6.循环

// 传统写法 已经在Swift3中淘汰
for var i = 0; i < 10; i++ {
    print(i)
}

for i in 0..<10 {
    print(i)
}

for i in 0...10 {
    print(i)
}

for _ in 0..<10 {
    print("hello")
}

var a = 0
while a < 10 {
    print(b)
    // a++已经在Swift3之后淘汰
    a = a + 1
}

var b = 0
repeat {
    print(b)
    b = b + 1
} while b < 20