In its simplest form, the definition of a function named name looks like this:
function name body endfunction
A valid function name is like a valid variable name: a sequence of letters, digits and underscores, not starting with a digit. Functions share the same pool of names as variables.
The function body consists of Octave statements. It is the most important part of the definition, because it says what the function should actually do.
For example, here is a function that, when executed, will ring the bell on your terminal (assuming that it is possible to do so):
function wakeup printf ("\a"); endfunction
Once this function is defined, you can ask Octave to evaluate it by typing the name of the function.
Normally, you will want to pass some information to the functions you define. The syntax for passing parameters to a function in Octave is
function name (arg-list) body endfunction
where arg-list is a comma-separated list of the function’s arguments. When the function is called, the argument names are used to hold the argument values given in the call. The list of arguments may be empty, in which case this form is equivalent to the one shown above.
To print a message along with ringing the bell, you might modify the
wakeup to look like this:
function wakeup (message) printf ("\a%s\n", message); endfunction
Calling this function using a statement like this
wakeup ("Rise and shine!");
will cause Octave to ring your terminal’s bell and print the message
‘Rise and shine!’, followed by a newline character (the ‘\n’
in the first argument to the
In most cases, you will also want to get some information back from the functions you define. Here is the syntax for writing a function that returns a single value:
function ret-var = name (arg-list) body endfunction
The symbol ret-var is the name of the variable that will hold the value to be returned by the function. This variable must be defined before the end of the function body in order for the function to return a value.
Variables used in the body of a function are local to the function. Variables named in arg-list and ret-var are also local to the function. See Global Variables, for information about how to access global variables inside a function.
For example, here is a function that computes the average of the elements of a vector:
function retval = avg (v) retval = sum (v) / length (v); endfunction
If we had written
avg like this instead,
function retval = avg (v) if (isvector (v)) retval = sum (v) / length (v); endif endfunction
and then called the function with a matrix instead of a vector as the argument, Octave would have printed an error message like this:
error: value on right hand side of assignment is undefined
because the body of the
if statement was never executed, and
retval was never defined. To prevent obscure errors like this,
it is a good idea to always make sure that the return variables will
always have values, and to produce meaningful error messages when
problems are encountered. For example,
avg could have been
written like this:
function retval = avg (v) retval = 0; if (isvector (v)) retval = sum (v) / length (v); else error ("avg: expecting vector argument"); endif endfunction
There is still one remaining problem with this function. What if it is
called without an argument? Without additional error checking, Octave
will probably print an error message that won’t really help you track
down the source of the error. To allow you to catch errors like this,
Octave provides each function with an automatic variable called
nargin. Each time a function is called,
automatically initialized to the number of arguments that have actually
been passed to the function. For example, we might rewrite the
avg function like this:
function retval = avg (v) retval = 0; if (nargin != 1) usage ("avg (vector)"); endif if (isvector (v)) retval = sum (v) / length (v); else error ("avg: expecting vector argument"); endif endfunction
Octave automatically reports an error for functions written in .m file code if they are called with more arguments than expected. Octave does not automatically report an error if a function is called with too few arguments, since functions in general may have default arguments, but any attempt to use a variable that has not been given a value will result in an error. Functions can check the arguments they are called with to avoid such problems and to provide more context-specific error messages.
Report the number of input arguments to a function.
Called from within a function, return the number of arguments passed to the function. At the top level, return the number of command line arguments passed to Octave.
If called with the optional argument fcn—a function name or handle—return the declared number of arguments that the function can accept.
If the last argument to fcn is varargin the returned value is
negative. For example, the function
union for sets is declared as
function [y, ia, ib] = union (a, b, varargin) and nargin ("union") ⇒ -3
nargin does not work on compiled functions
(.oct files) such as built-in or dynamically loaded functions.
Return the name of the n-th argument to the calling function.
If the argument is not a simple variable name, return an empty string.
Examples which will return
"" are numbers (
y/2), and cell or structure indexing
inputname is only useful within a function. When used at the command
line or within a script it always returns an empty string.
By default, return an empty string if the n-th argument is not a valid variable name. If the optional argument ids_only is false, return the text of the argument even if it is not a valid variable name. This is an Octave extension that allows the programmer to view exactly how the function was invoked even when the inputs are complex expressions.
Query or set the internal variable that controls whether internal output from a function is suppressed.
If this option is disabled, Octave will display the results produced by evaluating expressions within a function body that are not terminated with a semicolon.
When called from inside a function with the
"local" option, the
variable is changed locally for the function and any subroutines it calls.
The original variable value is restored when exiting the function.