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
获取和设置下标(字符串和子字符串)- Swift 4.2
Swift 4.2, Xcode 10
我的回答是基于@ alcarlson的回答。 唯一的区别是你可以返回一个Substring或String(在某些情况下，一个字符)。您还可以获取和设置下标。 最后，我的回答比@alecarlson的回答更麻烦，也更长，因此，我建议你把它放在源文件中。
扩展:
public extension String {
public subscript (i: Int) -> Character {
get {
return self[index(startIndex, offsetBy: i)]
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ..< end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ... end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ..< end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> String {
get {
return "\(self[index(startIndex, offsetBy: i)])"
}
set (c) {
let n = index(startIndex, offsetBy: i)
self.replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ..< end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ... end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ..< end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> Substring {
get {
return Substring("\(self[index(startIndex, offsetBy: i)])")
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
}
public extension Substring {
public subscript (i: Int) -> Character {
get {
return self[index(startIndex, offsetBy: i)]
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ..< end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ... end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ..< end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> String {
get {
return "\(self[index(startIndex, offsetBy: i)])"
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ..< end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ... end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ..< end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> Substring {
get {
return Substring("\(self[index(startIndex, offsetBy: i)])")
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
}
其他回答
Swift的String类型没有提供characterAtIndex方法，因为Unicode字符串有几种编码方式。你要用UTF8, UTF16，还是别的?
您可以通过检索String来访问CodeUnit集合。utf8和String。utf16属性。您还可以通过检索String来访问UnicodeScalar集合。unicodeScalars财产。
在NSString实现的精神中，我返回一个unichar类型。
extension String
{
func characterAtIndex(index:Int) -> unichar
{
return self.utf16[index]
}
// Allows us to use String[index] notation
subscript(index:Int) -> unichar
{
return characterAtIndex(index)
}
}
let text = "Hello Swift!"
let firstChar = text[0]
Swift 4.2或更高版本
使用String的indexes属性进行范围和部分范围下标
作为@LeoDabus nice answer的变体，我们可以为DefaultIndices添加一个额外的扩展，目的是允许我们在为后者实现自定义下标(通过Int专用范围和部分范围)时使用String的indexes属性。
extension DefaultIndices {
subscript(at: Int) -> Elements.Index { index(startIndex, offsetBy: at) }
}
// Moving the index(_:offsetBy:) to an extension yields slightly
// briefer implementations for these String extensions.
extension String {
subscript(range: Range<Int>) -> SubSequence {
let start = indices[range.lowerBound]
return self[start..<indices[start...][range.count]]
}
subscript(range: ClosedRange<Int>) -> SubSequence {
let start = indices[range.lowerBound]
return self[start...indices[start...][range.count]]
}
subscript(range: PartialRangeFrom<Int>) -> SubSequence {
self[indices[range.lowerBound]...]
}
subscript(range: PartialRangeThrough<Int>) -> SubSequence {
self[...indices[range.upperBound]]
}
subscript(range: PartialRangeUpTo<Int>) -> SubSequence {
self[..<indices[range.upperBound]]
}
}
let str = "foo bar baz bax"
print(str[4..<6]) // "ba"
print(str[4...6]) // "bar"
print(str[4...]) // "bar baz bax"
print(str[...6]) // "foo bar"
print(str[..<6]) // "foo ba"
感谢@LeoDabus指出我在使用索引属性作为字符串下标的(其他)替代方案的方向!
swift 2.0子字符串更新
public extension String {
public subscript (i: Int) -> String {
return self.substringWithRange(self.startIndex..<self.startIndex.advancedBy(i + 1))
}
public subscript (r: Range<Int>) -> String {
get {
return self.substringWithRange(self.startIndex.advancedBy(r.startIndex)..<self.startIndex.advancedBy(r.endIndex))
}
}
}
你可以通过将字符串转换为数组，并通过下标获得特定的索引，如下所示
var str = "Hello"
let s = Array(str)[2]
print(s)
获取和设置下标(字符串和子字符串)- Swift 4.2
Swift 4.2, Xcode 10
我的回答是基于@ alcarlson的回答。 唯一的区别是你可以返回一个Substring或String(在某些情况下，一个字符)。您还可以获取和设置下标。 最后，我的回答比@alecarlson的回答更麻烦，也更长，因此，我建议你把它放在源文件中。
扩展:
public extension String {
public subscript (i: Int) -> Character {
get {
return self[index(startIndex, offsetBy: i)]
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ..< end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ... end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ..< end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> String {
get {
return "\(self[index(startIndex, offsetBy: i)])"
}
set (c) {
let n = index(startIndex, offsetBy: i)
self.replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ..< end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ... end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ..< end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> Substring {
get {
return Substring("\(self[index(startIndex, offsetBy: i)])")
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
}
public extension Substring {
public subscript (i: Int) -> Character {
get {
return self[index(startIndex, offsetBy: i)]
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ..< end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> Substring {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return self[start ... end]
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ... end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> Substring {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return self[startIndex ..< end]
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> String {
get {
return "\(self[index(startIndex, offsetBy: i)])"
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
public subscript (bounds: CountableRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ..< end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ..< end, with: s)
}
}
public subscript (bounds: CountableClosedRange<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: CountablePartialRangeFrom<Int>) -> String {
get {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
return "\(self[start ... end])"
}
set (s) {
let start = index(startIndex, offsetBy: bounds.lowerBound)
let end = index(endIndex, offsetBy: -1)
replaceSubrange(start ... end, with: s)
}
}
public subscript (bounds: PartialRangeThrough<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ... end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ... end, with: s)
}
}
public subscript (bounds: PartialRangeUpTo<Int>) -> String {
get {
let end = index(startIndex, offsetBy: bounds.upperBound)
return "\(self[startIndex ..< end])"
}
set (s) {
let end = index(startIndex, offsetBy: bounds.upperBound)
replaceSubrange(startIndex ..< end, with: s)
}
}
public subscript (i: Int) -> Substring {
get {
return Substring("\(self[index(startIndex, offsetBy: i)])")
}
set (c) {
let n = index(startIndex, offsetBy: i)
replaceSubrange(n...n, with: "\(c)")
}
}
}
