这是一个简单的两层网络：affine - relu - affine - softmax
（关于网络结构的具体描述都在函数文档里了）
from builtins import range
from builtins import object
import numpy as np
from cs231n.layers import *
from cs231n.layer_utils import *
class TwoLayerNet(object):
"""
A two-layer fully-connected neural network with ReLU nonlinearity and
softmax loss that uses a modular layer design. We assume an input dimension
of D, a hidden dimension of H, and perform classification over C classes.
The architecure should be affine - relu - affine - softmax.
Note that this class does not implement gradient descent; instead, it
will interact with a separate Solver object that is responsible for running
optimization.
The learnable parameters of the model are stored in the dictionary
self.params that maps parameter names to numpy arrays.
"""
def __init__(self, input_dim=3*32*32, hidden_dim=100, num_classes=10,
weight_scale=1e-3, reg=0.0):
"""
Initialize a new network.
Inputs:
- input_dim: An integer giving the size of the input
- hidden_dim: An integer giving the size of the hidden layer
- num_classes: An integer giving the number of classes to classify
- dropout: Scalar between 0 and 1 giving dropout strength.
- weight_scale: Scalar giving the standard deviation for random
initialization of the weights.
- reg: Scalar giving L2 regularization strength.
"""
self.params = {}
self.reg = reg
############################################################################
# TODO: Initialize the weights and biases of the two-layer net. Weights #
# should be initialized from a Gaussian with standard deviation equal to #
# weight_scale, and biases should be initialized to zero. All weights and #
# biases should be stored in the dictionary self.params, with first layer #
# weights and biases using the keys 'W1' and 'b1' and second layer weights #
# and biases using the keys 'W2' and 'b2'. #
############################################################################
self.params['W1'] = weight_scale*np.random.randn(input_dim,hidden_dim)
self.params['b1'] = np.zeros(hidden_dim)
self.params['W2'] = weight_scale*np.random.randn(hidden_dim,num_classes)
self.params['b2'] = np.zeros(num_classes)
############################################################################
# END OF YOUR CODE #
############################################################################
def loss(self, X, y=None):
"""
Compute loss and gradient for a minibatch of data.
Inputs:
- X: Array of input data of shape (N, d_1, ..., d_k)
- y: Array of labels, of shape (N,). y[i] gives the label for X[i].
Returns:
If y is None, then run a test-time forward pass of the model and return:
- scores: Array of shape (N, C) giving classification scores, where
scores[i, c] is the classification score for X[i] and class c.
If y is not None, then run a training-time forward and backward pass and
return a tuple of:
- loss: Scalar value giving the loss
- grads: Dictionary with the same keys as self.params, mapping parameter
names to gradients of the loss with respect to those parameters.
"""
scores = None
############################################################################
# TODO: Implement the forward pass for the two-layer net, computing the #
# class scores for X and storing them in the scores variable. #
############################################################################
a1,cache1 = affine_relu_forward(X,self.params['W1'],self.params['b1'])
scores,cache2 = affine_forward(a1,self.params['W2'],self.params['b2'])
#求最终scores的过程中，使用cache记录下一些中间计算结果，以便后面求梯度
############################################################################
# END OF YOUR CODE #
############################################################################
# If y is None then we are in test mode so just return scores
if y is None:
return scores
loss, grads = 0, {}
############################################################################
# TODO: Implement the backward pass for the two-layer net. Store the loss #
# in the loss variable and gradients in the grads dictionary. Compute data #
# loss using softmax, and make sure that grads[k] holds the gradients for #
# self.params[k]. Don't forget to add L2 regularization! #
# #
# NOTE: To ensure that your implementation matches ours and you pass the #
# automated tests, make sure that your L2 regularization includes a factor #
# of 0.5 to simplify the expression for the gradient. #
############################################################################
loss_without_reg, dscores = softmax_loss(scores, y)
loss = loss_without_reg + 0.5 * self.reg * (np.sum(self.params['W1']**2) + \
np.sum(self.params['W2']**2))
da2, grads['W2'], grads['b2'] = affine_backward(dscores, cache2)
#
grads['W2'] += self.reg*cache2[1]
dx, grads['W1'], grads['b1'] = affine_relu_backward(da2, cache1)
grads['W1'] += self.reg*cache1[0][1]
############################################################################
# END OF YOUR CODE #
############################################################################
return loss, grads