Programming

Passing Optional Arguments to Nested Functions

You can use optional input and output arguments with nested functions, but you should be aware of how MATLAB interprets varargin, varargout, nargin, and nargout under those circumstances.

varargin and varargout are variables and, as such, they follow exactly the same scoping rules as any other MATLAB variable. Because nested functions share the workspaces of all outer functions, varargin and varargout used in a nested function can refer to optional arguments passed to or from the nested function, or passed to or from one of its outer functions.

nargin and nargout, on the other hand, are functions and when called within a nested function, always return the number of arguments passed to or from the nested function itself.

Using varargin and varargout

varargin or varargout used in a nested function can refer to optional arguments passed to or from that function, or to optional arguments passed to or from an outer function.

• If a nested function includes varargin or varargout in its function declaration line, then the use of varargin or varargout within that function returns optional arguments passed to or from that function.
• If varargin or varargout are not in the nested function declaration but are in the declaration of an outer function, then the use of varargin or varargout within the nested function returns optional arguments passed to the outer function.

In the example below, function C is nested within function B, and function B is nested within function A. The term varargin{1} in function B refers to the second input passed to the primary function A, while varargin{1} in function C refers to the first argument, z, passed from function B:

• function x = A(y, varargin)   % Primary function A
  B(nargin, y * rand(4))
  
     function B(argsIn, z)      % Nested function B
     if argsIn >= 2
        C(z, varargin{1}, 4.512, 1.729)
     end
  
        function C(varargin)    % Nested function C
        if nargin >= 2
           x = varargin{1}
        end
        end   % End nested function C
     end   % End nested function B
  end   % End primary function A
  

Using nargin and nargout

When nargin or nargout appears in a nested function, it refers to the number of inputs or outputs passed to that particular function, regardless of whether or not it is nested.

In the example shown above, nargin in function A is the number of inputs passed to A, and nargin in function C is the number of inputs passed to C. If a nested function needs the value of nargin or nargout from an outer function, you can pass this value in as a separate argument, as done in function B.

Example of Passing Optional Arguments to Nested Functions

This example references the primary function's varargin cell array from each of two nested functions. (Because the workspace of an outer function is shared with all functions nested within it, there is no need to pass varargin to the nested functions.)

Both nested functions make use of the nargin value that applies to the primary function. Calling nargin from the nested function would return the number of inputs passed to that nested function, and not those that had been passed to the primary. For this reason, the primary function must pass its nargin value to the nested functions.

• function meters = convert2meters(miles, varargin)
  % Converts MILES (plus optional FEET and INCHES input)
  % values to METERS.
  
  if nargin < 1 || nargin > 3
     error('1 to 3 input arguments are required');
  end
  
     function feet = convert2Feet(argsIn)
     % Nested function that converts miles to feet and adds in 
     % optional FEET argument.
     
     feet = miles .* 5280;
     
     if argsIn >= 2
        feet = feet + varargin{1};
     end
     end  % End nested function convert2Feet
  
     function inches = convert2Inches(argsIn)
     % Nested function that converts feet to inches and adds in 
     % optional INCHES argument.
     
     inches = feet .* 12;
     
     if argsIn == 3
        inches = inches + varargin{2};
     end
     end  % End nested function convert2Inches
  
  feet = convert2Feet(nargin);
  inches = convert2Inches(nargin);
  
  meters = inches .* 2.54 ./ 100;
  end  % End primary function convert2meters
  

• convert2meters(5)
  ans =
    8.0467e+003
  

• convert2meters(5, 2000, 4.7)
  ans =
    8.6564e+003
  

Passing Variable Numbers of Arguments

Returning Output Arguments

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