Variable Length Argument List

Sometimes we need pass along an unknown number of arguments before actually calling the function, such as searching for the minimum of maximum of a list of numbers, or calculating their average. Most programming language are able to first construct an array or list and then pass the constructed object as a single argument. But we can also pass an undefined length of arguments, and this post just summarizes this approach. Different languages provide different ways to deal with this case, and here I am discussing C/C++, Java and Python.

In C/C++, we need the type va_list and macros va_start, va_arg, and va_end. All these are included in the header file <stdarg.h> or <cstdarg>. Below is an example.

In Java, however, we directly append an ellipse after the type name, and then we can give a variable name (which is actually is an array and can be iterated via enhanced for loop in Java).

In Python, programmers have more power than passing an array of arguments: it allows a dictionary of arguments, by which we actually passing both a set of identifier names and their corresponding values. This offers more options to use the variable length arguments.

In summary,

1. The variable length parameters are always placed at the end of parameter list of the function.
2. In C/C++, at least one parameter must be passed and declared in the list. There is no such requirement in Java or Python. Also, in C/C++, the comma or space is not required after the last given parameter.
3. In C/C++, the variable arguments are able to be converted to different types via va_arg, and also this conversion is required for each argument. However, Java will first construct an array of the same type for all the passed arguments. Of course, you can cast the elements if necessary.
4. In Python, the arguments in calling function may become a little bit complex. The general order is that non-keyword arguments should be placed before keyword arguments. Also, there may be explicitly defined or predefined tuple or dict that will be passed as variable arguments, and in general these are always placed after "in-place" arguments. Here is an example.

   def: foo(arg1, arg2, *nonkws, **kws):
       #### function body
   
   pre_Tuple = (pre_nonkw1, pre_nonkw2)
   pre_Dict = {'pre_kw1':pre_v1, 'pre_kw2':pre_v2}
   
   # call function foo()
   foo(val1, val2, nkw1, nkw2, kw1=v1, kw2=v2, *(expl_nkw1, expl_nkw2), **{'expl_kw1':expl_v1, 'expl_kw2':expl_v2})

   or

   foo(val1, val2, nkw1, nkw2, kw1=v1, kw2=v2, *pre_Tuple, **pre_Dict)

   
   The only possibility to reorder the argument list is

   foo(val1, val2, nkw1, nkw2, *(expl_nkw1, expl_nkw2), kw1=v1, kw2=v2, **{'expl_kw1':expl_v1, 'expl_kw2':expl_v2})

   or

   foo(val1, val2, nkw1, nkw2, *pre_Tuple, kw1=v1, kw2=v2, **pre_Dict)

   
   Explicitly defined tuple or dict cannot co-exist with predefined tuple or dict. Only one for each is possible.