Supplied objective function must return a scalar value

Question

I am trying to code a ML algorithm in Matlab. These are my different functions:

sigmoid.m:

function g = sigmoid(z)
g = zeros(size(z));
g = 1 ./ (1+exp(z));

costFunction.m

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

m = length(y); % number of training examples
z = -X * theta;
g = sigmoid(z);
J = 1/m * ((-y * log(g)') - ((1 - y) * log(1 - g)'));

grad = zeros(size(theta'));
grad = (1/m) * (X' * (g - y));

ex2.m (This is the main file of my project and I put the relative lines I get this error message)

options = optimset('GradObj', 'on', 'MaxIter', 400);

[theta, cost] = ...
    fminunc(@(t)(costFunction(t, X, y)), initial_theta, options);

The error message:

Error using fminunc (line 348) Supplied objective function must return a scalar value.

Error in ex2 (line 97) fminunc(@(t)(costFunction(t, X, y)), initial_theta, options);

I don't know is there enough information above or not? If not, let me know to add extra information.

Answer 1

I changed the following line of code:

J = 1/m * ((-y * log(g)') - ((1 - y) * log(1 - g)'));

To the following line of code:

J = 1/m * (((-y)' * log(g)) - ((1 - y)' * log(1 - g)));

And problem solved! The y and g were 100*1 matrices and with previous code I had J=100*100 matrix, but with new code I have J=1*1 matrix or scalar number and problem solved!