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[英]How to apply change from gradient descent to weights in my Neural Network?
[英]gradient descent in neural network training plateauing
我一直在嘗試在python中實現一個基本的back-propogation神經網絡,並完成了初始化和訓練權重集的編程。 然而,在我訓練的所有集合上,誤差(均方)總是收斂到一個奇怪的數字 - 錯誤總是在進一步的迭代中減少,但從未真正接近零。
任何幫助將非常感激。
import csv
import numpy as np
class NeuralNetwork:
layers = 0
shape = None
weights = []
layerIn = []
layerOut = []
def __init__(self, shape):
self.shape = shape
self.layers = len(shape) - 1
for i in range(0,self.layers):
n = shape[i]
m = shape[i+1]
self.weights.append(np.random.normal(scale=0.2, size = (m,n+1)))
def sgm(self, x):
return 1/(1+np.exp(-x))
def dersgm(self, x):
y = self.sgm(x)
return y*(y-1)
def run(self, input):
self.layerIn = []
self.layerOut = []
for i in range(self.layers):
if i == 0:
layer = self.weights[0].dot(np.vstack((input.transpose(), np.ones([1,input.shape[0]]))))
else:
layer = self.weights[i].dot(np.vstack((self.layerOut[-1], np.ones([1,input.shape[0]]))))
self.layerIn.append(layer)
self.layerOut.append(self.sgm(layer))
return self.layerOut[-1].T
def backpropogate(self, input, y, learning_rate):
deltas = []
y_hat = self.run(input)
#Calculate deltas
for i in reversed(range(self.layers)):
#for last layer
if i == self.layers-1:
error = y_hat - y
msq_error = sum(.5 * ((error) ** 2))
#returns delta, k rows for k inputs, m columns for m nodes
deltas.append(error * self.dersgm(y_hat))
else:
error = deltas[-1].dot(self.weights[i+1][:,:-1])
deltas.append(self.dersgm(self.layerOut[i]).T * error)
#Calculate weight-deltas
wdelta = []
ordered_deltas = list(reversed(deltas)) #reverse order because created backwards
#returns weight deltas, k rows for k nodes, m columns for m next layer nodes
for i in range(self.layers):
if i == 0:
#add bias
input_with_bias = np.vstack((input.T, np.ones(input.shape[0])))
#some over n rows of deltas for n training examples to get one delta for all examples
#for all nodes
wdelta.append(ordered_deltas[i].T.dot(input_with_bias.T))
else:
with_bias = np.vstack((self.layerOut[i-1], np.ones(input.shape[0])))
wdelta.append(ordered_deltas[i].T.dot(with_bias.T))
#update_weights
def update_weights(self, weight_deltas, learning_rate):
for i in range(self.layers):
self.weights[i] = self.weights[i] +\
(learning_rate * weight_deltas[i])
update_weights(self, wdelta, learning_rate)
return msq_error
#end backpropogate
def train(self, input, target, lr, run_iter):
for i in range(run_iter):
if i % 100000 == 0:
print self.backpropogate(input, target, lr)
以下場景中的誤差函數不能為0,因為誤差函數為0將要求點與曲線完美匹配。
擁有更多神經元肯定會減少誤差,因為該功能可以具有更復雜和精確的形狀。 但是,如果您對數據非常適合,則會出現稱為過度擬合的問題,如下圖所示。 從左到右,曲線要么不適合數據集,要么幾乎正確擬合,然后右邊的過度擬合。
右邊的場景會導致錯誤為0,但這不是必需的,你想避免這種情況。 怎么樣?
確定網絡中神經元數量是否理想(具有良好擬合)的最簡單方法是通過反復試驗。 將您的數據分成訓練數據(80% - 訓練網絡)和測試數據(20% - 僅保留用於在訓練后測試網絡)。 雖然只訓練訓練數據,但可以在測試數據集上繪制性能。
您還可以使用第三個數據集進行驗證,請參閱: 神經網絡中的訓練,驗證和測試集之間的差異是什么?
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