Matlab 一次回归与全回归中函数句柄的混淆
我正在参加Andrew Ng在coursera上的机器学习课程,我很困惑为什么一个特定的例子适用于“一对所有”分类:Matlab 一次回归与全回归中函数句柄的混淆,matlab,machine-learning,function-handle,Matlab,Machine Learning,Function Handle,我正在参加Andrew Ng在coursera上的机器学习课程,我很困惑为什么一个特定的例子适用于“一对所有”分类: 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
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
% logisitc 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);
% You need to return the following variables correctly
all_theta = zeros(num_labels, n + 1);
% 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 use
% 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);
%
initial_theta = zeros(n + 1, 1);
options = optimset('GradObj', 'on', 'MaxIter', 50);
for i = 1:num_labels
c = i * ones(size(y));
fprintf('valores')
[theta] = fmincg (@(t)(lrCostFunction(t, X, (y == c), lambda)), initial_theta, options);
all_theta(i,:) = theta;
end
% =========================================================================
end
我对这一行特别困惑:[theta]=fmincg(@(t)(lrCostFunction(t,X,(y==c,lambda)),initial_theta,options)代码>lrCostFunction
被定义为具有参数θ、X、y、lambda
,因此我不知道t
在那里做什么
另外,将θ
括在括号中的目的是什么:[theta]
任何有关单步执行此代码的帮助都将非常有用。谢谢。您正在查看一行,其中定义了匿名函数。匿名函数类似于函数的简写定义,以
@
开头,后跟此函数的参数(在您的示例中是t
)。此参数t
作为第一个参数传递给函数lrCostFunction()
,实际上是theta
参数。也就是说,你要求函数fmincg()
最小化这个匿名函数的输出,这个匿名函数是围绕lrCostFunction()
的一个包装器,这样你就可以在使用匿名函数定义之外定义的X
、y
和lambda
的同时最小化过θ
为了更好地理解匿名函数,可以拆分代码:
func_handle = @(t)(lrCostFunction(t, X, (y == c), lambda)) % anonymous function
func_handle(initial_theta); % returns the cost at the initial_theta
[theta] = fmincg(func_handle, initial_theta, options);
有关匿名函数的详细信息,请参见官方网站
theta
周围的括号是多余的。Andrew Ng在他的作业以及他的倍频程/MATLAB教程中对此进行了深入的阐述。还要检查副本。此外,用[]
包围的theta
是多余的。通常情况下,您有一个可以返回多个输出的函数,因此您通常会在逗号分隔的列表中包含希望由周围的[]
捕获的每个变量。在这种情况下,函数只返回一个变量,因此MATLAB中的语法糖允许我们在一个变量的情况下删除[]
。否则,用[]
包围输出变量并省略它们并没有什么区别。