我正在Swift中制作一个计算器应用程序,一旦获得答案,我想将其显示在UILabel中。唯一的问题是我想限制所说的答案为8个字符。这是我的代码:
let answerString = "(answer)"
println(answer)
calculatorDisplay.text = answerString.substringToIndex(advance(answerString.startIndex, 8))
这不会返回任何编译器错误,但在运行时我得到:
fatal error: can not increment endIndex
如有任何帮助,我们将不胜感激。
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
使用第二个可以确保结果在字符串的有效范围内:
let truncatedText = answerString.substringToIndex(advance(answerString.startIndex, 8, answerString.endIndex))
Swift 2/Xcode 7:的更新
let truncatedText = answerString.substringToIndex(answerString.startIndex.advancedBy(8, limit: answerString.endIndex))
但一个更简单的解决方案是
let truncatedText = String(answerString.characters.prefix(8))
Swift 3/Xcode 8 beta 6的更新:从Swift 3开始,"收藏移动他们的索引",相应的代码现在是
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。