How to compute Cost function for linear regression

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Muhammad Kundi
Muhammad Kundi 2019-6-22
评论： Abdulwaliyi 2022-12-12
Hi,
I am trying to compute cost function
I am using the following code:
function J = computeCost(X, y, theta)
%COMPUTECOST Compute cost for linear regression
% J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
% parameter for linear regression to fit the data points in X and y
% Initialize some useful values
m = length(y); % number of training examples
% You need to return the following variables correctly
J = 0;
% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
% You should set J to the cost.
h = X * theta;
sError = (h - y) .^ 2;
J = sum(sError) / (2 * m);
% =========================================================================
end
.But I am getting the following error
Error in computeCost (line 7)
m = length(y); % number of training examples
.Please help
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Muhammad Kundi
Muhammad Kundi 2019-6-23
sorry...I forgot to mention...I just hit run to execute the program and I get this error message..
Do I have to run the function by inserting the values of x,y and theta???

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回答（10 个）

João Marlon Souto Ferraz
I believe than this is happen because you are running and trying to compile the function. Just save and to call than function from the other script.
Mahdi Badriyan
Mahdi Badriyan 2020-5-25
hi
J = sum(sError) / (2 .* m);
in this way your code will be corrected.
Sanskar Choudhary
Sanskar Choudhary 2021-12-28
open file submit.m and then run it.(don't run computeCost.m file, this will not work because its just function)
Ghayyur Abbas
Ghayyur Abbas 2019-8-29
initialize x y theta with values in the function
Heng Dao
Heng Dao 2021-2-6
((h)-y).^2 Try try hope can help u
Prasad Kute
Prasad Kute 2021-7-23
function J = computeCost(X, y, theta)
%COMPUTECOST Compute cost for linear regression
% J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
% parameter for linear regression to fit the data points in X and y
% Initialize some useful values
m = length(y); % number of training examples
% You need to return the following variables correctly
J = 0;
% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
% You should set J to the cost.
J = (1/(2*m))*sum(((X*theta) - y).^2)
Try this, you will get it correct.
Image Analyst
Image Analyst 2022-2-4
You say "sorry...I forgot to mention...I just hit run to execute the program and I get this error message.."
Well that is the whole problem. When you have a function that expects inputs like X, y, theta then you need to supply those inputs. You can't just click the green run triangle and expect somehow that doing that will automagically invent values for X, y, and theta. So you need to do something like
% Declare inputs:
X = rand(1,10);
y = rand(1, 10);
theta = 42;
% Now run the function:
result = computeCost(X, y, theta)
fprintf('Done with the program!\n');
%==========================================================================================
% Define the function:
function J = computeCost(X, y, theta)
%COMPUTECOST Compute cost for linear regression
% J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
% parameter for linear regression to fit the data points in X and y
% Initialize some useful values
m = length(y); % number of training examples
% You need to return the following variables correctly
J = 0;
% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
% You should set J to the cost.
h = X * theta;
sError = (h - y) .^ 2;
J = sum(sError) / (2 * m);
end
Image Analyst
Image Analyst 2022-2-6
As long as y is defined (like you assigned something to y before you called the function) then that line
m = length(y); % number of training examples
should work. However you didn't give the entire error message. You gave the line the error occurred on but not the actual error description. Why not? You need to include all the red text, not just some of it.
By the way you need to assign something for y, you can't just click the green Run triangle on the toolbar without assigning all the input variables.
Jia He
Jia He 2022-2-6
If you meet errors in submitting, and your function is correct. Try to delete its .mlx file and submit it again.
MAZEN ALHARBI
MAZEN ALHARBI 2022-4-5
function J = computeCost(X, y, theta)
%COMPUTECOST Compute cost for linear regression
% J = COMPUTECOST(X, y, theta) computes the cost of using theta as the
% parameter for linear regression to fit the data points in X and y
% Initialize some useful values
% number of training examples
% You need to return the following variables correctly
data = load('ex1data1.txt'); % read comma separated data
y = data(:, 2);
m = length(y); % number of training examples
X = [ones(m, 1), data(:,1)]; % Add a column of ones to x
theta = zeros(2, 1);
% ====================== YOUR CODE HERE ======================
% Instructions: Compute the cost of a particular choice of theta
% You should set J to the cost.
J = 0;
J = 1/(2*m) * (X * theta - y)' * (X * theta - y); % vectorized form
% =========================================================================
end

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