逻辑回归成本函数

问题描述

function [J, grad] = costFunction(theta, X, y)
m = length(y);
h = sigmoid(X*theta);
sh = sigmoid(h);
grad = (1/m)*X'*(sh - y);
J = (1/m)*sum(-y.*log(sh) - (1 - y).*log(1 - sh));

end

我正在尝试为Logistic回归计算成本函数.有人可以告诉我为什么这不正确吗?

I'm trying to compute the cost function for logistic regression. Can someone please tell me why this isn't accurate?

更新:乙状结肠功能

function g = sigmoid(z)

g = zeros(size(z));
g = 1./(1 + exp(1).^(-z));

end

推荐答案

如Dan所述，您的costFunction调用了Sigmoid两次.首先，它在X*theta上执行sigmoid函数；然后它对sigmoid(X*theta)的结果再次执行S型函数.因此，sh = sigmoid(sigmoid(X*theta)).您的cost函数应该只调用一次sigmoid函数.

As Dan stated, your costFunction calls sigmoid twice. First, it performs the sigmoid function on X*theta; then it performs the sigmoid function again on the result of sigmoid(X*theta). Thus, sh = sigmoid(sigmoid(X*theta)). Your cost function should only call the sigmoid function once.

请参见下面的代码，我删除了sh变量，并在其他所有地方都将其替换为h.这样会使Sigmoid函数仅被调用一次.

See the code below, I removed the sh variable and replaced it with h everywhere else. This causes the sigmoid function to only be called once.

function [J, grad] = costFunction(theta, X, y)
m = length(y);
h = sigmoid(X*theta);
grad = (1/m)*X'*(h - y);
J = (1/m)*sum(-y.*log(h) - (1 - y).*log(1 - h));

end

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