How to show alert if radio button is not selected - swift

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iPhone

iOS

Xcode

Swift

I'm trying to create a signup page and what I want is when user forget to choose an option and select signup button must get alert "Firstly Select one option from above"

What I have tried:

else if !isValidEmail(testStr: email.text!)
        {
            let alert = UIAlertController(title: "Alert!", message: "Please enter correct Email ID", preferredStyle: .alert)
            alert.addAction(UIAlertAction(title: "OK", style: .default, handler: nil))
            self.present(alert, animated: true)
            return
        }

        else if (yes == 0)
        {
           // Here it is
            //How to show alert if radio button is not selected
        }