using variables in instance names

I am trying to create movie clips and text fields dynamically with actions script. That part is fine. But I need to know how to refer to these newly created clips.

For example if I use a loop to create 10 movie clip instances such as:

for(var i=0; i<10; i++) {
  duplicateMovieClip(_root.bar,"mc"+i,i);
}

That works fine.  Now lets say I want to change all of thier positions. How do I refer to each clip? I would assume you would do it like this but it does not work.

for(var i=0; i<10; i++) {
   setProperty (_root."mc"+i, _x, 10);
}

also, this doesn't work either:

for(var i=0; i<10; i++) {
  _root.setProperty ("mc"+i, _x, 10);
}

Basically, I need to use a variable in the object identifier like X to refer to my movie, but I have no way of saying that X is a variable. Such as; instead of mc1, mc2, mc3 ect. I could just use mcX.

Anyone know how to do this?
Thanks.