範圍運算符（.. <和...）在Swift字符串

Question

有人可以解釋爲什麼半開和關閉範圍不再在Swift 3中的字符串上工作相同？

此代碼：

var hello = "hello" 
let start = hello.index(hello.startIndex, offsetBy: 1) 
let end = hello.index(hello.startIndex, offsetBy: 4) 
let range = start..<end // <-- Half Open Range Operator still works 
let ell = hello.substring(with: range) 

但這並不：

var hello = "hello" 
let start = hello.index(hello.startIndex, offsetBy: 1) 
let end = hello.index(hello.startIndex, offsetBy: 4) 
let range = start...end // <-- Closed Range Operator does NOT work 
let ello = hello.substring(with: range) // ERROR 

這導致錯誤如下所示：

Cannot convert value of type 'ClosedRange<String.Index>' (aka 'ClosedRange<String.CharacterView.Index>') to expected argument type 'Range<String.Index>' (aka 'Range<String.CharacterView.Index>') 

Answer 1

• 爲什麼let range = start..<end工作？

NSRange是你需要的，如果你想使用substring(with:)（described here）。

這裏是Rangethe definition：

在可比類型的半開區間，從下限到， 但不包括上限。

要創建一個Range：

您可以通過使用半開區間操作 （.. <）創建範圍的實例。

所以婁代碼是完全正確的（和它的將工作）：

var hello = "hello" 
let start = hello.index(hello.startIndex, offsetBy: 1) 
let end = hello.index(hello.startIndex, offsetBy: 4) 
let range = start..<end // <-- Half Open Range Operator still works 
let ell = hello.substring(with: range) 

• 爲什麼let range = start...end沒有作品：

隨着波紋管你被迫ClosedRange成爲Range：

var hello = "hello" 
let start = hello.index(hello.startIndex, offsetBy: 1) 
let end = hello.index(hello.startIndex, offsetBy: 4) 
let range = start...end // <-- Closed Range Operator does NOT work 
let ello = hello.substring(with: range) // ERROR 

• 如何轉換ClosedRange至Range？

Converting between half-open and closed ranges

Answer 2

做你想要做什麼，不叫substring(with:)。直接下標：

var hello = "hello" 
let start = hello.index(hello.startIndex, offsetBy: 1) 
let end = hello.index(hello.startIndex, offsetBy: 4) 
let ello = hello[start...end] // "ello"