How to create an optional input parameter with special name?

I would like to use an optional input parameter. As far as I know, the standard solution is:

function myfunc(param1,param2,varargin)

however this is very ugly, because param1 and param2 can be named practically, but varargin is a system defined name. I understand, that in case of variable number of optional arguments this is not important, but for only one possible optional argument it would be practical to use a freely defined name. It is possible to solve this syntax problem somehow?

Answer 1

[Geoff Hayes]

Why not just call your optional third parameter as param3 and then check to see if it has been passed in or not? If the latter then you could initialize it to some default value. Something like

 function myFunc(param1,param2,param3)

 if ~exist('param3','var')
     % third parameter does not exist, so default it to something
      param3 = 42;
 end

 % remaining code

The above code checks to see if the third input exists or not. If not, then it is created and assigned a default value. The user can call your function as

 myFunc(12, 23);

or as

 myFunc(12,23,34);

[Stephen23]

Using nargin is much much faster than using exist.

[Richard]

What if I want to call

myFunc(param1,param3)

, assuming that I set up all three parameters as optional using

if ~exist('paramX','var')

statement?

Can I do something like this?

myFunc(param1=value1, param3=value3)

[Walter Roberson]

The matching of arguments to variables is done positionally, so there is no way to skip an argument.

One common method is to allow the user to pass [] to indicate a skipped argument (that will either not be used or will get a default value).

[Aaron Mailhot]

@Richard: if you want to have 'arbitrary variance' in multiple optional parameters, I suggest arranging your function to utilize parameter-value pairs.

Its a little convoluted, and there are probably utilities to help with it, BUT it's not too bad to arrange it yourself directly at the end of the day. E.g. call it this way (where 10 and 30 are the e.g. specific values you want to use):

myFunc('-param1', 10, '-param3', 30)

And then the top of your function would go something like:

function test_params(varargin)

    % Defaults
    param1 = 1;
    param2 = 2;
    param3 = 3;
    
    % Optionals
    for ii = 1:2:nargin
        if strcmp('-param1', varargin{ii})
            param1 = varargin{ii+1};
        elseif strcmp('-param2', varargin{ii})
            param2 = varargin{ii+1};
        elseif strcmp('-param3', varargin{ii})
            param3 = varargin{ii+1};
        end
    end
    
    % Test!
    disp(param1)
    disp(param2)
    disp(param3)
    
    % More stuff here
    % ...
    
end

[Aaron Mailhot]

For fun, you can even go nuts and call this with 'param3' first followed by 'param1'; it really is just 'users choice' now :).

Answer 2

[Martin Krajda]

See InputParser

https://de.mathworks.com/help/matlab/ref/inputparser.html