1.打印

print("hello world")

2.生命常量和变量

• 使用let声明常量，var变量

        let age = 18
        var name = "xingweixin"
        name = "fmy"
        var height:Double = 172.0 //后边加类型
        height = 160.0
        print(age)
        print(name)
        print(height)
        print("name:\(name)  age:\(age) height:\(height)")

3.if语句

let contentHeight = 40
let hasHeader = true
var rowHeight = contentHeight
if hasHeader {
    rowHeight = rowHeight + 50
} else {
    rowHeight = rowHeight + 20
}

4.for循环

//方式一
for index in 1...5 {
    print("\(index) * 5 = \(index * 5)")
}
//方式二a..<b
let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
    print("第 \(i + 1) 个人叫 \(names[i])")
}
//方式三
for name in names[2...] {
    print(name)
}
// Brian
// Jack

for name in names[...2] {
    print(name)
}
// Anna
// Alex
// Brian
//方式四
for name in names[..<2] {
    print(name)
}
// Anna
// Alex

5.字符串

• 初始化

var emptyString = ""               // 空字符串字面量
var anotherEmptyString = String()  // 初始化方法
// 两个字符串均为空并等价。

• 判断为空

if emptyString.isEmpty {
    print("Nothing to see here")
}

• 字符串的可变性

var variableString = "Horse"
variableString += " and carriage"
// variableString 现在为 "Horse and carriage"

let constantString = "Highlander"
constantString += " and another Highlander"
// 这会报告一个编译错误（compile-time error） - 常量字符串不可以被修改。

//在 Objective-C 和 Cocoa 中，需要通过选择两个不同的类（NSString 和 NSMutableString）来指定字符串是否可以被修改

• 拼接字符串

let string1 = "hello"
let string2 = " there"
var welcome = string1 + string2
// welcome 现在等于 "hello there"

var instruction = "look over"
instruction += string2
// instruction 现在等于 "look over there"

let exclamationMark: Character = "!"
welcome.append(exclamationMark)
// welcome 现在等于 "hello there!"