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octave: Ignoring Arguments

 
 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.
 
DONTPRINTYET       See also: nargout XREFnargout, *notevarargout:
DONTPRINTYET       See also: nargout XREFnargout, varargout

      XREFvarargout, nthargout XREFnthargout.
 

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