Question

i have asked this question before and it was closed because i was unable to provide the language it was written in. currently i am looking at these AS3 codes and the variables are prefixed with _ , may i know why? is it because of a convention, if so , why put a _? why do you even need to put a _?

/**
 * Enemy AI - Random movement
 * ---------------------
 * VERSION: 1.0
 * DATE: 1/25/2011
 * AS3
 * UPDATES AND DOCUMENTATION AT: http://www.FreeActionScript.com
 **/
package
{
    import flash.display.MovieClip;
    import flash.events.Event;
    import flash.events.MouseEvent;
    public class Main extends MovieClip
    {
        // player settings
        private var _moveSpeedMax:Number = 1000;
        private var _rotateSpeedMax:Number = 15;
        private var _decay:Number = .98;
        private var _destinationX:int = 150;
        private var _destinationY:int = 150;
        private var _minX:Number = 0;
        private var _minY:Number = 0;
        private var _maxX:Number = 550;
        private var _maxY:Number = 400;
        // player
        private var _player:MovieClip;
        // global
        private var _dx:Number = 0;
        private var _dy:Number = 0;
        private var _vx:Number = 0;
        private var _vy:Number = 0;
        private var _trueRotation:Number = 0;
        /**
         * Constructor
         */
        public function Main()
        {
            // create player object
            createPlayer();
            // add listeners
            stage.addEventListener(Event.ENTER_FRAME, enterFrameHandler);
        }
        /**
         * Creates player
         */
        private function createPlayer():void
        {
            _player = new Player();
            _player.x = stage.stageWidth / 2;
            _player.y = stage.stageHeight / 2;
            stage.addChild(_player);
        }
        /**
         * EnterFrame Handlers
         */
        private function enterFrameHandler(event:Event):void
        {
            updateCollision();
            updatePosition();
            updateRotation();
        }
        /**
         * Calculate Rotation
         */
        private function updateRotation():void
        {
            // calculate rotation
            _dx = _player.x - _destinationX;
            _dy = _player.y - _destinationY;
            // which way to rotate
            var rotateTo:Number = getDegrees(getRadians(_dx, _dy));
            // keep rotation positive, between 0 and 360 degrees
            if (rotateTo > _player.rotation + 180) rotateTo -= 360;
            if (rotateTo < _player.rotation - 180) rotateTo += 360;
            // ease rotation
            _trueRotation = (rotateTo - _player.rotation) / _rotateSpeedMax;
            // update rotation
            _player.rotation += _trueRotation;
        }
        /**
         * Calculate Position
         */
        private function updatePosition():void
        {
            // update velocity
            _vx += (_destinationX - _player.x) / _moveSpeedMax;
            _vy += (_destinationY - _player.y) / _moveSpeedMax;
            // if close to target
            if (getDistance(_dx, _dy) < 50)
            {
                getRandomDestination();
            }
            // apply decay (drag)
            _vx *= _decay;
            _vy *= _decay;
            // update position
            _player.x += _vx;
            _player.y += _vy;
        }
        /**
         * updateCollision
         */
        protected function updateCollision():void
        {
            // Check X
            // Check if hit top
            if (((_player.x - _player.width / 2) < _minX) && (_vx < 0))
            {
              _vx = -_vx;
            }
            // Check if hit bottom
            if ((_player.x + _player.width / 2) > _maxX && (_vx > 0))
            {
              _vx = -_vx;
            }
            // Check Y
            // Check if hit left side
            if (((_player.y - _player.height / 2) < _minY) && (_vy < 0))
            {
              _vy = -_vy
            }
            // Check if hit right side
            if (((_player.y + _player.height / 2) > _maxY) && (_vy > 0))
            {
              _vy = -_vy;
            }
        }
        /**
         * Calculates a random destination based on stage size
         */
        private function getRandomDestination():void
        {
            _destinationX = Math.random() * (_maxX - _player.width) + _player.width / 2;
            _destinationY = Math.random() * (_maxY - _player.height) + _player.height / 2;
        }
        /**
         * Get distance
         * @param   delta_x
         * @param   delta_y
         * @return
         */
        public function getDistance(delta_x:Number, delta_y:Number):Number
        {
            return Math.sqrt((delta_x*delta_x)+(delta_y*delta_y));
        }
        /**
         * Get radians
         * @param   delta_x
         * @param   delta_y
         * @return
         */
        public function getRadians(delta_x:Number, delta_y:Number):Number
        {
            var r:Number = Math.atan2(delta_y, delta_x);
            if (delta_y < 0)
            {
                r += (2 * Math.PI);
            }
            return r;
        }
        /**
         * Get degrees
         * @param   radians
         * @return
         */
        public function getDegrees(radians:Number):Number
        {
            return Math.floor(radians/(Math.PI/180));
        }
    }
}

Answer 1

It’s a naming convention that typically denotes a private class level variable. It’s supposed to make the code easier to read by allowing you to be able to determine the scope of the variable.