KEYWORDS Function Keywords

Section: Functions and Scripts


A feature of IDL that FreeMat has adopted is a modified form of keywords. The purpose of keywords is to allow you to call a function with the arguments to the function specified in an arbitrary order. To specify the syntax of keywords, suppose there is a function with prototype
  function [out_1,...,out_M] = foo(in_1,...,in_N)

Then the general syntax for calling function foo using keywords is

  foo(val_1, val_2, /in_k=3)

which is exactly equivalent to

  foo(val_1, val_2, [], [], ..., [], 3),

where the 3 is passed as the k-th argument, or alternately,

  foo(val_1, val_2, /in_k)

which is exactly equivalent to

  foo(val_1, val_2, [], [], ..., [], logical(1)),

Note that you can even pass reference arguments using keywords.


The most common use of keywords is in controlling options for functions. For example, the following function takes a number of binary options that control its behavior. For example, consider the following function with two arguments and two options. The function has been written to properly use and handle keywords. The result is much cleaner than the MATLAB approach involving testing all possible values of nargin, and forcing explicit empty brackets for don't care parameters.

function c = keyfunc(a,b,operation,printit)
  if (~isset('a') | ~isset('b')) 
    error('keyfunc requires at least the first two 2 arguments'); 
  if (~isset('operation'))
    % user did not define the operation, default to '+'
    operation = '+';
  if (~isset('printit'))
    % user did not specify the printit flag, default is false
    printit = 0;
  % simple operation...
  eval(['c = a ' operation ' b;']);
  if (printit) 
    printf('%f %s %f = %f\n',a,operation,b,c);

Now some examples of how this function can be called using keywords.

--> keyfunc(1,3)                % specify a and b, defaults for the others

ans = 

--> keyfunc(1,3,/printit)       % specify printit is true
1.000000 + 3.000000 = 4.000000

ans = 

--> keyfunc(/operation='-',2,3) % assigns a=2, b=3

ans = 

--> keyfunc(4,/operation='*',/printit) % error as b is unspecified
In /home/basu/dev/branches/FreeMat4/help/tmp/keyfunc.m(keyfunc) at line 3
    In scratch() at line 1
    In base(base)
    In base()
    In global()
Error: keyfunc requires at least the first two 2 arguments