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.
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
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));
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
ans does not figure in the count of output
arguments, so the value of
nargout may be zero.
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
[a, b, c] = f ()
a = 1 b = (0x0) c = 2
along with a warning.
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.
nargout to return 0 inside the function
[s, t] = f ()
nargout to return 2 inside the function
In the second usage,
nargout (@histc) # or nargout ("histc") using a string input
will return 2, because
histc has two outputs, whereas
will return -2, because
imread has two outputs and the second is
nargout does not work for built-in functions and
returns -1 for all anonymous functions.