# Backpropagation

```# https://en.wikipedia.org/wiki/Backpropagation
import numpy as np

# https://en.wikipedia.org/wiki/Sigmoid_function
def sigmoid(x):
return 1.0 / (1 + np.exp(-x))

assert(sigmoid(0) == 0.5)

def sigmoid_dx(x):
return x * (1.0 - x)

assert(sigmoid_dx(0.5) == 0.25)

# data
x = np.array([[0,0,1], [0,1,1], [1,0,1], [1,1,1]])
y = np.array([[0], [1], [1], [0]])

w_1 = np.random.rand(x.shape[1],4)
w_2 = np.random.rand(4,1)

output = np.zeros(y.shape)

# feedforward
def feedforward(x, w_1, w_2, output):
layer_1 = sigmoid(np.dot(x, w_1))
output = sigmoid(np.dot(layer_1, w_2))

return layer_1, w_1, w_2, output

# backpropagation
def backpropagation(layer_1, w_1, w_2, x, y, output):
w_2_dx = np.dot(np.transpose(layer_1), (2 * (y - output) * sigmoid_dx(output)))
w_1_dx = np.dot(np.transpose(x),  (np.dot(2 * (y - output) * sigmoid_dx(output), np.transpose(w_2)) * sigmoid_dx(layer_1)))

# update weights
w_1 += w_1_dx
w_2 += w_2_dx

return layer_1, w_1, w_2, x, y, output

# run
for i in range(1500):
layer_1, w_1, w_2, output = feedforward(x, w_1, w_2, output)
layer_1, w_1, w_2, x, y, output = backpropagation(layer_1, w_1, w_2, x, y, output)

print(output)
# [[0.01021434]
#  [0.97674973]
#  [0.97726171]
#  [0.02572076]]
```