From: 7stud -- <bbxx789_05ss@...>
Date: 2008-02-20T18:53:23+09:00
Subject: Re: What does *args do?

Philip Brocoum wrote:
> I sometimes see funcs declared with def fun (blah, *args)
> 
> What does the *args do? Thanks!

Normally, if you define a method with two parameter variables, for 
instance:

def show(x, y)  #x, y are parameter variables
  puts x
  puts y
end

show(1, 2)

--output:--
1
2


...then you *must* call the method with two arguments(1 and 2 are the 
arguments).  Otherwise you get an error, for example:

show(1, 2, 3)

--output:--
'show': wrong number of arguments (3 for 2) (ArgumentError)


The * allows you to call a method and specify more arguments:

def show(x, *arr)
  puts x

  arr.each do |elmt|
    puts elmt
  end

end

show(1, 2, 3)
puts
show(1, 2, 3, 4)

--output:--
1
2
3

1
2
3
4


You still need to call show() with at least one argument so that ruby 
can assign the first argument to the x parameter variable, but after 
that you can specify any number of arguments in the method call.  The 
extra arguments are gathered up into an array and then assigned to the 
variable name after the *, in this case that would be: arr.

If you have a method call like the following:

show(1, 2, 3, 4)  #def show(x, *arr)

ruby assigns the argument 1 to the parameter variable x, and then ruby 
creates an array to hold 2, 3, 4:

[2, 3, 4]

and then ruby assigns that array to the parameter variable arr:

arr = [2, 3, 4]


In effect the * in this definition:

def show(x, *arr)

says to ruby, "Please assign the first argument in the method call to x, 
then gather up any additional arguments, stick them into an array, and 
assign the array to the variable name to my right.




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