CS224d assignment 1【Neural Network Basics】

refer to:

机器学习公开课笔记(5)：神经网络(Neural Network)

CS224d笔记3——神经网络

深度学习与自然语言处理(4)_斯坦福cs224d 大作业测验1与解答

CS224d Problem set 1作业

softmax:

def softmax(x):

    assert len(x.shape) > 1
    x -= np.max(x, axis=1, keepdims=True)
    x = np.exp(x) / np.sum(np.exp(x), axis=1, keepdims=True)

    return x

sigmoid & sigmoid_grad:

def sigmoid(x):

    result = 1.0 / (1.0 + np.exp(-x))

    return result

def sigmoid_grad(f):

    f=f*(1.0-f)

    return f

gradcheck_naive:

def gradcheck_naive(f, x):
    """
    Gradient check for a function f
    - f should be a function that takes a single argument and outputs the
        cost and its gradients
    - x is the point (numpy array) to check the gradient at
    """ 

    rndstate = random.getstate()
    random.setstate(rndstate)
    fx, grad = f(x) # Evaluate function value at original point
    h = 1e-4

    # Iterate over all indexes in x
    it = np.nditer(x, flags=['multi_index'], op_flags=['readwrite'])
    while not it.finished:
        ix = it.multi_index

        ### try modifying x[ix] with h defined above to compute numerical gradients
        ### make sure you call random.setstate(rndstate) before calling f(x) each
        ### time, this will make it
        ### possible to test cost functions with built in randomness later
        ### YOUR CODE HERE:
        old_val = x[ix]
        x[ix] = old_val - h
        random.setstate(rndstate)
        ( fxh1, _ ) = f(x)

        x[ix] = old_val + h
        random.setstate(rndstate)
        ( fxh2, _ ) = f(x)

        numgrad = (fxh2 - fxh1)/(2*h)
        x[ix] = old_val
        ### END YOUR CODE

        # Compare gradients
        reldiff = abs(numgrad - grad[ix]) / max(1, abs(numgrad), abs(grad[ix]))
        if reldiff > 1e-5:
            print "Gradient check failed."
            print "First gradient error found at index %s" % str(ix)
            print "Your gradient: %f \t Numerical gradient: %f" % (grad[ix], numgrad)
            return

        it.iternext() # Step to next dimension

    print "Gradient check passed!"

neural.py

import numpy as np
import random

from q1_softmax import softmax
from q2_sigmoid import sigmoid, sigmoid_grad
from q2_gradcheck import gradcheck_naive

def forward_backward_prop(data, labels, params, dimensions):
    """
    Forward and backward propagation for a two-layer sigmoidal network 

    Compute the forward propagation and for the cross entropy cost,
    and backward propagation for the gradients for all parameters.
    """

    ### Unpack network parameters (do not modify)
    ofs = 0
    Dx, H, Dy = (dimensions[0], dimensions[1], dimensions[2])

    W1 = np.reshape(params[ofs:ofs+ Dx * H], (Dx, H))
    ofs += Dx * H
    b1 = np.reshape(params[ofs:ofs + H], (1, H))
    ofs += H
    W2 = np.reshape(params[ofs:ofs + H * Dy], (H, Dy))
    ofs += H * Dy
    b2 = np.reshape(params[ofs:ofs + Dy], (1, Dy))

    N, D = data.shape

    # data --> N x D
    # W1 --> D x H
    # b1 --> 1 x H
    # W2 --> H x V
    # b2 --> 1 x V
    # labels --> N x V

    ### YOUR CODE HERE: forward propagation
    Z1 = np.dot(data, W1) + b1 # N x H
    A1 = sigmoid(Z1) # N x H
    Z2 = np.dot(A1, W2) + b2 # N x V
    A2 = softmax(Z2) # N x V

    # cross entropy cost

    #first method
    #B = np.exp(Z2) # N x V
    #b = np.sum(B, axis=1) + 1e-8 # N x 1
    #z = np.log(b) # N x 1
    #cost = np.sum(z) - np.sum(Z2 * labels)
    #cost /= N

    #second method
    cost = - np.sum(np.log(A2[labels == 1]))/N
    ### END YOUR CODE
    #cost = b2[0,-1]

    ### YOUR CODE HERE: backward propagation            formula:
    delta2 = A2 - labels # N x V                                 delta2=A2-y
    gradb2 = np.sum(delta2, axis=0) # 1 x V                      gradb2<--delta2
    gradb2 /= N # 1 x V
    gradW2 = np.dot(A1.T, delta2) # H x V                        gradW2=A1.T*delta2
    gradW2 /= N # H x V
    delta1 =  sigmoid_grad(A1) * np.dot(delta2, W2.T)# N x H     delta1=f'(A1)*delta2*W2.T
    gradb1 = np.sum(delta1, axis=0) # 1 x H                      gradb1<--delta1
    gradb1 /= N # 1 x H
    gradW1 = np.dot(data.T, delta1) # D x H                      gradW1=X.T*delta1
    gradW1 /= N # D x H
    ### END YOUR CODE

    ### Stack gradients (do not modify)
    grad = np.concatenate((gradW1.flatten(), gradb1.flatten(),
        gradW2.flatten(), gradb2.flatten()))

    return cost, grad

def sanity_check():
    """
    Set up fake data and parameters for the neural network, and test using
    gradcheck.
    """
    print "Running sanity check..."

    N = 20
    dimensions = [10, 5, 10]
    data = np.random.randn(N, dimensions[0])   # each row will be a datum 20*10
    labels = np.zeros((N, dimensions[2]))
    for i in xrange(N):
        labels[i,random.randint(0,dimensions[2]-1)] = 1 #one-hot vector

    params = np.random.randn((dimensions[0] + 1) * dimensions[1] + (
        dimensions[1] + 1) * dimensions[2], )

    gradcheck_naive(lambda params: forward_backward_prop(data, labels, params,
        dimensions), params)

if __name__ == "__main__":
    sanity_check()