Go语言趣学指南-lesson3

循环和分支

本章学习目标
学会使用计算机通过if和switch做选择
学会使用for循环重复执行指定的代码
学会基于条件实现循环和分支处理

3.1 真或假

• Booleas 类型

  • 两个预声明长量true和flase用于判断是真是假
  • 区别于python和js把空字符和空列表也表示假

• Strings,Contains

  • 来自strings包的Contains函数可以判断某个字符串是否包含另外的字符串

• 例子(3.1)

  package main
  
  import (
  	"fmt"
  	"strings"
  )
  
  func main() {
  	fmt.Println("You fond yourself in a dimly lit cavern.")
  	var command = "walk outside"
  	var exit = strings.Contains(command, "outside")
  	fmt.Println("You leave the cave;", exit)
  }
  
  执行结果：
  
  You fond yourself in a dimly lit cavern.
  You leave the cave; true
  

3.2 比较

• 如果我们比较两个值、得到的结果也是true或者false

• 比较运算符：

  • ==
  • <=
  • <
  • !=
  • 大于等于
  • 大于

• 例子(3.2)

  package main
  
  import (
  	"fmt"
  )
  
  func main() {
  	fmt.Println("There is a sign near the entrance that reads 'No Minors'")
  	var age = 41
  	var minor = age < 18
  	fmt.Printf("At age %v, am I a minor? %v\n", age, minor)
  }
  
  执行结果：
  
  There is a sign near the entrance that reads 'No Minors'
  At age 41, am I a minor? false
  

3.3 用if做分支

• 直接看例子(3.3)

  package main
  
  import (
  	"fmt"
  )
  
  func main() {
  	var command = "go east"
  	if command == "go east" {
  		fmt.Println("You head further up the mountain.")
  	} else if command == "go inside" {
  		fmt.Println("You enter the cave where you live out the rest of your life")
  	} else {
  		fmt.Println("Didn't quite get that.")
  	}
  }
  
  
  执行结果:
  
  You head further up the mountain.
  

• 在这里else if 和else都是可选择的

• else if 可以为多个

3.4 逻辑运算符

• || 表示或、&& 表示与，它们用来同时检查多个条件

• 例子(3.4)

  package main
  
  import (
  	"fmt"
  )
  
  func main() {
  	fmt.Println("The year is 2100, should you leap?")
  	var year = 2100
  	var leap = year%4 == 0 || (year%4 == 0 && year%100 != 0)
  	if leap {
  		fmt.Println("Look before you leap!")
  	} else {
  		fmt.Println("Keep your feet on the ground")
  	}
  }
  
  
  执行结果：
  
  The year is 2100, should you leap?
  Look before you leap!
  

• 逻辑或中会使用一种叫做短路逻辑（当第一个条件为true时就不会走第二个条件）

• 取反逻辑运算符

  • ！，可以把true变成false，反之亦然

3.5 使用switch做分支

• 例子(3.5)

  package main
  
  import (
  	"fmt"
  )
  
  func main() {
  	var command = "go east"
  	switch command {
  	case "go east":
  		fmt.Println("You head further up the mountain.")
  	case "go inside":
  		fmt.Println("You enter the cave where you live out the rest of your life")
  	case "read sign":
  		fmt.Println("The sign reads 'No Minors'.")
  	default:
  		fmt.Println("Didn't quite get that.")
  	}
  }
  
  
  执行结果：
  
  You head further up the mountain.
  

• Switch 语句也可以对数字进行匹配

• 还有一个fallthrough关键字，它用来执行下一个case

  • 例子(3.5.1)

    package main
    
    import (
    	"fmt"
    )
    
    func main() {
    	var room = "lake"
    	switch {
    	case room == "cave":
    		fmt.Println("this cave")
    	case room == "lake":
    		fmt.Println("this lake")
    		fallthrough
    	case room == "underwater":
    		fmt.Println("执行fall")
    	}
    }
    
    
    执行结果：
    
    this lake
    执行fall
    

3.6 使用循环做重复

• for 关键字可以让你的代码重复执行

• for 后边不跟条件，那就是无限循环

  • 可以用break跳出循环

  • 例子

    package main
    
    import (
    	"fmt"
    	"time"
    )
    
    func main() {
    	var count = 10
    	for {
    		fmt.Println(count)
    		if count <= 0 {
    			break
    		}
    		time.Sleep(time.Second)
    		count--
    	}
    	fmt.Println("end")
    }
    
    
    执行结果：
    
    10
    9
    8
    7
    6
    5
    4
    3
    2
    1
    0
    end