How many keywords are there in go language?

There are 25 keywords in the Go language, including: break (exit the loop), default (select the structure default item), func (define the function), interface (define the interface), case (select the structure label) , const (define constant), continue, select, chan, continue (skip this loop), defer (delay execution content), go, map, struct, else, goto, package, switch, etc.

The operating environment of this tutorial: Windows 7 system, GO version 1.18, Dell G3 computer.

1. Overview

Keywords (also called reserved words) are sequences of characters that are reserved by programming languages ​​and are not used by programmers as identifiers.

Keywords are words that are given special meanings by the Go language and can also be called reserved words.

There are a total of 25 keywords in the Go language. The reason why the keywords in the Go language are deliberately kept so few is to simplify the code analysis during the compilation process. Like other languages, keywords cannot be used as identifiers.

##break Exit the loopdefaultSelect structure default items (switch, select)funcDefinition FunctioninterfaceDefine interfaceselectchannelcaseSelect structure tagchanDefine channelconstConstantcontinueSkip this loopdeferDelay execution Content (finishing work)goConcurrent executionmapmap typestructDefine structureelseSelect structuregotoJump statementpackagePackage##switch##fallthroughProcess controlifSelection structurerangeGet elements from slice, map and other structurestypeDefine the typeforLoopimportImport packagereturnReturnvarDefine variablesThe 25 keywords of Go can be used according to their functions Divided into 3 categories, namely package management, program entity declaration and definition, and program flow control.

Reserved keywords	Description
Selection structure

CategoryKeywordsProgram Statement import, packagechan, const, func, interface, map, struct, type, vargo, select, break, case, continue, default, defer, else, fallthrough, for, goto, if, range, return, switch

	##Program entity declaration and definition
	Program control flow
	Note: In the Go language, the declaration and definition of program entities are based on its data type system. For example, the keywords chan, func, interface, map and struct correspond to the Go language's composite data types Channel (channel), Function (function), Interface (interface), Map (dictionary) and Struct (structure) respectively. There are 15 keywords in total for program control flow. Among them, go and select are mainly used for concurrent programming in Go language. 2. Package management 2.1 import #import is used to import packages, so Exported identifiers from the package can be used. The import format is as follows: import _ "package path" import . "package path" import alias "package path" import ( _ "package path" . "package path" alias "package path" ) The package path can be preceded by one of the three modifiers. Underscores are blank identifiers, which means that the identifiers in the package are not used, and only the side effects of the package are needed, that is, the initialization expression of the package-level variables is calculated and the init initialization function of the imported package is executed. A dot replaces a package alias, indicating that access to exported identifiers in a package does not require the package name. alias represents the alias of the package. Import examples are as follows: 导入声明 Sin的本地名 import "lib/math" math.Sin import m "lib/math" m.Sin import . "lib/math" Sin 2.2 package package is used to declare the name of the package , needs to be placed at the front of all codes in the go file. A package consists of one or more go source files, which must be placed in the same directory, and the package names of these go files in the same directory can only have one name. The declaration format is as follows: package <packagename> packagename cannot be a blank identifier_. 3. Program entity declaration and definition 3.1 chan chan is used to declare channel (channel). A channel provides a mechanism for two concurrently executing functions to synchronize and communicate by passing values ​​of specific element types. Uninitialized channel value is nil. The declaration format is as follows: chan T // 可以被用来发送和接收类型T的值 chan<- T // 只能被用来发送浮点数 <-chan T // 只能被用来接收整数 where the <-operator specifies the direction of the channel, sending or receiving. If no direction is given, the channel is bidirectional. A channel can be forced to be send-only or receive-only by type conversion or assignment. The initialization of the channel can be achieved through the make function, and its result value serves as a reference to the underlying data structure. The buffer size can be set for the channel during initialization. The default value is zero, indicating an unbuffered or synchronized channel. ci := make(chan int) // 整数类型的无缓冲信道 cj := make(chan int, 0) // 整数类型的无缓冲信道 cp := make(chan *os.File, 100) // 指向文件指针的带缓冲信道 3.2 const const is used to define constants. Once created, it cannot be assigned or modified. const can appear anywhere the keyword var can appear. The way to declare a constant is the same as the way to declare a variable with var. The format is as follows: const name = value const name T = value const ( name = value name T = value ) Note that const in Golang does not support things like C/ Modifying the parameters and return values ​​of functions in C means that the following statements are illegal. func test(const name *string) func test(name *string) const *string 3.3 func func is used to define functions. Go functions support variable parameters and multiple return values, but do not support default parameters. If a function has multiple return value formal parameters, they need to be enclosed in parentheses. The definition format is as follows: func funcName(){} //无参无返回值 func funcName(t T) T {} //有参有返回值 func funcName(t T, list ...T) (T1,T1) {} //有变参有多个返回值 What needs to be noted in the code format is that the first brace in the function body The function name must match. This is Go's mandatory requirement for code format, and it is also true for other statements, such as if else statements, for statements, switch statements, select statements, etc. 3.4 interface interface is used to define the interface. An interface is a set of methods. If a type implements all the method sets in an interface, then the type implements this interface. Interface type variables can store values ​​of any type that implements the interface. In particular, interface{} represents an empty interface type. By default, all types implement the empty interface, so interface{} can receive any type of value. An example is as follows: //空接口 interface{} //一个简单的File接口 type File interface { Read(b Buffer) bool Write(b Buffer) bool Close() } 3.5 map map is used to declare mapping variables. Mapping is a container type, which is an unordered group of elements of the same type. The corresponding value can be obtained through a unique key. You can use make to create map variables. Uninitialized map values ​​are nil. There are several main ways to create map variables: //创建0容量的map var myMap = make(map[T1] T2) //创建指定容量的map var myMap = make(map[T]T2, cap) //创建并初始化map var myMap = map[string]int { "dable" : 27, "cat" : 28, } Usage example: package main import "fmt" func main() { nameAge := make(map[string]int) nameAge["bob"] = 18 //增 nameAge["tom"] = 16 //增 delete(nameAge, "bob") //删 nameAge["tom"] = 19 //改 v := nameAge["tom"] //查 fmt.Println("v=",v) v, ok := nameAge["tom"] //查，推荐用法 if ok { fmt.Println("v=",v,"ok=",ok) } for k, v :=range nameAge { //遍历 fmt.Println(k, v) } } Output result: v= 19 v= 19 ok= true tom 19 3.6 struct struct is used to define the structure. A structure is a container type and is a collection of multiple values ​​of the same or different types. package main import "fmt" type Vertex struct { X, Y int } var ( v1 = Vertex{1, 2} // 类型为 Vertex v2 = Vertex{X: 1} // Y:0 被省略 v3 = Vertex{} // X:0 和 Y:0 p = &Vertex{1, 2} // 类型为 *Vertex ) func main() { fmt.Printf("%#v %#v %#v %#v\n", v1, v2, v3, p) } Output result: main.Vertex{X:1, Y:2} main.Vertex{X:1, Y:0} main.Vertex{X:0, Y:0} &main.Vertex{X:1, Y:2} 3.7 type type 用于定义类型，比如定义struct、interface与等价类型。 //定义struct type Person struct { name string } //定义接口 type Person interface { speak(word string) } //定义等价类型，rune等价于int32 type rune int32 3.8 var var 用于定义变量，语法格式主要有： var name T //name默认为类型T的零值 var name = value //根据值value推断变量name的类型 var name T = value //赋初始值时指明类型 var name1, name2 T //同时定义多个同类型变量 //同时定义多个不同类型的变量 var ( name string ="dable" age int = 18 ) 定义变量可以使用:=来替代var，但是:=运算符只能用于函数体内。 4.程序流程控制 4.1 for range break continue （1）for 与 range for是Go中唯一用于循环结构的关键词。有三个使用方式，分别是单个循环条件，经典的初始化/条件/后续形式，还有和range关键词结合使用来遍历容器类对象（数组、切片、映射）。 //单条件 i := 1 for i <= 3 { fmt.Println(i) i = i + 1 } //初始化/条件/后续形式 //注意Go中没有前置自增与自减运算符，即++i是非法的 for i:=0; i < 3; i++ { fmt.Println(i) } //for range形式遍历数组 array :=[...]int{0,1,2,3,4,5} for i, v :=range array{ fmt.Println(i,v) } （2）break break用于终止最内层的"for"、“switch"或"select"语句的执行。break可以携带标签，用于跳出多层。如果存在标签，则标签必须放在"for”、"switch"或"select"语句开始处。 //终止for L: for i < n { switch i { case 5: break L } } （3）continue continue通常用于结束当前循环，提前进入下一轮循环。也可以像break一样携带标签，此时程序的执行流跳转到标签的指定位置，可用于跳出多层"for"、“switch"或"select”，提前进入下一轮的执行。示例如下： //提前进入下一轮循环 for i:=0; i < 3; i++ { if i == 1 { continue } fmt.Println(i) } //输出结果 0 2 //提前进入标签处for的下一轮循环 L: for i:=0; i < 2; i++ { for j:=0; j < 3; j++{ if j == 1 { continue L } fmt.Println(i, j) } } //输出结果 0 0 1 0 4.2 goto goto用于将程序的执行转移到与其标签相应的语句。可以使用goto退出多层"for"、“switch"或"select”，功能类似于break携带标签。 //终止for L: for i < n { switch i { case 5: goto L } } 注意事项： （1）执行"goto"不能在跳转过程中跳过变量的定义，不然会报编译错误。例如： goto L //编译报错 v := 3 L: fmt.Println(v) （2）在块外的goto语句不能跳转至该块中的标签。例如： if n%2 == 1 { goto L1 } for n > 0 { f() n-- L1: f() n-- } 是错误的，因为标签 L1 在"for"语句的块中而 goto 则不在。 （3）程序设计时，应尽量避免使用goto语句，因为程序执行流的随意跳转会破坏结构化设计风格，导致代码可读性下降。 4.3 switch case default fallthrough 这四个关键词是结合使用的。switch语句提供多路执行，表达式或类型说明符与switch中的case相比较从而决定执行哪一分支。default用于给出默认分支，即所有的case分支都不满足时执行default分支。Go中的switch语句在执行完某个case子句后，不会再顺序地执行后面的case子句，而是结束当前switch语句。使用fallthrough可以继续执行后面的case与default子句。 下面分别以表达式选择或类型选择为例演示switch case default fallthrough的用法。 //表达式选择 switch tag { default: s3() //default子句可以出现在switch语句中的任意位置，不一定是最后一个 case 0, 1, 2, 3: s1() //case表达式可以提供多个待匹配的值，使用逗号分隔 case 4, 5, 6, 7: s2() } switch x := f(); { case x < 0: return -x //case表达式无需为常量 default: return x } switch { //缺失的switch表达式意为"true" case x < y: f1() fallthrough //强制执行下一个case子句 case x < z: f2() //此处没有fallthrough，switch执行流在此终止 case x == 4: f3() } //类型选择 switch i := x.(type) { case int: printInt(i) // i 的类型为 int case float64: printFloat64(i) // i 的类型为 float64 case func(int) float64: printFunction(i) // i 的类型为 func(int) float64 case bool, string: printString("type is bool or string") // i 的类型为 bool or string default: printString("don&#39;t know the type") // i 的类型未知 } 4.4 if else if与else实现条件控制，与C有许多相似之处，但也有其不同之处。变化主要有三点： （1）可省略条件表达式的括号； （2）支持初始化语句，可定义代码块局部变量； （3）if与else块中只有一条语句也需要添加大括号； （4）起始大括号必须与if与else同行。 if err := file.Chmod(0664); err != nil { log.Print(err) return err } 4.5 return defer （1）return return用于函数执行的终止并可选地提供一个或多个返回值。 任何在函数F中被推迟的函数会在F 返回给其调用者前执行。函数可以通过return返回多个值。如果返回值在函数返回形参中指定了名字，那么return时可不带返回值列表。 //无返回值 func noResult() { return } //单返回值 func simpleF() int { return 2 } //多返回值 func complexF2() (float64, float64) { re = 7.0 im = 4.0 return re, im } //返回值已具名 unc complexF3() (re float64, im float64) { re = 7.0 im = 4.0 return } （2）defer defer语句用于预设一个函数调用，即推迟函数的执行。 该函数会在执行 defer 的函数返回之前立即执行。它显得非比寻常， 但却是处理一些事情的有效方式，例如无论以何种路径返回，都必须释放资源的函数。 典型的例子就是解锁互斥和关闭文件。 //将文件的内容作为字符串返回。 func Contents(filename string) (string, error) { f, err := os.Open(filename) if err != nil { return "", err } defer f.Close() // f.Close 会在函数结束后运行 var result []byte buf := make([]byte, 100) for { n, err := f.Read(buf[0:]) result = append(result, buf[0:n]...) if err != nil { if err == io.EOF { break } return "", err // 我们在这里返回后，f 就会被关闭 } } return string(result), nil // 我们在这里返回后，f 就会被关闭 } 推迟诸如 Close 之类的函数调用有两点好处：第一， 它能确保你不会忘记关闭文件。如果你以后又为该函数添加了新的返回路径时， 这种情况往往就会发生。第二，它意味着“关闭”离“打开”很近， 这总比将它放在函数结尾处要清晰明了。 使用defer时，需要注意两点： （a）被推迟函数的实参（如果该函数为方法则还包括接收者）在推迟执行时就会求值，而不是在调用执行时才求值。这样不仅无需担心变量值在函数执行时被改变， 同时还意味着可以给被推迟的函数传递不同参数。下面是个简单的例子。 for i := 0; i < 5; i++ { defer fmt.Printf("%d ", i) } （b）被推迟的函数按照后进先出（LIFO）的顺序执行，因此以上代码在函数返回时会打印 4 3 2 1 0。 4.6 go go用于创建Go程（goroutine），实现并发编程。Go程是与其它Go程并发运行在同一地址空间的函数，相比于线程与进程，它是轻量级的。Go程在多线程操作系统上可实现多路复用，因此若一个线程阻塞，比如说等待I/O，那么其它的线程就会运行。Go程的设计隐藏了线程创建和管理的诸多复杂性。 在函数或方法前添加 go 关键字能够在新的Go程中调用它。当调用完成后，该Go程也会安静地退出。效果有点像Unix Shell中的 & 符号，它能让命令在后台运行。 package main import ( "fmt" "time" ) func main() { go func(){ fmt.Println("in first goroutine") }() go func(){ fmt.Println("in second goroutine") }() fmt.Println("main thread start sleep, and other goroutine start execute") time.Sleep(10*time.Second) } 输出结果： main thread start sleep, and other goroutine start execute in second goroutine in first goroutine 注意，从输出结果可以看出，go程的执行顺序和创建的顺序是没有关系的，也就是说存在多个go程时，其执行的顺序是随机的。 4.7 select select语句用来选择哪个case中的发送或接收操作可以被立即执行。它类似于switch语句，但是它的case涉及channel有关的I/O操作。也就是说select就是用来监听和channel有关的IO操作，它与select, poll, epoll相似，当IO操作发生时，触发相应的动作，实现IO多路复用。 package main import "fmt" func main(){ ch1 := make(chan int, 1) ch2 := make(chan int, 1) ch1 <- 3 ch2 <- 5 select { case <- ch1: fmt.Println("ch1 selected.") case <- ch2: fmt.Println("ch2 selected.") default: //如果ch1与ch2没有数据到来，则进入default处理流程。如果没有default子句，则select一直阻塞等待ch1与ch2的数据到来 fmt.Println("default") } } 输出结果： ch1 selected. //或者 ch2 selected. 从输出结果可以看出，当存在多个case满足条件，即有多个channel存在数据时，会随机的选择一个执行。 【相关推荐：Go视频教程、编程教学】

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