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

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!

Answer 1

[Cedric Wannaz]

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

[Jennifer Taylor]

Perfect, thank you! Somehow ishandle never came up in my search.

[Cedric Wannaz]

You're welcome! It's a bit confusing, for function handles you'd use isa(f, 'function_handle') instead.

[Walter Roberson]

From R2014b onwards it is better to use http://www.mathworks.com/help/matlab/ref/isgraphics.html

[Cedric Wannaz]

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!