Swift Xcode 6 If Else statement check letters or numbers

I am trying to write and if else statement.

I need it to check if atextField is empty or has letters and if the textfield is empty or has letters is triggers a UIAlertWIndow.

I have tried boolean methods to no avail.

ANy idea?

This is what I have so far:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {

    if self.numOfGuestsData.text == ""  {


        alert()
        println("Something Amish")

       let alertController = UIAlertController(title: "Split The Bill", message:
       "Please Enter The Number of Guests!", preferredStyle: UIAlertControllerStyle.Alert)
        alertController.addAction(UIAlertAction(title: "Okay", style: UIAlertActionStyle.Default,handler: nil))
      presentViewController(alertController, animated: true, completion: nil)


    } else if self.numOfGuestsData.text != "" {


        let subTotalVC = segue.destinationViewController as subTotalAmountViewController

        subTotalVC.numOfGuests = numOfGuestsData.text
        println("everything is Ok Here")

    }




    }

also if I take out the alert window the if else statement fails for some reason.

Like if I write it like this:

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject!) {

    if self.numOfGuestsData.text == ""  {


        alert()
        println("Something Amish")



    } else if self.numOfGuestsData.text != "" {


        let subTotalVC = segue.destinationViewController as subTotalAmountViewController

        subTotalVC.numOfGuests = numOfGuestsData.text
        println("everything is Ok Here")

    }




    }

It just goes to the else...