(octave.info)Ignoring Arguments

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11.5 Ignoring Arguments
=======================

In the formal argument list, it is possible to use the dummy placeholder
‘~’ instead of a name.  This indicates that the corresponding argument
value should be ignored and not stored to any variable.

     function val = pick2nd (~, arg2)
       val = arg2;
     endfunction

   The value of ‘nargin’ is not affected by using this declaration.

   Return arguments can also be ignored using the same syntax.  For
example, the sort function returns both the sorted values, and an index
vector for the original input which will result in a sorted output.
Ignoring the second output is simple—don’t request more than one output.
But ignoring the first, and calculating just the second output, requires
the use of the ‘~’ placeholder.

     x = [2, 3, 1];
     [s, i] = sort (x)
     ⇒
     s =

        1   2   3

     i =

        3   1   2

     [~, i] = sort (x)
     ⇒
     i =

        3   1   2

   When using the ‘~’ placeholder, commas—not whitespace—must be used to
separate output arguments.  Otherwise, the interpreter will view ‘~’ as
the logical not operator.

     [~ i] = sort (x)
     parse error:

       invalid left hand side of assignment

   Functions may take advantage of ignored outputs to reduce the number
of calculations performed.  To do so, use the ‘isargout’ function to
query whether the output argument is wanted.  For example:

     function [out1, out2] = long_function (x, y, z)
       if (isargout (1))
         ## Long calculation
         ...
         out1 = result;
       endif
       ...
     endfunction

 -- : isargout (K)
     Within a function, return a logical value indicating whether the
     argument K will be assigned to a variable on output.

     If the result is false, the argument has been ignored during the
     function call through the use of the tilde (~) special output
     argument.  Functions can use ‘isargout’ to avoid performing
     unnecessary calculations for outputs which are unwanted.

     If K is outside the range ‘1:max (nargout)’, the function returns
     false.  K can also be an array, in which case the function works
     element-by-element and a logical array is returned.  At the top
     level, ‘isargout’ returns an error.

     See also: Note: nargout, *note varargout:
     XREFvarargout, Note: nthargout.