Correct use of tilde operator for input arguments

Question

Function:

My MATLAB function has one output and several input arguments, most of which are optional, i.e.:

output=MyFunction(arg1,arg2,opt1,opt2,...,optN)

What I want to do:

I'd like to give only arg1, arg2 and the last optional input argument optN to the function. I used the tilde operator as follows:

output=MyFunction(str1,str2,~,~,...,true)

Undesired result:

That gives the following error message:

Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.

The error points to the comma after the first tilde, but I don't know what to make of it to be honest.

Problem identification:

• I use MATLAB 2013b, which supports the tilde operator.

• According to MATLAB's documentation the above function call should work:

  You can ignore any number of function inputs, in any position in the argument list. Separate consecutive tildes with a comma...

• I guess there are a few workarounds, such as using '' or [] as inputs, but I'd really like to understand how to correctly use '~' because actually leaving inputs out allows me to use exist() when checking the input arguments of a function.

If you need any further info from me, please let me know.

Thank you very much!

Answer 1

The tilde is only for function declaration. Matlab's mlint recommends to replace unused arguments by ~. The result is a function declared like this function output = MyFunction(a, b, ~, c). This is a very bad practice.

Since you have a function where the parameters are optional, you must call the function with empty arguments output=MyFunction(str1,str2,[],[],...,true).

A better way to do it is to declare the function with the varargin argument and prepare your function for the different inputs:

function output = MyFunction(varargin)

if nargin == 1
    % Do something for 1 input
elseif nargin == 2
    % Do something for 3 inputs
elseif nargin == 3
    % Do something for 3 inputs
else
    error('incorrect number of input arguments')
end

It is even possible to declare your function as follows:

function output = MyFunction(arg1, arg2, varargin)

The declaration above will tell Matlab that you are expecting at least two parameters.

See the documentation of nargin here.

... and the documentation of varargin here

Answer 2

To have variable number of inputs, use varargin. Use it together with nargin.

Example:

function varlist2(X,Y,varargin)
   fprintf('Total number of inputs = %d\n',nargin);

   nVarargs = length(varargin);
   fprintf('Inputs in varargin(%d):\n',nVarargs)
   for k = 1:nVarargs
      fprintf('   %d\n', varargin{k})
   end