11.4 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.

It is possible to use the nthargout function to obtain only some of the return values or several at once in a cell array. See Cell Array Objects.

: arg = nthargout (n, fcn, …) ¶

: arg = nthargout (n, ntot, fcn, …) ¶

Return the nth output argument of the function specified by the function handle or string fcn.

Any additional arguments are passed directly to fcn. The total number of arguments to call fcn with can be passed in ntot; by default ntot is n. The input n can also be a vector of indices of the output, in which case the output will be a cell array of the requested output arguments.

The intended use of nthargout is to avoid intermediate variables. For example, when finding the indices of the maximum entry of a matrix, the following two compositions of nthargout

m = magic (5);
cell2mat (nthargout ([1, 2], @ind2sub, size (m),
                     nthargout (2, @max, m(:))))
⇒ 5   3

are completely equivalent to the following lines:

m = magic (5);
[~, idx] = max (M(:));
[i, j] = ind2sub (size (m), idx);
[i, j]
⇒ 5   3

It can also be helpful to have all output arguments collected in a single cell array as the following code demonstrates:

USV = nthargout ([1:3], @svd, hilb (5));

See also: nargin, nargout, varargin, varargout, isargout.

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 hist functions are examples of built-in functions that behave differently depending on the value of nargout. For example, hist will draw a histogram when called with no output variables, but if called with outputs it will return the frequency counts and/or bin centers without creating a plot.

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.

: n = nargout () ¶

: n = nargout (fcn) ¶

Report the number of output arguments from a function.

Called from within a function, return the number of values the caller expects to receive. At the top level, nargout with no argument is undefined and will produce an error.

If called with the optional argument fcn—a function name or handle—return the number of declared output values that the function can produce.

If the final output argument is varargout the returned value is negative.

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.

In the second usage,

nargout (@histc)   # or nargout ("histc") using a string input

will return 2, because histc has two outputs, whereas

nargout (@imread)

will return -2, because imread has two outputs and the second is varargout.

Programming Note. nargout does not work for built-in functions and returns -1 for all anonymous functions.

See also: nargin, varargout, isargout, nthargout.