How to set an optional function input as a first input?

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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!

Accepted Answer

Cedric Wannaz
Cedric Wannaz on 11 Sep 2015
Edited: Cedric Wannaz on 11 Sep 2015
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) ;
ax = [] ;
% - 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 ;
% - ...
Cedric Wannaz
Cedric Wannaz on 12 Sep 2015
Thank you Walter, I had missed this information. It makes me realize that I should make a synthesis of all releases information since .. a long time ago, and scan/update/refactor a lot of projects!

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