Local variable "this"

programming flash
1

So basic as3 evaluation…

var r = 0;
trace( this["r"] );
//output: 0

but how about…

var r = 0;
function foo(){
  var r = 3;
  trace( this["r"] );
}
foo();
//output: 0

how do you access a local variable via evaluating a string? (in other words, the output would be 3, stored in the local variable, not 0)