代碼來自網(wǎng)絡(luò)，做了一下修改調(diào)整，邊界處理等。
新建一個文件StringExtension0.swift 代碼如下，打開注釋即可（直接粘貼代碼，空格沒了。。。）。
//
//? StringExtension0.swift
//
//? Created by mac on 2021/2/20.
//下標(biāo)操作字符串
/*
測試用例
let aStr = "0123456789";
let str0 = aStr[index: 10]; //獲取第幾位的字符
let str1 = aStr[19...30];? //從19到30，包含30
let str2 = aStr[15..<56];? //從15到56不包含56
let str3 = aStr[...7];? ? // 0到7，包含7
let str4 = aStr[9...];? ? //9到最后，包含9
let str5 = aStr[3,5];? ? //從3開始，截取5位
let str6 = aStr[from: 14]; //從14開始 同 aStr[14...];
print("str0 \(str0)");
print("str1 \(str1)");
print("str2 \(str2)");
print("str3 \(str3)");
print("str4 \(str4)");
print("str5 \(str5)");
print("str6 \(str6)");
*/
//extension String {
//? ? /// 根據(jù)下標(biāo)獲取某個下標(biāo)字符
//? ? subscript(index index: Int) -> String {
//? ? ? ? if index < 0 || index >= self.count{
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let startIndex = self.index(self.startIndex, offsetBy: index)
//? ? ? ? let endIndex = self.index(startIndex, offsetBy: 1)
//? ? ? ? return String(self[startIndex..
//? ? }
//? ? /// 根據(jù)range獲取字符串 a[1...3]
//? ? subscript(r: ClosedRange) -> String {
//? ? ? ? guard r.lowerBound < count else{
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let start = index(startIndex, offsetBy: max(r.lowerBound, 0))
//? ? ? ? let end = index(startIndex, offsetBy: min(r.upperBound, count - 1))
//? ? ? ? return String(self[start...end])
//? ? }
//? ? /// 根據(jù)range獲取字符串 a[0..<2]
//? ? subscript(r: Range) -> String {
//? ? ? ? guard r.lowerBound < count else{
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let start = index(startIndex, offsetBy: max(r.lowerBound, 0))
//? ? ? ? let end = index(startIndex, offsetBy: min(r.upperBound, count))
//? ? ? ? return String(self[start..
//? ? }
//? ? /// 根據(jù)range獲取字符串 a[...2]
//? ? subscript(r: PartialRangeThrough) -> String {
//? ? ? ? let end = index(startIndex, offsetBy: min(r.upperBound, count - 1))
//? ? ? ? return String(self[startIndex...end])
//? ? }
//? ? /// 根據(jù)range獲取字符串 a[5...]
//? ? subscript(r: PartialRangeFrom) -> String {
//? ? ? ? guard r.lowerBound < count else{
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let start = index(startIndex, offsetBy: max(r.lowerBound, 0))
//? ? ? ? let end = index(startIndex, offsetBy: count - 1)
//? ? ? ? return String(self[start...end])
//? ? }
//? ? /// 根據(jù)range獲取字符串 a[..<3]
//? ? subscript(r: PartialRangeUpTo) -> String {
//? ? ? ? let end = index(startIndex, offsetBy: min(r.upperBound, count))
//? ? ? ? return String(self[startIndex..
//? ? }
//? ? //根據(jù)range截取a[3,5] 從3開始截取5位
//? ? subscript(start: Int, count: Int)->String{
//? ? ? ? guard start < count else {
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let begin = index(startIndex, offsetBy: max(0, start))
//? ? ? ? let end = index(startIndex, offsetBy: min(self.count, start + count))
//? ? ? ? return String(self[begin..
//? ? }
//
//? ? subscript(from from: Int)->String{
//? ? ? ? guard from < count else {
//? ? ? ? ? ? return ""
//? ? ? ? }
//? ? ? ? let start = self.index(endIndex, offsetBy: from - count)
//? ? ? ? return String(self[start..
//? ? }
//}
