【吳恩達】機器學習ex3程式設計練習
阿新 • • 發佈:2018-12-10
1.
function [J, grad] = lrCostFunction(theta, X, y, lambda) %LRCOSTFUNCTION Compute cost and gradient for logistic regression with %regularization % J = LRCOSTFUNCTION(theta, X, y, lambda) computes the cost of using % theta as the parameter for regularized logistic regression and the % gradient of the cost w.r.t. to the parameters. % Initialize some useful values m = length(y); % number of training examples % You need to return the following variables correctly J = 0; grad = zeros(size(theta)); % ====================== YOUR CODE HERE ====================== % Instructions: Compute the cost of a particular choice of theta. % You should set J to the cost. % Compute the partial derivatives and set grad to the partial % derivatives of the cost w.r.t. each parameter in theta % % Hint: The computation of the cost function and gradients can be % efficiently vectorized. For example, consider the computation % % sigmoid(X * theta) % % Each row of the resulting matrix will contain the value of the % prediction for that example. You can make use of this to vectorize % the cost function and gradient computations. % % Hint: When computing the gradient of the regularized cost function, % there're many possible vectorized solutions, but one solution % looks like: % grad = (unregularized gradient for logistic regression) % temp = theta; % temp(1) = 0; % because we don't add anything for j = 0 % grad = grad + YOUR_CODE_HERE (using the temp variable) % Z=sum((theta'.*X)')';%依然是X的維度 h=sigmoid(Z); J=sum((-y)'*log(h)-(1-y)'*log(1-h))/m; J=J+lambda*sum(theta([2:end],:).^2)/m/2; grad1=X(:,1)'*(h-y)/m; grad2=X(:,[2:size(theta)])'*(h-y)/m+lambda*theta([2:size(theta)],:)/m; grad=[grad1,grad2']; grad = grad(:); end
總結:在程式設計的時候,遇到了一個問題就是Z的求解時,應該是theta向量對應到每個資料集的對應特徵相乘,所以需要用到.*的形式。除此之外,在grad求解的時候,X不是整個X,而是每個對應有一個特徵,也就是資料集的一列。
2.
function [all_theta] = oneVsAll(X, y, num_labels, lambda) %ONEVSALL trains multiple logistic regression classifiers and returns all %the classifiers in a matrix all_theta, where the i-th row of all_theta %corresponds to the classifier for label i % [all_theta] = ONEVSALL(X, y, num_labels, lambda) trains num_labels % logistic regression classifiers and returns each of these classifiers % in a matrix all_theta, where the i-th row of all_theta corresponds % to the classifier for label i % Some useful variables m = size(X, 1); n = size(X, 2); initial_theta = zeros(n + 1, 1); % You need to return the following variables correctly all_theta = zeros(num_labels, n + 1); options = optimset('GradObj', 'on', 'MaxIter', 50); % Add ones to the X data matrix X = [ones(m, 1) X]; % ====================== YOUR CODE HERE ====================== % Instructions: You should complete the following code to train num_labels % logistic regression classifiers with regularization % parameter lambda. % % Hint: theta(:) will return a column vector. % % Hint: You can use y == c to obtain a vector of 1's and 0's that tell you % whether the ground truth is true/false for this class. % % Note: For this assignment, we recommend using fmincg to optimize the cost % function. It is okay to use a for-loop (for c = 1:num_labels) to % loop over the different classes. % % fmincg works similarly to fminunc, but is more efficient when we % are dealing with large number of parameters. % % Example Code for fmincg: % % % Set Initial theta % initial_theta = zeros(n + 1, 1); % % % Set options for fminunc % options = optimset('GradObj', 'on', 'MaxIter', 50); % % % Run fmincg to obtain the optimal theta % % This function will return theta and the cost % [theta] = ... % fmincg (@(t)(lrCostFunction(t, X, (y == c), lambda)), ... % initial_theta, options); % for i=1:num_labels, initial_theta = zeros(n + 1, 1); options = optimset('GradObj', 'on', 'MaxIter', 50); [theta] =fmincg (@(t)(lrCostFunction(t, X, (y == i), lambda)),initial_theta, options); all_theta(i,:)=theta'; end end
總結:這裡遇到的問題是如何求解theta。在我們求得J,grad之後,可以使用優化演算法一步步迭代求得theta。因此這裡其實就是每個分類呼叫優化演算法即可。但是要注意,優化演算法的引數在每一次分類呼叫時的初始化問題。因為我們在解析問題的時候,每一個分類對於其他分類來說是完全獨立的。
3.預測
function p = predictOneVsAll(all_theta, X) %PREDICT Predict the label for a trained one-vs-all classifier. The labels %are in the range 1..K, where K = size(all_theta, 1). % p = PREDICTONEVSALL(all_theta, X) will return a vector of predictions % for each example in the matrix X. Note that X contains the examples in % rows. all_theta is a matrix where the i-th row is a trained logistic % regression theta vector for the i-th class. You should set p to a vector % of values from 1..K (e.g., p = [1; 3; 1; 2] predicts classes 1, 3, 1, 2 % for 4 examples) m = size(X, 1); num_labels = size(all_theta, 1); % You need to return the following variables correctly p = zeros(size(X, 1), 1); % Add ones to the X data matrix X = [ones(m, 1) X]; % ====================== YOUR CODE HERE ====================== % Instructions: Complete the following code to make predictions using % your learned logistic regression parameters (one-vs-all). % You should set p to a vector of predictions (from 1 to % num_labels). % % Hint: This code can be done all vectorized using the max function. % In particular, the max function can also return the index of the % max element, for more information see 'help max'. If your examples % are in rows, then, you can use max(A, [], 2) to obtain the max % for each row. % h=X*all_theta'; h_max=max(h,[],2); for i=1:size(X,1), for j=1:num_labels, if h_max(i)==h(i,j), p(i)=j; break; end end end
function p = predict(Theta1, Theta2, X)
%PREDICT Predict the label of an input given a trained neural network
% p = PREDICT(Theta1, Theta2, X) outputs the predicted label of X given the
% trained weights of a neural network (Theta1, Theta2)
% Useful values
m = size(X, 1);
num_labels = size(Theta2, 1);
% You need to return the following variables correctly
p = zeros(size(X, 1), 1);
% ====================== YOUR CODE HERE ======================
% Instructions: Complete the following code to make predictions using
% your learned neural network. You should set p to a
% vector containing labels between 1 to num_labels.
%
% Hint: The max function might come in useful. In particular, the max
% function can also return the index of the max element, for more
% information see 'help max'. If your examples are in rows, then, you
% can use max(A, [], 2) to obtain the max for each row.
%
X=[ones(size(X,1),1),X];
a1=sigmoid(X*Theta1');
a1=[ones(size(a1,1),1),a1];
a2=sigmoid(a1*Theta2');
a3=max(a2,[],2);
for i=1:size(X,1),
for j=1:num_labels,
if a3(i)==a2(i,j),
p(i)=j;
break;
end
end
end
總結:多分類問題,就是選取n個分類總概率最大的那一個作為最終結果。max[A,[],2]是求解A矩陣中每一行最大的值。A(1,2)表示第一行第二列對應的值。