Assigning a function that does not exist to a new function?

Hello everyone,

I new to Matlab and I am trying to understand the implementation of the following program.

http://www.miketipping.com/downloads.htm

In this program function CONTROLS is taking the value of SB2_ControlSettings; however, SB2_ControlSettings is not defined anywhere. The name of the file is SB2_ControlSettings.m though. Then the program goes about using every variable of this newly created function and use it as members of a class.

% SB2_CONTROLSETTINGS  Set parameters to control the SPARSEBAYES algorithm
function CONTROLS = SB2_ControlSettings
%%Define parameters which influence the underlying operation of the 
%%SparseBayes inference algorithm

 % TOLERANCES
% 
% Any Q^2-S "relevance factor" less than this is considered to be zero
% 
CONTROLS.ZeroFactor      = 1e-12;
%
% If the change in log-alpha for the best re-estimation is less than this,
% we consider termination
% 
CONTROLS.MinDeltaLogAlpha  = 1e-3;

If anybody could help me, I would highly appreciate it.

Have a wonderful day :),

Answer 1

[Aquatris]

In Matlab functions are defined as

function y = funName(x)

which takes the input x and outputs y. So in the pasted code, the function is called "SB2_ControlSettings" and does not take any input. The output is the "Controls" variable. I believe this function is just to adjust settings for further in the code.

[Romeo Acevedo]

Thanks, Aquatris for your answer! Would you mind clarifying something for me further?

I am still confused on why CONTROLS is used as a class object later on (with all variables being declared as CONTROLS.x or CONTROLS.y .

Is this function declaration another way of declaring a class object?

[Aquatris]

In Matlab the "CONTROLS" variable is a structure variable, not a class. The structure variables are usually preferred when you want to store several related variables within a single variable.

Keep in mind this is not C language and do not assume the syntax is the same :)

Answer 2

[Stephen23]

Your misunderstand several fundamental aspects of MATLAB functions and variable assignment.

"In this program function CONTROLS is taking the value of SB2_ControlSettings;"

No, The function you have shown is actually SB2_ControlSettings. It returns one output argument, which is named Controls. How to define functions is clearly explained in the documentation:

https://www.mathworks.com/help/matlab/ref/function.html

"however, SB2_ControlSettings is not defined anywhere."

The name of the function is always defined on the line with function, just like with your code.

"The name of the file is SB2_ControlSettings.m though."

Yes, and that is the name of the function. These should be the same (and I highly recommend that they are).

"Then the program goes about using every variable of this newly created function and use it as members of a class."

No, there is no class being defined here. The variable Controls is a structure:

https://www.mathworks.com/help/matlab/ref/struct.html

The structure Controls does not exist at the start of the function, but it is implicitly created by the first time it is allocated to.

[Romeo Acevedo]

Thanks, Stephen Cobeldick for your enlightening answer! There is still one aspect of this function that still confuses me.

You addressed that this function is also a structure; however, I thought that structures needed the keyword "struct" to have correctly Matlab syntax. Would you mind clarifying for me why is this case different?

Thanks again :)

[Stephen23]

"...I thought that structures needed the keyword "struct" to have correctly Matlab syntax."

Using struct is just one way of creating a structure. The documentation shows that it is possible to implicitly create a structure simply by allocating to it:

https://www.mathworks.com/help/matlab/matlab_prog/create-a-structure-array.html

"Would you mind clarifying for me why is this case different?"

The answer to your question is "by definition": because that is what MATLAB is designed to do.