Go web programming practice (2) -- process control statement

Flow control statement

Every language will introduce process control statements ,Go Language also has these . Such as branch structure if-else、for loop 、for-range loop 、switch-case sentence 、goto sentence 、break Statements and continue sentence . below , Bloggers will give examples to introduce how to use .

if-else sentence

Basically ,Go Linguistic if-else Statements and Java The language is basically the same . such as , Here we use the size of the value , Choose output , The code is as follows ：

func main() {
    
	var b int=9
	if b>9{
    
		fmt.Println(" Greater than ")
	}else if(b==9){
    
		fmt.Println(" be equal to ")
	}else{
    
		fmt.Println(" Less than ")
	}
}

for Loop statement

Different from other languages ,Go The language is only for loop , No, while and do-while loop . And its use method is similar to C The language is very close to . below , We cycle the output 1-100 Number , The code is as follows ：

var i int
for i=0;i<=100 ;i++  {
    
	fmt.Println(i)
}

use for Cycle to achieve do-while

Of course , If you want to use other languages do-while sentence , We can for The loop is written like this ：

var i int=0
for{
    
	i++
	if i>50{
    
		break
	}
}

use for Cycle to achieve while

alike , We can also put for Cycle to achieve while The effect of the cycle , The code is as follows ：

i :=0
for i<=10{
    
	fmt.Println(i)
	i++
}

break Specify to jump out of the loop

Did you learn C Linguistic , I should know Go Language can be used directly goto Jump to the specified code . and for Inside the loop , We can also pass break Statement directly determines which loop to terminate . such as ：

var i,j int
JumpLoop1:
	for i=1;i<100;i++{
    
		JumpLoop2:
			for j=0;j<i;j++{
    
				fmt.Println(j)
				if i>10{
    
					break JumpLoop1
				}else{
    
					break JumpLoop2
				}
			}
	}

Those who are interested can be deleted else Test them separately , Look at the end result . But generally speaking , If you don't specify JumpLoop1, Default to the innermost break Will only jump out of the innermost for loop .

continue sentence

Like any other language , Sometimes we just need to jump out of this cycle , therefore Go Language also provides us continue sentence . Of course ,continue Statements can also be followed by tags , For example, this cycle without jumping out of the innermost layer , And jump out of the outermost cycle , It can be written like this ：

var i, j int
JumpTag:
	for i = 1; i < 100; i++ {
    
		for j = 0; j < i; j++ {
    
			fmt.Println(j)
			if i > 10 {
    
				continue JumpTag
			}
		}
	}

This code executes , To i=10,j=9 after , Only output 0. because i>10 when , The cycle is j=0 Output once , Out of the outermost cycle , Therefore, there will be no other value .

for-range loop

In other programming languages , We usually have a problem with map To iterate . alike ,Go Language provides us with for-range loop , It is also an iterative structure . The syntax is as follows ：

for key,value :=range  Composite variable value {
    
	//.... Logical statement 
}

Traversal array

First , We need to understand the definition of array , It is the same as the definition of ordinary variables , Just one more "[]" Number .

// No initial value definition 
var num []int
// There is an initial value definition 
var num =[]int{
    1,2,3,4,5,6,7,8,9,0}

below , Let's show how to traverse the above array . The code is as follows ：

// Traversal in the case of declaring an array 
var num =[]int{
    1,2,3,4,5,6,7,8,9,2}
for key,value :=range num{
    
	fmt.Println("key=",key,"value=",value)
}
// Directly traverse the temporarily created array 
for key,value :=range []int{
    1,2,3,4,5,6,7,8,9,2}{
    
	fmt.Println("key=",key,"value=",value)
}

Traversal string

Like some languages ,Go The string of a language is actually an array of single characters , We can also in the program , Yes Go String traversal . The code is as follows ：

var name string="liyuanjinglyj"
for key,value :=range name{
    
	fmt.Printf("key=%d , value=%c \n",key,value)
}

Traverse map

about map It's even simpler , When traversing ,key,value Exactly corresponding to its key value pair . The specific traversal method is as follows ：

// obtain key,value
m:=map[string] string{
    
	"name":"liyuanjing",
	"age":"29",
}
for key,value :=range m{
    
	fmt.Printf("key=%s , value=%s \n",key,value)
}
// Get only value
m:=map[string] string{
    
	"name":"liyuanjing",
	"age":"29",
}
for _,value :=range m{
    
	fmt.Println(value)
}

In the second way key Change to "_", This underline is an anonymous variable , It can be understood as a placeholder . Anonymous variables themselves do not participate in space allocation , It doesn't take the name of a variable . alike , Also can be value Using anonymous variables , Get only key.

Traversal channel （channel）

What is a channel , We will introduce later , Here we define a channel directly , See how to traverse .

a := make(chan int)
go func() {
    
	a <- 1
	a <- 2
	a <- 3
	close(a)
}()
for v := range a {
    
	fmt.Println(v)
}

The logic of the above code ：

1. Create an integer channel and instantiate
2. By keyword go Start a goroutine
3. Pass the number into the channel , The function is to push data into the channel 123
4. End and close the channel
5. use for-range Loop to channel a Traversal , That is, continue to receive data from the channel until the channel is closed

switch-case sentence

Go The language has improved C Linguistic switch-case sentence , The expression does not have to be constant , It doesn't even have to be an integer . and case And case Between them are independent code segments , No need to pass break Statement jumps out of the current case, To avoid execution to the next case.

var name =" Jay Chou "
switch name {
    
case " Jay Chou ":
	fmt.Println(" singer ")
case " Wang Zhaojun ":
	fmt.Println(" Messenger of reconciliation ")
default:
	fmt.Println(" Check no one ")
}

The running result of the above code will only print the singer , No, break The following statement will not be executed , This blogger thinks Go The language design is good .

One branch has multiple values

Of course , Actually switch-case Statement case, It's not just about choosing one word , For example, some stars , Even if the actor is a singer , Do you use if and Judge ？

var name =" singer "
switch name {
    
case " singer "," actor ":
	fmt.Println(" Jackson Yi ")
case " The writer ":
	fmt.Println(" Yanye Qisheng ")
default:
	fmt.Println(" Nothing ")
}

Branch expressions

case Statements can be more than constants , Can also be combined with if Add expressions as well . Examples are as follows ：

var num =22
switch{
    
case num>0 && num==22 :
	fmt.Println(" yes 22 you 're right ")
default:
	fmt.Println(" Nothing ")
}

As shown in the above code ,switch under these circumstances , There is no need to add variables for judgment .

goto sentence

Go The most famous sentence in the language , Namely goto sentence . adopt goto Statement can jump directly to the specified label , Unconditional jump between codes . in addition ,goto Statements can also be used to jump out of a loop , Avoid repeated exits and other scenarios .

In the first article , We go through break Jump straight out to the outermost loop , Now we change the code at that time , Use goto To end the loop .

func main() {
    
var i, j int
for i = 1; i < 100; i++ {
    
	for j = 0; j < i; j++ {
    
		fmt.Println(j)
		if i > 10 {
    
			goto endTag
		}
	}
}
return
endTag:
	fmt.Println(" The loop ends ")
}

As shown in the above code , We directly specify to jump to endTag At the label , This label is also named by yourself .

But here's the thing ,endTag There's a... on the label return, This is because if the conditions are not met, it will not be implemented endTag Tag statement , Otherwise , Even if it ends , The program will also be executed in sequence .

goto It's so convenient , What application scenarios do readers think of ？

I believe readers are in other languages , Thrown all kinds of exceptions , In particular, many network requests are actually abnormal , But is it very annoying to throw one after another ？

and goto It's easy , You can jump directly to the specified exception , In this way, all the same exceptions can be solved with one exception .

err :=getEmail()
if err !=nil{
    
	goto endTag
}
err =getUser()
if err !=nil{
    
	goto endTag
}
endTag:
	fmt.Println(err)
// exception handling 

here , We assume that there is 2 A way , Get user name and mailbox , But in fact, they throw the same exception , Then you can jump directly to the specified label , Handle the same exception . Instead of each throwing an exception and writing it again .