Same trace,two results,why?

Question

//.....................
trace(channelbtn[channellistnum+channelshownum]);//line1
programtween_y.onMotionStopped = function(){
trace(channelbtn[channellistnum+channelshownum]);//line2
//line1,trace right result,but line2 trace undefined,why???
"channelbtn[channellistnum+channelshownum]" is a button
object, "channellistnum" and "channelshownum" are two
numbers,they've been defined above line1,so line1 trace right
result,but why line2 trace undefined?what's the different between
these two function ways below:
1. function a(){}
2. a = function(){}
Thank you for reply.......

Answer 1

thank Osh_it,now i know where the problem is:
In AS2.0,function like this way need Delegate!!!!This is very
important,because the object is undefined in this function way.
programtween_y.onMotionStopped = function(){
trace(channelbtn[channellistnum+channelshownum]);//line2
so,we should use this function like this:
programtween_y.onMotionStopped = Delegate.create(this,aaa);
private function aaa(){
trace(channelbtn[channellistnum+channelshownum]);//line2
It's ok,now!!!