Problems with "/ ' +

I’m having problems with this line:

"_root.mc"+i+".thex" = random(800);

it just doesnt work. I want “i” to be a variable number. But I don’t really know how to work with “+ i +”, maybe somebody could tell me something about the use of ", +, / and '.

Associative array referencing:

_root["mc"+i].thex

http://www.kirupaforum.com/forums/showthread.php?t=12082

i don;t really get it yet. i want a duplicated mc to get his thex from _root.mc1 (or _root.mc2). Here is my code in frame 1:

amount = 5;
while (amount>1) {
	duplicateMovieClip(_root.mc, "mc"+i, i);
	setProperty("mc"+i, _y, i*20);
	_root["mc"+i].thex = random(800);
	trace(_root["mc"+i].thex);//_root.thex);
	setProperty("mc"+i, _alpha, random(100));
	i++;
	amount--;
}

and this is the code for the mc:

onClipEvent (load) {
	thex = _root.thex;
	speed = 4;	
}
onClipEvent (enterFrame) {
	x = thex - _x
	_x += x/speed;
}