Neural Network Basics

import numpy as np
# 2-layer neural net from scratch learning XOR
rng=np.random.default_rng(0); X=np.array([[0,0],[0,1],[1,0],[1,1]]); y=np.array([[0],[1],[1],[0]])
W1,b1=rng.normal(size=(2,8)),np.zeros(8); W2,b2=rng.normal(size=(8,1)),np.zeros(1); sig=lambda z:1/(1+np.exp(-z))
for ep in range(5000):
    h=np.tanh(X@W1+b1); o=sig(h@W2+b2); d2=o-y; d1=(d2@W2.T)*(1-h**2)
    W2-=0.1*h.T@d2; b2-=0.1*d2.sum(0); W1-=0.1*X.T@d1; b1-=0.1*d1.sum(0)
    if ep%1000==0: print("epoch",ep,"loss",round(float(((o-y)**2).mean()),4))
print("XOR predictions:",o.round(2).ravel())