Swift count string size in for loop

for my homework assignment I have to convert the sentence the user has typed in 3 different ways.

• Convert all letters to uppercase(completed)
• Convert all letters to lowercase(completed)
• Convert the uppercase letters to lowercase and vice versa(having trouble)

I believe my logic is correct but I don't know where I am wrong.

This is my code:

//
//  ViewController.swift
//  Lower Upper Converter
//
//  Created by Mac User on 9/27/15.
//  Copyright © 2015 Omid Nassir. All rights reserved.
//

import UIKit

class ViewController: UIViewController {

   private var input = ""
    private var i = 0
    var sentense = [String]()
    //var sentenseChar: [Character] = []

   //input fields
    @IBOutlet weak var input1: UITextField!

    @IBOutlet weak var input2: UITextField!




    //caps button
    @IBAction func capsBtn(sender: AnyObject)
    {
        if input1.text != nil
        {
            input = input1.text!.uppercaseString
            input2.text = input
        }
        else
        {
            input2.text = ""
        }
    }

    //lower case button
    @IBAction func lowsBtn(sender: AnyObject)
    {
        if input1.text != nil
        {
            input = input1.text!.lowercaseString
            input2.text = input
        }
        else
        {
            input2.text = ""
        }
    }


    //word converter button
    @IBAction func capsLowsBtn(sender: AnyObject)
    {

        if input1.text != nil
        {

          for i=0; i < count(input1.text); i++
          {
            input = advance(input1.startIndex, i)
            str[input]
            sentense[i] = advance(input.startIndex, i)
          }

          }

    }//end of convert function


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

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

I am getting an error message:

 Cannot invoke 'count' with an argument list of type '(String?)'

for this statement: for i=0; i < count(input1.text); i++