在swift中修剪字符串结束，在运行时获得错误 [英] Trim end off of string in swift, getting error at runtime

问题描述

我正在Swift中制作一个计算器应用程序，一旦得到我的答案，我想在UILabel中显示它。唯一的问题是我想将所述答案限制为8个字符。这是我的代码：

I'm making a calculator app in Swift, once my answer is obtained I want to display it in a UILabel. Only problem is I want to limit said answer to 8 characters. Here is my code:

let answerString = "\(answer)"
    println(answer)
    calculatorDisplay.text = answerString.substringToIndex(advance(answerString.startIndex, 8))

这不是返回任何编译器错误，但在运行时我得到：

This does not return any compiler errors but at runtime I get:

fatal error: can not increment endIndex

非常感谢任何和所有帮助。

Any and all help would be greatly appreciated.

推荐答案

有两种不同的 advance（）函数：

/// Return the result of advancing `start` by `n` positions. ...
func advance<T : ForwardIndexType>(start: T, n: T.Distance) -> T

/// Return the result of advancing start by `n` positions, or until it
/// equals `end`. ...
func advance<T : ForwardIndexType>(start: T, n: T.Distance, end: T) -> T

使用第二个，您可以确保结果在字符串的有效范围内：

Using the second one you can ensure that the result is within the valid bounds of the string:

let truncatedText = answerString.substringToIndex(advance(answerString.startIndex, 8, answerString.endIndex))

更新 Swift 2 / Xcode 7：

let truncatedText = answerString.substringToIndex(answerString.startIndex.advancedBy(8, limit: answerString.endIndex))

但更简单的解决方案是

let truncatedText = String(answerString.characters.prefix(8))

更新 Swift 3 / Xcode 8 beta 6：截至Swift 3，集合移动
他们的索引，相应的代码现在

Update for Swift 3/Xcode 8 beta 6: As of Swift 3, "collections move their index", the corresponding code is now

let to = answerString.index(answerString.startIndex,
                            offsetBy: 8,
                            limitedBy: answerString.endIndex)
let truncatedText = answerString.substring(to: to ?? answerString.endIndex)

更简单的解决方案

let truncatedText = String(answerString.characters.prefix(8))

仍然有效。

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