[Yugong series] go teaching course 005 variables in July 2022

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One 、 Variable

1. Definition of variables

Variables come from mathematics , Used to describe the data storage space in a computer . Variable can be accessed by variable name . In imperative language , Variables are usually variable ; But in pure functional languages （ Such as Haskell） in , Variables can be immutable （immutable） Of . In some languages , Variables may be explicitly defined to represent variable states 、 An abstraction with storage space （ If in Java and Visual Basic in ）; But other languages may use other concepts （ Such as C The object of ） To refer to this abstraction , Without strictly defining “ Variable ” Accurate extension of .

2. Role of variables

The function of variables is to mark and store data in memory .

Memory , Full name memory . It is used to store the data during the operation of the computer .

In order to better store data , The memory is divided into different storage units as follows ：

Take a storage unit out of memory , Store the corresponding data ：

For example, the above red area , Variable name = The name of the area , Data exists in the region

3. Variable declaration and initialization

3.1 Declaration of variables

Go It's a static language , All variables must be declared before use . The meaning of the declaration is to tell the compiler the boundary information of the memory that the variable can operate on , This boundary is usually provided by the type information of variables . stay Go In language , There is a general variable declaration method like this ：

Use of variables

fmt.Println(a)

3.2 Initialization of a variable

You can assign values when defining variables , This process is called variable initialization

var age int=10

3.3 The assignment of a variable

You can assign values to variables after they are defined , Declare before assign .

var age,num int 
age=10
num=20
fmt.Println(age,num) //10,20

Assign a variable to another variable as follows ：

var age int =10
var num int
num=age
fmt.Println(num) //10

Be careful ： Assigning a value to a variable will overwrite the old value

3.4 Case study ： Exchange the values of two variables

Temporary variable

package main
import "fmt"
func main(){
    
	a := 1
	b := 5
	var t int 
	t = a
	a = b
	b = t
	fmt.Println("a = ", a, "b = ", b )
}

Do not use temporary variables

package main
import "fmt"
func main(){
    
	a := 1
	b := 5
	a = a + b
	b = a - b
	a = a - b
	fmt.Println("a = ", a, "b = ", b )
}

Direct assignment

package main
import "fmt"
func main() {
    
    a := []int{
    1, 2}
    b := []int{
    3, 4, 5}
    a, b = b, a
    fmt.Println(`a：`, a)
    fmt.Println(`b：`, b)
}

summary

• Variable declarations ：var Variable name variable type
• Declare multiple variables ：var Variable name 1, Variable name … type
• Declare an integer variable , The default value is 0
• Output statements can use only one Println function , The middle part is separated by English half width commas !
• You can set the value of a variable , Assign to another variable , And the old value in the variable is overwritten by the new value .