Index of UIButton in Collection using Swift

I have Swift code that looks like what's below. I have a collection of four buttons that I've put into an IBOutlet Collection. Those four buttons also link to a @IBAction. When I press a button I am able to see the title of the button that was pressed, but I also want to know the index for that button that was press, so that I can disable that particular button.

How can I find the index of the button that was pressed.

Thanks.

---

import UIKit

class ViewController: UIViewController {

@IBOutlet var answerButtons: [UIButton]!

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.


@IBAction func pressedButton(sender: UIButton) {
    let buttonTitle = sender.titleForState(UIControlState.Normal)
    println("Pressed Button \(buttonTitle)")
    listbuttons()
}

func listbuttons() {
    for button in answerButtons {
        let buttonTitle = button.titleForState(UIControlState.Normal)

        println("Button Title: \(buttonTitle) at Index: \(button)")
    }
}