11.2 Multiple Return Values

Unlike many other computer languages, Octave allows you to define functions that return more than one value. The syntax for defining functions that return multiple values is

function [ret-list] = name (arg-list)
  body
endfunction

where name, arg-list, and body have the same meaning as before, and ret-list is a comma-separated list of variable names that will hold the values returned from the function. The list of return values must have at least one element. If ret-list has only one element, this form of the function statement is equivalent to the form described in the previous section.

Here is an example of a function that returns two values, the maximum element of a vector and the index of its first occurrence in the vector.

function [max, idx] = vmax (v)
  idx = 1;
  max = v (idx);
  for i = 2:length (v)
    if (v (i) > max)
      max = v (i);
      idx = i;
    endif
  endfor
endfunction

In this particular case, the two values could have been returned as elements of a single array, but that is not always possible or convenient. The values to be returned may not have compatible dimensions, and it is often desirable to give the individual return values distinct names.

In addition to setting nargin each time a function is called, Octave also automatically initializes nargout to the number of values that are expected to be returned. This allows you to write functions that behave differently depending on the number of values that the user of the function has requested. The implicit assignment to the built-in variable ans does not figure in the count of output arguments, so the value of nargout may be zero.

The svd and lu functions are examples of built-in functions that behave differently depending on the value of nargout.

It is possible to write functions that only set some return values. For example, calling the function

function [x, y, z] = f ()
  x = 1;
  z = 2;
endfunction

as

[a, b, c] = f ()

produces:

a = 1

b = [](0x0)

c = 2

along with a warning.

Built-in Function: nargout ()

Built-in Function: nargout (fcn_name)

Within a function, return the number of values the caller expects to receive. If called with the optional argument fcn_name, return the maximum number of values the named function can produce, or -1 if the function can produce a variable number of values.

For example,

f ()

will cause nargout to return 0 inside the function f and

[s, t] = f ()

will cause nargout to return 2 inside the function f.

At the top level, nargout is undefined.

See also: nargin, varargin, varargout.

Function File: msgstr = nargchk (minargs, maxargs, nargs)

Function File: msgstr = nargchk (minargs, maxargs, nargs, "string")

Function File: msgstruct = nargchk (minargs, maxargs, nargs, "struct")

Return an appropriate error message string (or structure) if the number of inputs requested is invalid.

This is useful for checking to see that the number of input arguments supplied to a function is within an acceptable range.

See also: nargoutchk, error, nargin, nargout.

Function File: msgstr = nargoutchk (minargs, maxargs, nargs)

Function File: msgstr = nargoutchk (minargs, maxargs, nargs, "string")

Function File: msgstruct = nargoutchk (minargs, maxargs, nargs, "struct")

Return an appropriate error message string (or structure) if the number of outputs requested is invalid.

This is useful for checking to see that the number of output arguments supplied to a function is within an acceptable range.

See also: nargchk, error, nargout, nargin.