xcode Swift 4 中 textField 的前 4 个字符的子字符串
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substring of the first 4 characters from a textField in Swift 4
提问by JamLis
I'm trying to create a substring of the first 4 characters entered in a textField in Swift 4 on my iOS app.
我正在尝试在我的 iOS 应用程序的 Swift 4 中的 textField 中创建前 4 个字符的子字符串。
Since the change to Swift 4 I'm struggling with basic String parsing.
自从更改为 Swift 4 以来，我一直在努力进行基本的字符串解析。
So based on Apple documentation I'm assuming I need to use the substring.index function and I understand the second parameter (offsetBy) is the number of characters to create a substring with. I'm just unsure how I tell Swift to start at the beginning of the string.
因此，根据 Apple 文档，我假设我需要使用 substring.index 函数，并且我知道第二个参数 (offsetBy) 是用于创建子字符串的字符数。我只是不确定如何告诉 Swift 从字符串的开头开始。
This is the code so far:
这是到目前为止的代码：
let postcode = textFieldPostcode.text
let newPostcode = postcode?.index(STARTATTHEBEGININGOFTHESTRING, offsetBy: 4)
I hope my explanation makes sense, happy to answer any questions on this.
我希望我的解释有意义，很高兴回答有关此的任何问题。
Thanks,
谢谢，
回答by vadian
In Swift 4 you can use
在 Swift 4 中你可以使用
let string = "Hello World"
let first4 = string.prefix(4) // Hell
The type of the result is a new type Substringwhich behaves very similar to String. However if first4is supposed to leave the current scope – for example as a return value of a function – it's recommended to create a Stringexplicitly:
结果的类型是一种新类型Substring，其行为与String. 但是，如果first4应该离开当前范围——例如作为函数的返回值——建议String显式创建：
let first4 = String(string.prefix(4)) // Hell
See also SE 0163 String Revision 1
另请参阅SE 0163 字符串修订版 1
回答by Mohammad Sadegh Panadgoo
In Swift 4:
在 Swift 4 中：
let postcode = textFieldPostcode.text!
let index = postcode.index(postcode.startIndex, offsetBy: 4)
let newPostCode = String(postcode[..<index])
