code
'''
import numpy as np
import matplotlib.pyplot as plt
N = 100 # 每个类中的样本点
D = 2 # 维度
K = 3 # 类别个数
X = np.zeros((N*K,D)) # 样本input
y = np.zeros(N*K, dtype='uint8') # 类别标签
for j in range(K):
ix = range(N*j,N*(j+1))
r = np.linspace(0.0,1,N) # radius
t = np.linspace(j*4,(j+1)*4,N) + np.random.randn(N)*0.2 # theta
X[ix] = np.c_[r*np.sin(t), r*np.cos(t)]
y[ix] = j
# 可视化一下我们的样本点
plt.scatter(X[:, 0], X[:, 1], c=y, s=40, cmap=plt.cm.Spectral)
#plt.show()
W = 0.01 * np.random.randn(D,K)
b = np.zeros((1,K))
#需要自己敲定的步长和正则化系数
step_size = 1e-0
reg = 1e-3 #正则化系数
#梯度下降迭代循环
num_examples = X.shape[0]
for i in range(200):
# 计算类别得分, 结果矩阵为[N x K]
scores = np.dot(X, W) + b
# 计算类别概率
exp_scores = np.exp(scores)
probs = exp_scores / np.sum(exp_scores, axis=1, keepdims=True) # [N x K]
# 计算损失loss(包括互熵损失和正则化部分)
corect_logprobs = -np.log(probs[range(num_examples),y])
data_loss = np.sum(corect_logprobs)/num_examples
reg_loss = 0.5*reg*np.sum(W*W)
loss = data_loss + reg_loss
if i % 10 == 0:
print ("iteration %d: loss %f" % (i, loss))
# 计算得分上的梯度
dscores = probs
dscores[range(num_examples),y] -= 1
dscores /= num_examples
# 计算和回传梯度
dW = np.dot(X.T, dscores)
db = np.sum(dscores, axis=0, keepdims=True)
dW += reg*W # 正则化梯度
#参数更新
W += -step_size * dW
b += -step_size * db
#评估准确度
scores = np.dot(X, W) + b
predicted_class = np.argmax(scores, axis=1)
print ('training accuracy: %.2f' % (np.mean(predicted_class == y)))
'''
Neural Network
# 随机初始化参数
h = 100 # 隐层大小
W = 0.01 * np.random.randn(D,h)
b = np.zeros((1,h))
W2 = 0.01 * np.random.randn(h,K)
b2 = np.zeros((1,K))
# 手动敲定的几个参数
step_size = 1e-0
reg = 1e-3 # 正则化参数
# 梯度迭代与循环
num_examples = X.shape[0]
for i in range(10000):
hidden_layer = np.maximum(0, np.dot(X, W) + b) #使用的ReLU神经元
scores = np.dot(hidden_layer, W2) + b2
# 计算类别概率
exp_scores = np.exp(scores)
probs = exp_scores / np.sum(exp_scores, axis=1, keepdims=True) # [N x K]
# 计算互熵损失与正则化项
corect_logprobs = -np.log(probs[range(num_examples),y])
data_loss = np.sum(corect_logprobs)/num_examples
reg_loss = 0.5*reg*np.sum(W*W) + 0.5*reg*np.sum(W2*W2)
loss = data_loss + reg_loss
if i % 1000 == 0:
print ("iteration %d: loss %f" % (i, loss))
# 计算梯度
dscores = probs
dscores[range(num_examples),y] -= 1
dscores /= num_examples
# 梯度回传
dW2 = np.dot(hidden_layer.T, dscores)
db2 = np.sum(dscores, axis=0, keepdims=True)
dhidden = np.dot(dscores, W2.T)
dhidden[hidden_layer <= 0] = 0
# 拿到最后W,b上的梯度
dW = np.dot(X.T, dhidden)
db = np.sum(dhidden, axis=0, keepdims=True)
# 加上正则化梯度部分
dW2 += reg * W2
dW += reg * W
# 参数迭代与更新
W += -step_size * dW
b += -step_size * db
W2 += -step_size * dW2
b2 += -step_size * db2
#计算分类准确度
hidden_layer = np.maximum(0, np.dot(X, W) + b)
scores = np.dot(hidden_layer, W2) + b2
predicted_class = np.argmax(scores, axis=1)
print ('training accuracy: %.2f' % (np.mean(predicted_class == y)))
