matlab,machine-learning , Why can't I calculate CostFunction J

## Question:

Tag: matlab,machine-learning

This is my implementation of `CostFunctionJ`:

``````    function J = CostFunctionJ(X,y,theta)

m = size(X,1);
predictions = X*theta;
sqrErrors =(predictions - y).^2;

J = 1/(2*m)* sum(sqrErrors);
``````

But when I try to enter the command in MATLAB as:

``````>> X = [1 1; 1 2; 1 3];
>> y = [1; 2; 3];
>> theta = [0,1];
>> J = CostFunctionJ(X,y,theta)
``````

It gives me the following error:

``````Error using  *
Inner matrix dimensions must agree.
Error in CostFunctionJ (line 4)
predictions = X*theta;
``````

As suggested in the comments, the error is because `x` is of dimension `3x2` and `theta` of dimension `1x2`, so you can't do `X*theta`.

I suspect you want:

``````theta = [0;1]; % note the ; instead of ,
% theta is now of dimension 2x1
% X*theta is now a legit operation
``````

which would result in:

``````>> predictions = X*theta
predictions =

1
2
3
``````

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