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
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
其他回答
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)")
}
}
}
我刚想出了一个巧妙的变通办法
var firstChar = Array(string)[0]
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
斯威夫特5.3
我觉得这很优雅。“Hacking with Swift”的Paul Hudson提出了以下解决方案:
@available (macOS 10.15, * )
extension String {
subscript(idx: Int) -> String {
String(self[index(startIndex, offsetBy: idx)])
}
}
然后，要从字符串中获取一个字符，你只需做:
var string = "Hello, world!"
var firstChar = string[0] // No error, returns "H" as a String
注意:我只是想补充，这将返回一个字符串指出在评论。我认为这对Swift用户来说可能是意想不到的，但我经常需要一个字符串来直接在我的代码中使用，而不是字符类型，所以它确实简化了我的代码，避免了后来从字符到字符串的转换。
在Swift 5中，不扩展字符串:
var str = "ABCDEFGH"
for char in str {
if(char == "C") { }
}
以上Swift代码与Java代码相同:
int n = 8;
var str = "ABCDEFGH"
for (int i=0; i<n; i++) {
if (str.charAt(i) == 'C') { }
}
