Catalog

Conditional statement if

Conditional statement switch

Type Switch

Conditional statement select

Syntax of select statement:

Typical use of select

1. Timeout judgment

2. exit

3. judge whether the channel is blocked

Loop statement for

Conditional statement if

Conditional statement switch

   switch marks {
      case 90: grade = "A"
      case 80: grade = "B"
      case 50,60,70 : grade = "C"
      default: grade = "D"  
   }

Type Switch

Switch statements can also be used for type switch to determine the type of variables actually stored in an interface variable.

The Type Switch syntax format is as follows:

switch x.(type){
    case type:
       statement(s)      
    case type:
       statement(s)
    /* You can define any number of case s */
    default: /* Optional */
       statement(s)
}

Example:

package main

import "fmt"

func main() {
    var x interface{}
    //Writing one:
    switch i := x.(type) { // With initialization statement
    case nil:
        fmt.Printf(" x Types :%T\r\n", i)
    case int:
        fmt.Printf("x yes int type")
    case float64:
        fmt.Printf("x yes float64 type")
    case func(int) float64:
        fmt.Printf("x yes func(int) type")
    case bool, string:
        fmt.Printf("x yes bool or string type")
    default:
        fmt.Printf("Unknown type")
    }
    //Writing two
    var j = 0
    switch j {
    case 0:
    case 1:
        fmt.Println("1")
    case 2:
        fmt.Println("2")
    default:
        fmt.Println("def")
    }
    //Writing three
    var k = 0
    switch k {
    case 0:
        println("fallthrough")
        fallthrough
        /*
            Go The switch is very flexible. The expression does not have to be a constant or an integer. The process is from top to bottom until a match is found;
            If a switch has no expression, it matches true.
            Go In it, switch is equivalent to each case with break at the end,
            After the match is successful, it will not automatically execute other case s down, but jump out of the whole switch,
            But you can use fallthrough to enforce the following case code.
        */
    case 1:
        fmt.Println("1")
    case 2:
        fmt.Println("2")
    default:
        fmt.Println("def")
    }
    //Writing three
    var m = 0
    switch m {
    case 0, 1:
        fmt.Println("1")
    case 2:
        fmt.Println("2")
    default:
        fmt.Println("def")
    }
    //Writing four
    var n = 0
    switch { //Omit the conditional expression when if...else if...else
    case n > 0 && n < 10:
        fmt.Println("i > 0 and i < 10")
    case n > 10 && n < 20:
        fmt.Println("i > 10 and i < 20")
    default:
        fmt.Println("def")
    }
}

The above code execution result is:

    x Types :<nil>
    fallthrough
    1
    1
    def

Conditional statement select

Syntax of select statement:

select {
    case communication clause  :
       statement(s);      
    case communication clause  :
       statement(s);
    /* You can define any number of case s */
    default : /* Optional */
       statement(s);
}

The syntax of the select statement is described below:

Typical use of select

1. Timeout judgment

//For example, in the following scenario, global respan is used to accept the response. If the time exceeds 3S and there is no data returned in respan, the second case will execute
var resChan = make(chan int)
// do request
func test() {
    select {
    case data := <-resChan:
        doData(data)
    case <-time.After(time.Second * 3):
        fmt.Println("request time out")
    }
}

func doData(data int) {
    //...
}

2. exit

//In the main thread (cooperation process), it is as follows:
var shouldQuit=make(chan struct{})
fun main(){
    {
        //loop
    }
    //...out of the loop
    select {
        case <-c.shouldQuit:
            cleanUp()
            return
        default:
        }
    //...
}

//In another cooperation process, if the operation encounters illegal operation or unhandled error, the data notification program is sent to shouldQuit to stop operation
close(shouldQuit)

3. Judge whether the channel is blocked

//In some cases, there is a requirement that you do not want the channel cache to be full
ch := make (chan int, 5)
//...
data: =0
select {
case ch <- data:
default:
    //Do something like discard data. Depending on demand
}

Loop statement for

The For loop of the Go language has three forms, only one of which uses semicolons.

•     for init; condition; post { }
•     for condition { }
•     for { }

    init: Generally, it is an assignment expression to assign initial value to the control variable;

    condition: Relation expression or logic expression, cycle control condition;

    post: Generally, it is an assignment expression, which increments or decrements the control variable.

 

•     for The statement is executed as follows:
  •     ①Expression first init Initial value;
  •     ②Discriminant assignment expression init Whether the given condition Condition, if the value is true and the loop condition is met, execute the statement in the loop body, and then execute post，Enter the second cycle, and judge again condition；Otherwise judgment condition If the condition is not met, it will terminate for Loop, execute out of loop statements.
     

s := "abc"

for i, n := 0, len(s); i < n; i++ { // The common for loop supports initialization statements.
    println(s[i])
}

n := len(s)
for n > 0 {                // Replace while (n > 0) {}
    println(s[n])        // Replace for (; n > 0;) {}
    n-- 
}

for {                    // Replace while (true) {}
    println(s)            // Replace for (;;) {}
}

Don't expect the compiler to understand you. It's a good idea to calculate all the results in the initialization statement.

package main

func length(s string) int {
    println("call length.")
    return len(s)
}

func main() {
    s := "abcd"

    for i, n := 0, length(s); i < n; i++ {     // Avoid calling the length function multiple times.
        println(i, s[i])
    } 
}

Output:

    call length.
    0 97
    1 98
    2 99
    3 100

Reference link:

http://www.topgoer.com/%E6%B5%81%E7%A8%8B%E6%8E%A7%E5%88%B6/%E5%BE%AA%E7%8E%AF%E8%AF%AD%E5%8F%A5for.html