相关疑难解决方法(0)

我有两个问题:

let amount:String? = amountTF.text

1. amount?.characters.count <= 0

它给出了错误:

Binary operator '<=' cannot be applied to operands of type 'String.CharacterView.IndexDistance?' (aka 'Optional<Int>') and 'In

2. let am = Double(amount)

它给出了错误:

Cannot invoke initializer for type 'Double' with an argument list of type '(String?)'

我不知道如何解决这个问题.