我怎样才能得到字符串的第n个字符?我尝试了括号([])访问器,没有运气。
var string = "Hello, world!"
var firstChar = string[0] // Throws error
错误:'下标'是不可用的:不能下标String与Int,请参阅文档注释讨论
我怎样才能得到字符串的第n个字符?我尝试了括号([])访问器,没有运气。
var string = "Hello, world!"
var firstChar = string[0] // Throws error
错误:'下标'是不可用的:不能下标String与Int,请参阅文档注释讨论
当前回答
Swift 2.0的Xcode 7通用种子
var text = "Hello, world!"
let firstChar = text[text.startIndex.advancedBy(0)] // "H"
对于第n个字符,将0替换为n-1。
编辑:Swift 3.0
text[text.index(text.startIndex, offsetBy: 0)]
注意,有更简单的方法来获取字符串中的某些字符
例如:let firstChar = text.characters.first
其他回答
允许负指数
它总是有用的,不必总是写string[string]。长度- 1]用于在使用下标扩展名时获取最后一个字符。这(Swift 3)扩展允许负索引,范围和CountableClosedRange。
extension String {
var count: Int { return self.characters.count }
subscript (i: Int) -> Character {
// wraps out of bounds indices
let j = i % self.count
// wraps negative indices
let x = j < 0 ? j + self.count : j
// quick exit for first
guard x != 0 else {
return self.characters.first!
}
// quick exit for last
guard x != count - 1 else {
return self.characters.last!
}
return self[self.index(self.startIndex, offsetBy: x)]
}
subscript (r: Range<Int>) -> String {
let lb = r.lowerBound
let ub = r.upperBound
// quick exit for one character
guard lb != ub else { return String(self[lb]) }
return self[self.index(self.startIndex, offsetBy: lb)..<self.index(self.startIndex, offsetBy: ub)]
}
subscript (r: CountableClosedRange<Int>) -> String {
return self[r.lowerBound..<r.upperBound + 1]
}
}
如何使用:
var text = "Hello World"
text[-1] // d
text[2] // l
text[12] // e
text[0...4] // Hello
text[0..<4] // Hell
对于更彻底的程序员:在这个扩展中包括一个防止空字符串的保护
subscript (i: Int) -> Character {
guard self.count != 0 else { return '' }
...
}
subscript (r: Range<Int>) -> String {
guard self.count != 0 else { return "" }
...
}
还有另一种选择,在String声明中解释过
extension String : BidirectionalCollection {
subscript(i: Index) -> Character { return characters[i] }
}
斯威夫特3:
extension String {
func substring(fromPosition: UInt, toPosition: UInt) -> String? {
guard fromPosition <= toPosition else {
return nil
}
guard toPosition < UInt(characters.count) else {
return nil
}
let start = index(startIndex, offsetBy: String.IndexDistance(fromPosition))
let end = index(startIndex, offsetBy: String.IndexDistance(toPosition) + 1)
let range = start..<end
return substring(with: range)
}
}
"ffaabbcc".substring(fromPosition: 2, toPosition: 5) // return "aabb"
在Swift 3中,没有对String类的扩展,就像我能做的那样简单!
let myString = "abcedfg"
let characterLocationIndex = myString.index(myString.startIndex, offsetBy: 3)
let myCharacter = myString[characterLocationIndex]
本例中的myCharacter为“3”。
在项目中包含此扩展
extension String{
func trim() -> String
{
return self.trimmingCharacters(in: NSCharacterSet.whitespaces)
}
var length: Int {
return self.count
}
subscript (i: Int) -> String {
return self[i ..< i + 1]
}
func substring(fromIndex: Int) -> String {
return self[min(fromIndex, length) ..< length]
}
func substring(toIndex: Int) -> String {
return self[0 ..< max(0, toIndex)]
}
subscript (r: Range<Int>) -> String {
let range = Range(uncheckedBounds: (lower: max(0, min(length, r.lowerBound)),
upper: min(length, max(0, r.upperBound))))
let start = index(startIndex, offsetBy: range.lowerBound)
let end = index(start, offsetBy: range.upperBound - range.lowerBound)
return String(self[start ..< end])
}
func substring(fromIndex: Int, toIndex:Int)->String{
let startIndex = self.index(self.startIndex, offsetBy: fromIndex)
let endIndex = self.index(startIndex, offsetBy: toIndex-fromIndex)
return String(self[startIndex...endIndex])
}
然后像这样使用函数
let str = "Sample-String"
let substring = str.substring(fromIndex: 0, toIndex: 0) //returns S
let sampleSubstr = str.substring(fromIndex: 0, toIndex: 5) //returns Sample