102 lines
3.7 KiB
Matlab
102 lines
3.7 KiB
Matlab
function [J grad] = nnCostFunction(nn_params, ...
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input_layer_size, ...
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hidden_layer_size, ...
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num_labels, ...
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X, y, lambda)
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%NNCOSTFUNCTION Implements the neural network cost function for a two layer
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%neural network which performs classification
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% [J grad] = NNCOSTFUNCTON(nn_params, hidden_layer_size, num_labels, ...
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% X, y, lambda) computes the cost and gradient of the neural network. The
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% parameters for the neural network are "unrolled" into the vector
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% nn_params and need to be converted back into the weight matrices.
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%
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% The returned parameter grad should be a "unrolled" vector of the
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% partial derivatives of the neural network.
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%
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% Reshape nn_params back into the parameters Theta1 and Theta2, the weight matrices
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% for our 2 layer neural network
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Theta1 = reshape(nn_params(1:hidden_layer_size * (input_layer_size + 1)), ...
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hidden_layer_size, (input_layer_size + 1));
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Theta2 = reshape(nn_params((1 + (hidden_layer_size * (input_layer_size + 1))):end), ...
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num_labels, (hidden_layer_size + 1));
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% Setup some useful variables
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m = size(X, 1);
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% You need to return the following variables correctly
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J = 0;
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Theta1_grad = zeros(size(Theta1));
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Theta2_grad = zeros(size(Theta2));
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% ====================== YOUR CODE HERE ======================
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% Instructions: You should complete the code by working through the
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% following parts.
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%
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% Part 1: Feedforward the neural network and return the cost in the
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% variable J. After implementing Part 1, you can verify that your
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% cost function computation is correct by verifying the cost
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% computed in ex4.m
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%
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yy = zeros(m, num_labels);
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for i = 1:m
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yy(i, y(i)) = 1;
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endfor
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a1 = [ones(m, 1), X];
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z2 = a1*Theta1';
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a2 = [ones(m, 1), sigmoid(z2)];
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z3 = a2*Theta2';
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a3 = hx = sigmoid(z3);
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J = sum(sum(-yy.*log(hx)-(1.-yy).*log(1.-hx)))/m;
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J += lambda*(sum(sum(Theta1(:, 2:end).^2)) + sum(sum(Theta2(:, 2:end).^2)))/(2*m);
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% Part 2: Implement the backpropagation algorithm to compute the gradients
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% Theta1_grad and Theta2_grad. You should return the partial derivatives of
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% the cost function with respect to Theta1 and Theta2 in Theta1_grad and
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% Theta2_grad, respectively. After implementing Part 2, you can check
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% that your implementation is correct by running checkNNGradients
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%
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% Note: The vector y passed into the function is a vector of labels
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% containing values from 1..K. You need to map this vector into a
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% binary vector of 1's and 0's to be used with the neural network
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% cost function.
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%
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% Hint: We recommend implementing backpropagation using a for-loop
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% over the training examples if you are implementing it for the
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% first time.
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%
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delta3 = a3 - yy;
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delta2 = delta3*Theta2.*[ones(m, 1), sigmoidGradient(z2)];
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delta2 = delta2(:, 2:end);
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Theta2_grad += (delta3'*a2)/m;
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Theta1_grad += (delta2'*a1)/m;
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% Part 3: Implement regularization with the cost function and gradients.
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%
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% Hint: You can implement this around the code for
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% backpropagation. That is, you can compute the gradients for
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% the regularization separately and then add them to Theta1_grad
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% and Theta2_grad from Part 2.
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%
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Theta2_grad(:, 2:end) += lambda*Theta2(:, 2:end)/m;
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Theta1_grad(:, 2:end) += lambda*Theta1(:, 2:end)/m;
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% ----------------------------------------------------------
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% =========================================================================
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% Unroll gradients
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grad = [Theta1_grad(:) ; Theta2_grad(:)];
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end
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