如何在三元条件运算符中使用 Optional 变量？

Question

I want to use an Optional variable with the ternary conditional operator but it is throwing error this error: optional cannot be used as boolean. What am I doing wrong?

var str1: String?
var myBool:Bool
myBool = str1 ? true : false

Answer 1

You can not assign string value to bool but You can check it str1 is nil or not like this way :

myBool = str1 != nil ? true : false
print(myBool)

It will print false because str1 is empty.

Answer 2

Nil Coalescing Operator can be used as well. The code below uses the ternary conditional operator and forced unwrapping (a!) to access the value wrapped inside a when a is not nil, and to return b otherwise

Normal Ternary Operator :

output = a != nil ? a! : b output = a != nil ? a! : b Apple Developer Link : Please refer to Demo Link

In Swift 1.2 & 2, above line of code is replaced by a shorter format:

output = a ?? b

Demo Link : The nil coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil.

Answer 3

如果您想要的值是一个属性或一个可选的函数调用的结果（在 Swift 3.0 中），这甚至可以很好地工作：

return peripheral?.connected ?? false

Answer 4

Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (a ? b : c).

Example usage on tableView -

func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
     return section == 2 ?  4 :  1
}

indicates if section equal to 2 then it return 4 otherwise 1 on false.

Answer 5

To add on the already very good answers, sometimes you need the convenience of the ternary operator but also need to perform changes on the wrapped value, such as;

var fullName = lastName != nil ? "\(firsName) \(lasName!)" : firstName

But you don't want to force unwrap (even though this situation would be fine).

Then you can actually use Optional.map :

var fullName = lastName.map({ "\(firstName) \($0)" }) ?? firstName

The completion block passed to .map is only called if the wrapped value ( $0 ) is not nil . Otherwise, that function just returns nil , which you can then easily coalesce with the ?? operator.

Answer 6

In case the comparison is based on some condition

 let sliderValue = Float(self.preferenceData.getLocationRadius().characters.count > 1 ?self.preferenceData.getLocationRadius():"0.1")

Here the function getLocationRadius() returns a String. One more thing if we don't put a space between 1 and ? it results in an syntax error