[R] calling "exists" function inside another function is not working

[utkarshsinghal]

   Hi all,
   I want to define a function such that one of its argument if passed do one
   thing and if not passed do the second thing. So basically, I have to check
   whether the argument is passed or not inside the function. I am trying to
   use 'exists' function to do this.
   > f = function(a,b){exists("b")}
   > f(a=3, b=4)
   [1] TRUE
   This is working as expected.
   But if I do the following:
   > f(a=3)
   [1] TRUE
   Can anybody explain why this function is returning TRUE here.
-- 
Many thanks,
Utkarsh