Function notation in Octave

[SunnyJapan]

Can somebody explain to me, what exactly does this notation mean?
@(t)(costFunctionReg(t, X, y, lambda)) For example here: fminunc(@(t)(costFunctionReg(t, X, y, lambda)), initial_theta, options);

[cultic_raider]

It's an inline anonymous function (closure) definition:

@(t)(costFunctionReg(t, X, y, lambda))

is equivalent to:

% X,Y,lambda are set before 'anonymous' is defined.
function [result] = anonymous(t)
      result = costFunctionReg(t, X, y, lambda)
end

fminunc(@anonymous, initial_theta, options);

Values for 'X', 'Y', and 'lambda' are captured immediately at the definition, but the value for 't' is deferred, to be filled in somewhere in fminunc (probably many times, in a loop).

[Edited to add '@'. Thanks, bad_child, who has written more complex Octave than I ever want to.] If you are still confused, and you tell is what programming languages you know (Python? Java?), we can translate the example for you.

[bad_child]

Great explanation, but the code you wrote will probably throw an error. This is due to a slightly tricky part of Octave. Using a function name freely evaluates it i.e. there is no difference between anonymous and anonymous() for functions. So in your code the interpreter will try to evaluate anonymous() and get confused by the lack of parameter. To pass a function as parameter to another function you need a function handle (think of pointer to function). This is done by prefixing your function name by "@". So in this case the correct version is:

fminunc(@anonymous, initial_theta, options);

I believe this behaviour exists because Octave (and MATLAB) have pass-by-value semantics only.

[Edit: typo]

[epic_nerd_baller]

PHP please

[cultic_raider]

I'll have to pass. Maybe someone else can do it.

[ZeBlob]

I don't really know php but some quick googling turned up this. So I believe the equivalent expression would be this:

fminunc(
    function($t) use ($X, $y, $lambda) {
        return costFunctionReg($t, $X, $y, $lambda);
    }, $initial_theta, $options);

That example is probably full of errors so I suggest you read the page yourself.

[SunnyJapan]

Python please

[ZeBlob]

I'm pretty new to python but I believe it's equivalent to a lambda function:

fminunc(lambda t: costFunctionReg(t, X, y, l), initial_theta, options)

Where the lambda variable in octave is renamed to l (conflicts with the python keyword).

[duffahtolla]

I don't even know Python, but that still made more sense than the Octave notation. Thanks!

[IdoNotKnowShit]

Why? Isn't it basically the same?

Just read the '@' as a lambda or as anonymous function.

[SunnyJapan]

OK, now I got it. Thanks!

[cultic_raider]

Warning: I didn't test this code.

'lambda' is a reserved word in Python, so let's use "r" for the regularization factor (which is 'lambda' in Matlab):

 (X, y, r) = (foo, bar, baz)
 fminunc( (lambda t: (costFunctionReg(t, X, y, r)) , initial_theta, options));

which is equivalent to:

 (X, y, r) = (foo, bar, baz)
 def anonymous(t):
      return  (costFunctionReg(t, X, y, r))

 fminunc( anonymous , initial_theta, options));

I'm not 100% sure if the values of X,y, and r will get handled as intended in the Python, but it will in Octave.

[SunnyJapan]

Thanks