MATLAB: How to set an optional function input as a first input

inputparserinputsMATLABmatlab function

I am trying to make a function with syntax similar to plot(ax, …), where the handle to an axis can be included before the required inputs, but is optional. Such as:
function add2plot(ax, x, y, varargin)
Everything I have found forces the required inputs to be first, including inputParser and manual parsing of varargin. So how does plot() do it?? I am using ver. 2013b, where the handle is a double, and thus hard to distinguish from my numerical x input. The addition of varargin makes using nargin complicated.
Any input is appreciated!

Best Answer

You can pass whatever you want to the parser; it doesn't get automatically what is passed to the function, but works on what you pass to its parse method. You have therefore the freedom to do something like:
 function foo( varargin )
    % - Extract first optional axes handle. Remove if present.
    if ~isempty( varargin ) && ishandle( varargin{1} )
       ax = varargin{1} ;
       varargin = varargin(2:end) ;
    else
       ax = [] ;
    end
    % - Build input parser and parse remaining args.
    parser = inputParser ;
    parser.StructExpand  = true ;
    parser.CaseSensitive = false ;
    parser.addRequired( .. ) ;
    .. etc.
    parser.parse( varargin{:} ) ;  % Pass only remaining args.
    args = parser.Results ;
    % - ...
 end
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