    Y_prediction = np.zeros((1,m))
    w = w.reshape(X.shape[0], 1)

    A = sigmoid(np.dot(w.T,X)+b)

    for i in range(A.shape[1]):

        Y_prediction[0,i] = 1 if A[0,i] > 0.5 else 0
        ###
        if A[0,i]>0.5:
            Y_prediction[0,i]=1
        else:
            Y_prediction[0,i]=0
        ###    
    assert(Y_prediction.shape == (1, m))
    
    return Y_prediction        

　　11.模型整理（其实就是把之前做的模块整合到一起），打印一下此时的cost

def model(X_train, Y_train, X_test, Y_test, num_iterations = 2000, learning_rate = 0.5, print_cost = False):

    w,b=initialize_with_zeros(X_train.shape[0])

    parameters, grads, costs = optimize(w,b,X_train,Y_train,num_iterations,learning_rate,print_cost)

    w = parameters["w"]
    b = parameters["b"]

    Y_prediction_test = predict(w,b,X_test)
    Y_prediction_train = predict(w,b,X_train)

    print("train accuracy: {} %".format(100 - np.mean(np.abs(Y_prediction_train - Y_train)) * 100))
    print("test accuracy: {} %".format(100 - np.mean(np.abs(Y_prediction_test - Y_test)) * 100))

    d = {"costs": costs,
         "Y_prediction_test": Y_prediction_test, 
         "Y_prediction_train" : Y_prediction_train, 
         "w" : w, 
         "b" : b,
         "learning_rate" : learning_rate,
         "num_iterations": num_iterations}
    
    return d

　　12.跑模型~

d = model(train_set_x, train_set_y, test_set_x, test_set_y, num_iterations = 2000, learning_rate = 0.005, print_cost = True)

　　13.收获的时候到了，看看你的。这个index可以随便换，只要是测试集里面的图片标签都可以（大概是），验收一下成果

index = 1
plt.imshow(test_set_x[:,index].reshape((num_px, num_px, 3)))
print ("y = " + str(test_set_y[0,index]) + ", you predicted that it is a \"" + classes[int(d["Y_prediction_test"][0,index])].decode("utf-8") +  "\" picture.")

　　14.做个图，看看loss(也就是cost)

costs = np.squeeze(d['costs'])
plt.plot(costs)
plt.ylabel('cost')
plt.xlabel('iterations (per hundreds)')
plt.title("Learning rate =" + str(d["learning_rate"]))
plt.show()

完结撒花！对了还有数据集我不知道怎么弄到博客园上，先试试，不行的话我再开一个专门上传这个数据集

　　15.补一个大佬给的加载文件的方法。

import numpy as np
import h5py
    
    
def load_dataset():
    train_dataset = h5py.File('datasets/train_catvnoncat.h5', "r")
    train_set_x_orig = np.array(train_dataset["train_set_x"][:]) # your train set features
    train_set_y_orig = np.array(train_dataset["train_set_y"][:]) # your train set labels

    test_dataset = h5py.File('datasets/test_catvnoncat.h5', "r")
    test_set_x_orig = np.array(test_dataset["test_set_x"][:]) # your test set features
    test_set_y_orig = np.array(test_dataset["test_set_y"][:]) # your test set labels

    classes = np.array(test_dataset["list_classes"][:]) # the list of classes
    
    train_set_y_orig = train_set_y_orig.reshape((1, train_set_y_orig.shape[0]))
    test_set_y_orig = test_set_y_orig.reshape((1, test_set_y_orig.shape[0]))
    
    return train_set_x_orig, train_set_y_orig, test_set_x_orig, test_set_y_orig, classes


#来自一位csdn大佬，有机会的话希望能给他点一点小红心