控制流：
1、if语句
let count = 5
if count >=3 {
print("yes")
}else{
print("no")
}2、switch语句
（1）Swift中不需要在case块中显示地使用break跳出switch。如果想要实现C风格的落入特性，可以给需要的case分支插入fallthrough语句
let fruit = "apple"
switch fruit{
case "apple":
print("good")
fallthrough
case "banana","orange":
print("great")
default:
print("bad")
}（2）case分支还可以进行区间匹配
let age = 5
switch age {
case 0...11:
print("正太")
case 12...30:
print("少年")
default:
print("大叔")
}（3）使用元组匹配（判断属于哪个象限）
let point = (2,2)
switch point {
case (0,0):
print("坐标在原点")
case (_,0):
print("坐标在x轴上")
case (0,_):
print("坐标在y轴上")
case (-3...3, -3...3):
print("坐标在长宽为6的正方形内")
default:
print("在什么地方")
}（4）case中还可以使用where关键字来做额外的判断条件
var height = 1.72
switch height{
case 1...3 where height == 1.72:
print("case 1")
case 1...3 where height == 2:
print("case 2")
default:
print("default")
}3，for循环语句
（1）for条件递增循环
for var i=1; i<100; i++ {
print("\(i)")
}（2）for-in循环
for i in 1..<100{
print("\(i)")
}
//遍历数组元素
let numbers = [1,2,4,7]
for num in numbers{
print("\(num)")
}
//遍历字典
let nameOfAge = ["lily":18, "Candy":24]
for (aName, iAge) in nameOfAge{
print("\(aName) is \(iAge)")
}
//遍历字符串的字符
for chare in "hangge".characters {
print(chare)
}
//使用foreach循环
(1...10).forEach
{
print($0)
}4，while循环语句
while i<100 {
i++
}
repeat{
i++
}while i<100
