AS3: class related questions and IF statment

package 
{
 
	import flash.display.MovieClip;
	import flash.events.MouseEvent;
 
	public class Main extends MovieClip
	{
		//why i have to declare these vars outside the Main function?
 
		//this var is for user, he can guesses the number
		// at the same time the program use it to compare in the (if) statment
		var currentGuess:uint;
		var startMessage:String;
 
		// this is the number stored and the user try to guess it
		var mysteryNumber:uint;
 
		public function Main ()
		{
			init ();
 
			function init ()
			{
				//trace ("Hello, from the init method!");
 
				//initialize variable
				startMessage = "I am thinking of a number between 1 and 100";
				mysteryNumber = 50;
 
				//initialize text field
				output_txt.text = startMessage;
				
				// why i have to define or give the input_txt empty value?
				input_txt.text = "";
				input_txt.backgroundColor = 0xffcccc;
				input_txt.restrict = "0-9";
 
				//why this command does not work
				stage.focus = input_txt;
 
				// add an event listener to a button
				guessButton_btn.addEventListener (MouseEvent.CLICK, onGuessButtonClick);
			}
 
			function onGuessButtonClick (e:MouseEvent):void
			{
				//assigning the input from the text field to the currentGuess var
 
				/*Its very easy to convert data types in AS3.0. All you need to do is
				surround the data you want to convert in parentheses and then add 
				the name of the type you want to convert it to.*/
				currentGuess = input_txt.text as uint;
 
				//when i type from 0 - 49, it says: That's too high 
				//but when i type 50 or above 50, it does not give me the
				//defined messege i specified
				if (currentGuess > mysteryNumber)
				{
					output_txt.text = "That's too high";
				}
				else if (currentGuess < mysteryNumber)
				{
					output_txt.text = "That's too low";
				}
				else
				{
					output_txt.text = "You got it";
				}
			}
		}
	}
}