[go]流程控制if&switch

if语法

 if statement; condition {
}

func main() {
	num := 10
	if num%2 == 0 {
		fmt.Println("even")
	} else {
		fmt.Println("odd")
	}
}

//even

func main() {
	if num := 10; num%2 == 0 {
		fmt.Println("even")
	} else {
		fmt.Println("odd")
	}
}

//even

func main() {
	num := 99
	if num < 50 {
		fmt.Println("<50")
	} else if num >= 51 && num <= 100 {
		fmt.Println("51<=num<=100")
	} else {
		fmt.Println(">100")
	}
}

//51<=num<=100

 for initialisation; condition; post {
}

// break: 终止for
func main() {
	for i := 0; i < 10; i++ {
		if i > 5 {
			break
		}
		fmt.Println(i)
	}
}

//1
//2
//3
//4
//5

// continue: 跳过本次循环
func main() {
	for i := 0; i < 10; i++ {
		if i %2==0 {
			continue
		}
		fmt.Println(i)
	}
}

//1
//3
//5
//7
//9

if与switch

• if改为switch

func main() {
	for i := 0; i < 3; i++ {
		if i == 0 {
			fmt.Println("0")
		}else if i == 1 || i == 2 {
			fmt.Println("1 or 2")
		} else {
			fmt.Println("not 0-2")
		}
	}
}

//可以改写为:

func main() {
	for i := 0; i < 3; i++ {
		switch {
		case i == 0:
			fmt.Println("0")
		case i == 1 || i == 2:
			fmt.Println("1 or 2")
		default:
			fmt.Println("not 0-2")
		}
	}
}

//也可以改写为这样
func main() {
	for i := 0; i < 3; i++ {
		switch i {
		case 0:
			fmt.Println("0")
		case 1, 2: //任意一个匹配即可
			fmt.Println("1 or 2")
		default:
			fmt.Println("not 0-2")
		}
	}
}

//对于条件较为复杂 使用
switch{
	case i>10 && i<20
}

• default分支说明

//default分支不是必须
func main() {
	for i := 0; i < 3; i++ {
		switch i {
		case 0:
			fmt.Println("0")
		case 1, 2:
			fmt.Println("1 or 2")
		}
	}
}

//default位置可选
func main() {
	for i := 0; i < 3; i++ {
		switch i {
		default:
			fmt.Println("not 0-2")
		case 0:
			fmt.Println("0")
		case 1, 2:
			fmt.Println("1 or 2")
		}
	}
}

//case条件不能重复
func main() {
	i:=0
	switch i {
	case 0:
		fmt.Println("0")
	case 0:
		fmt.Println("0")
	}
}

//.main.go:10:2: duplicate case 0 in switch

//case后隐含break, 无需手动去break, 一旦匹配一个分支, 即跳出整个switch
func main() {
	i := 3
	switch {
	case i > 0:
		fmt.Println("i>0")
	case i > 2:
		fmt.Println("i>2")
	}
}

//i>0

• fallthrough穿透(失效)下一层的条件, 执行下一层语句.

func main() {
    i := 1
    switch {
    case i > 0:
        fmt.Println("i>0")
        fallthrough
    case i > 2:
        fmt.Println("i>2")
    }
}

//i>0
//i>2

//fallthrough决定该层要不要穿透,如果该层无,则直接break,终结整个switch
func main() {
	i := 3
	switch {
	case i > 0:
		fmt.Println("i>0")
	case i > 2:
		fmt.Println("i>2")
		fallthrough
	default:
		fmt.Println("not 0 1 2 3")
	}
}

//i>0

//一旦一层被fallthrough, 其他的条件即使不成立, 也会被执行(无视条件了)
func main() {
	i := 1
	switch {
	case i > 0:
		fmt.Println("i>0")
		fallthrough
	case i > 2:
		fmt.Println("i>2")
	default:
		fmt.Println("not 0 1 2 3")
	}
}

//i>0
//i>2

func main() {
	i := 0
	switch {
	case i > 0:
		fmt.Println("i>0")
		fallthrough
	case i > 2:
		fmt.Println("i>2")
		fallthrough
	default:
		fmt.Println("not 0 1 2 3")
	}
}

//not 0 1 2 3

//fallthrough不允许放最后一个分支
func main() {
	i := 1
	switch {
	case i > 0:
		fmt.Println("i>0")
		fallthrough
	case i > 2:
		fmt.Println("i>2")
		fallthrough
	}
}

//.main.go:13:3: cannot fallthrough final case in switch

//switch满足条件处开始执行
func main() {
	s := "abcd"
	switch s[2] {
	case 'a':
		fmt.Println("a")
		fallthrough
	case 'b':
		fmt.Println("b")
		fallthrough
	case 'c':
		fmt.Println("c")
		fallthrough
	default:
		fmt.Println("default case")
	}
}
//c
//default case