ViewController in ios 5
===

I am a learner in writing iPhone app. I have a problem in ViewController.
And I'm using xcode 4.3.3, trying to implement iPhone 5.1 apps.
Every time I write some code about ViewController, it reports the same error: use of undeclared identifier "viewController". And if I declare it with type of UIViewController, it has runtime error.

Some codes are like this:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{   
    
    self.window.rootViewController = [[UINavigationController alloc]initWithRootViewController:viewController];
    // Override point for customization after application launch.
    return YES;
}

It is in AppDelegate.m file.

Is there any difference in using viewController in ios 4 and 5?
