I got the wrong answer even my code is right. Please help!

Question

Hai Luu 2020-2-8

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/504314-i-got-the-wrong-answer-even-my-code-is-right-please-help

评论： Walter Roberson 2020-4-17

--------------Main program---------------------------------

X = [ones(m, 1), data(:,1)]; % Add a column of ones to x
theta = zeros(2, 1); % initialize fitting parameters
% Some gradient descent settings
iterations = 1500;
alpha = 0.01;
fprintf('\nTesting the cost function ...\n')
% compute and display initial cost
J = computeCost(X, y, theta);
fprintf('With theta = [0 ; 0]\nCost computed = %f\n', J);
fprintf('Expected cost value (approx) 32.07\n');

--------------Calculating function J---------------------------------

function J = computeCost(X, y, theta)
m = length(y); 
h = X * theta;
J = (1/(2*m))*(sum(h-y).^2);
end

-------------------Here is my result----------------------------------------

Testing the cost function ...

With theta = [0 ; 0]

Cost computed = 1653.631660------------------> my result

Expected cost value (approx) 32.07 ----------------> this is the right answer

With theta = [-1 ; 2]

Cost computed = 4359.141958------------------->my result

Expected cost value (approx) 54.24----------------->this is the right answer

------------------------------------------------------------

17 个评论
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Walter Roberson 2020-2-9

No, h0(x) is defined in terms of the model theta_0 + theta_1*x_1 at the bottom of page 5. This conflicts with your implementation of h = X * theta in that the model does not multiply theta_0 by any X component.

But with you initializing theta to [0 0] then it is temporarily irrelevant, as you are going to have h0(x) being 0 for all x.

Using theta_0 conflicts with the notation of the definition of J in the middle of page 5, which clearly shows Sigma i = 1 to something, with i being used as the index of some variables. It is a conflict because i = 1 to something tells us that the lowest index is 1, not 0 -- unless, that is, for some hidden reason you are intended to ignore the first x entry, which would be unusual. But if the index is intended to start from 0 then the upper bound would have to be length minus 1, but that is not the case: the upper bound is the length itself. i = 1 to the length uses all of the values in a natural way -- but it does tell us that 1 is the first index, so theta_0 does not exist.

This conflict with the definition of h0 requires clarification from your instructor -- because if you interpret in the natural way that you did, you cannot possibly get the results that they want you to get.

supun dassanayake 2020-4-17

编辑：Walter Roberson 2020-4-17

在 MATLAB Online 中打开

The X val are:-

 X = [
          1.00          6.11
          1.00          5.53
          1.00          8.52
          1.00          7.00
          1.00          5.86
          1.00          8.38
          1.00          7.48
          1.00          8.58
          1.00          6.49
          1.00          5.05
          1.00          5.71
          1.00         14.16
          1.00          5.73
          1.00          8.41
          1.00          5.64
          1.00          5.38
          1.00          6.37
          1.00          5.13
          1.00          6.43
          1.00          7.07
          1.00          6.19
          1.00         20.27
          1.00          5.49
          1.00          6.33
          1.00          5.56
          1.00         18.95
          1.00         12.83
          1.00         10.96
          1.00         13.18
          1.00         22.20
          1.00          5.25
          1.00          6.59
          1.00          9.25
          1.00          5.89
          1.00          8.21
          1.00          7.93
          1.00          8.10
          1.00          5.61
          1.00         12.84
          1.00          6.35
          1.00          5.41
          1.00          6.88
          1.00         11.71
          1.00          5.77
          1.00          7.82
          1.00          7.09
          1.00          5.07
          1.00          5.80
          1.00         11.70
          1.00          5.54
          1.00          7.54
          1.00          5.31
          1.00          7.42
          1.00          7.60
          1.00          6.33
          1.00          6.36
          1.00          6.27
          1.00          5.64
          1.00          9.31
          1.00          9.45
          1.00          8.83
          1.00          5.18
          1.00         21.28
          1.00         14.91
          1.00         18.96
          1.00          7.22
          1.00          8.30
          1.00         10.24
          1.00          5.50
          1.00         20.34
          1.00         10.14
          1.00          7.33
          1.00          6.01
          1.00          7.23
          1.00          5.03
          1.00          6.55
          1.00          7.54
          1.00          5.04
          1.00         10.27
          1.00          5.11
          1.00          5.73
          1.00          5.19
          1.00          6.36
          1.00          9.77
          1.00          6.52
          1.00          8.52
          1.00          9.18
          1.00          6.00
          1.00          5.52
          1.00          5.06
          1.00          5.71
          1.00          7.64
          1.00          5.87
          1.00          5.31
          1.00          8.29
          1.00         13.39
          1.00          5.44];

Y:-