How to wrap an already existing function with a new function of the same name

I prefer jmetz's approach using builtin() when it can be applied, because it is clean and to the point. Unfortunately, many many functions are not found by builtin().

I found that I was able to wrap a function using a combination of the which -all and cd commands. I suspect that this approach can be adapted to a wide variety of applications.

In my example case, I wanted to (temporarily) wrap the interp1 function so that I could check for NaN output values. (The interp1 function will, by default, return a NaN under some conditions, such as if a query point is larger than the largest sample point.) Here's what I came up with:

function Vq = interp1(varargin)

   persistent interp1_builtin;

   if (isempty(interp1_builtin)) % first call: handle not set
      toolbox = 'polyfun';

      % get a list of all known instances of the function, and then 
      % select the first such instance that contains the toolbox name
      % in its path

      which_list = which('interp1','-all');
      for ii = 1:length(which_list)
         if (strfind(which_list{ii}, [filesep, toolbox, filesep]))
            base_path = fileparts(which_list{ii}); % path to the original function
            current_path = pwd;
            cd(base_path); % go to the original function's directory
            interp1_builtin = @interp1; % create a function handle to the original function
            cd(current_path); % go back to the working directory
            break
         end
      end
   end

   Vq = interp1_builtin(varargin{:});  % call the original function

   % test if the output was NaN, and print a message
   if (any(isnan(Vq)))
      dbstack;
      disp('ERROR: interp1 returned a NaN');
      keyboard
   end

end

See also: How to use MATLAB toolbox function which has the same name of a user defined function