MATLAB fminunc停止了，因为它不能减少目标公式？

Question

我正在尝试使用fminunc来获得逻辑回归中的最佳theta，但是我不断得到：

  

fminunc已停止，因为它无法减少目标函数   沿当前搜索方向。

我在网上搜索时发现，这通常是我在logistic_costFunction.m中实现的渐变错误的结果。我重新检查了工作，但找不到根本原因。

我不确定如何解决此问题，我们将不胜感激。

---

这是我的代码：

clear all; close all; clc;

%% Plotting data
x1 = linspace(0,3,50);
mqtrue = 5;
cqtrue = 30;
dat1 = mqtrue*x1+5*randn(1,50);

x2 = linspace(7,10,50);
dat2 = mqtrue*x2 + (cqtrue + 5*randn(1,50));

x = [x1 x2]'; % X

subplot(2,2,1);
dat = [dat1 dat2]'; % Y

scatter(x1, dat1); hold on;
scatter(x2, dat2, '*'); hold on;
classdata = (dat>40);

%% Compute Cost and Gradient

%  Setup the data matrix appropriately, and add ones for the intercept term
[m, n] = size(x);

% Add intercept term to x and X_test
x = [ones(m, 1) x];

% Initialize fitting parameters
initial_theta = zeros(n + 1, 1);

% Compute and display initial cost and gradient
[cost, grad] = logistic_costFunction(initial_theta, x, dat);

fprintf('Cost at initial theta (zeros): %f\n', cost);
fprintf('Gradient at initial theta (zeros): \n');
fprintf(' %f \n', grad);

%% ============= Part 3: Optimizing using fminunc  =============
%  In this exercise, you will use a built-in function (fminunc) to find the
%  optimal parameters theta.

%  Set options for fminunc
options = optimset('GradObj', 'on', 'MaxIter', 400);

%  Run fminunc to obtain the optimal theta
%  This function will return theta and the cost 
[theta, cost] = ...
    fminunc(@(t)(logistic_costFunction(t, x, dat)), initial_theta, options);

logistic_costFunction.m

function [J, grad] = logistic_costFunction(theta, X, y)

% Initialize some useful values
m = length(y); % number of training examples

grad = zeros(size(theta));

H = sigmoid(X*theta);
T = y.*log(H) + (1 - y).*log(1 - H);
J = -1/m*sum(T);

% ====================Compute grad==================

for i = 1 : m
    grad = grad + (H(i) - y(i)) * X(i,:)';
end

grad = 1/m*grad;

end

sigmoid.m

function g = sigmoid(z)
% Computes thes sigmoid of z

g = zeros(size(z));

g = 1 ./ (1 + (1 ./ exp(z)));

end