7.字符串

let str = "hello Objective-C"

var str = "hello Swift"

"""
静夜思
作者：李白
床前明月光，疑是地上霜。
举头望明月，低头思故乡。
"""

let str = "12345678"
let len2 = str.count  //8
let len3 = (str as NSString).length  //8

let str1 = "abc" + "cde"
var str = "abc"
str.append("cde")

let str = "abcdefg"
for char in str {
    print(char)
}
---------------------
let str = "abcdefg"
for (index, value) in str.enumerated() {
    print("\(index) --- \(value)")
}

let str = "abcdefg"
print(str.lowercased())  //全部小写
print(str.uppercased())  //全部大写

let str = "abcdefg"
print(str.contains("cd"))   //true
print(str.contains("c"))    //true
print(str.contains("o"))    //false

//字符串拆分(以单个或多个字符为分隔符)
let str = "abcdefg$$aaaa$$bbbb$$ccc"
let des1 = (str as NSString).components(separatedBy: "$$")
let des2 = str.components(separatedBy: "$$")
print(des1)  //["abcdefg", "aaaa", "bbbb", "ccc"]
print(des2) //["abcdefg", "aaaa", "bbbb", "ccc"]

let str = "abcdefg$$aaaa$$bbbb$$ccc"
let dest11 = str.replacingOccurrences(of: "$$", with: "**")
print(dest11)

let str = "Hello World"
//截取某字符串的前5个字符串
let sub1 = str.prefix(5)
//截取某字符串的后5个字符串
let str1 = str.suffix(5)

//也可以换种写法
let index2 = str.index(str.endIndex, offsetBy: -5)
let sub2 = str[index2..<str.endIndex]
//截取某字符串的第3个字符到第6个字符范围的字符串
let index3 = str.index(str.startIndex, offsetBy: 3)
let sub3 = str[str.startIndex..<index3]

let index4: String.Index = str.index(str.endIndex, offsetBy: -3)

//从某个位置开始 到某个位置结束
let sub4 = str[index3..<index4]