Programming

Function Arguments

When calling a function, the caller provides the function with any data it needs by passing the data in an argument list. Data that needs to be returned to the caller is passed back in a list of return values.

Semantically speaking, MATLAB always passes argument data by value. (Internally, MATLAB optimizes away any unnecessary copy operations.)

If you pass data to a function that then modifies this data, you will need to update your own copy of the data. You can do this by having the function return the updated value as an output argument.

This section covers

• Checking the Number of Input Arguments
• Passing Variable Numbers of Arguments
• Passing Optional Arguments to Nested Functions
• Returning Output Arguments

Checking the Number of Input Arguments

The nargin and nargout functions enable you to determine how many input and output arguments a function is called with. You can then use conditional statements to perform different tasks depending on the number of arguments. For example,

• function c = testarg1(a, b)
  if (nargin == 1)
      c = a .^ 2;
  elseif (nargin == 2)
      c = a + b;
  end
  

Given a single input argument, this function squares the input value. Given two inputs, it adds them together.

Here is a more advanced example that finds the first token in a character string. A token is a set of characters delimited by white space or some other character. Given one input, the function assumes a default delimiter of white space; given two, it lets you specify another delimiter if desired. It also allows for two possible output argument lists:

• function [token, remainder] = strtok(string, delimiters)
  % Function requires at least one input argument
  if nargin < 1
     error('Not enough input arguments.');
  end
  token = []; remainder = [];
  len = length(string);
  if len == 0
     return
  end
  
  % If one input, use white space delimiter
  if (nargin == 1)
     delimiters = [9:13 32]; % White space characters
  end
  i = 1;
  
  % Determine where non-delimiter characters begin
  while (any(string(i) == delimiters))
     i = i + 1;
     if (i > len), return, end
  end
  
  % Find where token ends
  start = i;
  while (~any(string(i) == delimiters))
     i = i + 1;
     if (i > len), break, end
  end
  finish = i - 1;
  token = string(start:finish);
  
  % For two output arguments, count characters after
  % first delimiter (remainder)
  if (nargout == 2)
     remainder = string(finish+1:end);
  end
  

The strtok function is a MATLAB M-file in the strfun directory.

Note    The order in which output arguments appear in the function declaration line is important. The argument that the function returns in most cases appears first in the list. Additional, optional arguments are appended to the list.

Identifying Dependencies

Passing Variable Numbers of Arguments

© 1994-2005 The MathWorks, Inc.