如何在swift中减去值

Question

我想减去tex_length和dotPosition值，我收到错误：

if let t = textField.text
 {            
    let dotPosition =  t.rangeOfString(".")?.endIndex.predecessor()
    let tex_length = countElements(textfield.text)
    print(dotPosition)
    print(tex_length)
    res = tex_length  - dotPosition ;           
 }