Go Language---Map

Go language---map78890185. Map usage:

Type persondb struct {Id string Name string Address string}fmt. Println ("=========map===========")//map is a bunch of unordered sets of key-value pairs that are built-in in Golang and can be used directly, unlike Java needs to introduce hashmap var personmap map[ STRING]PERSONDB//declares a map variable, string is the type of the key, PERSONDB is the type of the value Personmap = Make (Map[string]perso NDB, 10)//create a map with initial capacity of personmap["1"] = Persondb{id: "1", Name: "Zs", Address: "Fangshan District, Beijing"}// Assign a value to the map element personmap["2"] = Persondb{id: "2", Name: "ls", Address: "Beijing Haidian District"}//Use the built-in function Delete to delete the elements of key= "1" in Personmap, if not To the key, then nothing will happen, if Personmap is nil, then the wrong delete (Personmap, "1") if value, OK: = personmap["2"]; OK {//Find the key=2 element, then Ok=true,value is the value found; otherwise ok=false FMT. Println (Value)} PERSONMAP2: = map[string]persondb{"3": persondb{"3", "ww", "Hangzhou City West L. District"}}//Create and initialize a map personmap2[" 4 "] = persondb{" 4 "," ll "," Dongcheng District of Beijing "} for K, V: = range PERSONMAP2 {//Use range to traverse the elements of the map in FMT. Println (k, v) if k = = "3" {v.name = "WwUPdate "personmap2[" 3 "] = V//Can change value//The following method of assignment will be an error, only value can be assigned, cannot directly change its member variable//PERSONMAP2 ["3"]. Name = "Wwupdate"//cannot assign to struct field personmap2["3"]. Name in Map}} FMT.     Println (PERSONMAP2)

Output Result:
Fatal Error:concurrent Map writes
The result is that the map is not thread-safe.
As to why it is not thread-safe, analyze it later

Three. Map pass value or pass reference?

Run the following code:

//map传值还是传引用？ map和slice,channel一样，内部都有一个指向真正存储空间的指针，所以，即使传参时是对值的复制（传值），但都指向同一块存储空间。    var myMapV map[string]int    var myMap1 map[string]int    myMapV = map[string]int{"value": 0}    myMap1 = myMapV    fmt.Println("m =", myMapV)    fmt.Println("m1 =", myMap1)    myMap1["value"] = 1    fmt.Println("m =", myMapV)    fmt.Println("m1 =", myMap1)

Two. is the map thread-safe?

Run the following code:

//map不是线程安全的。     cMap := make(map[string]int)    for i := 0; i < 100; i++ {        go func() {            for j := 0; j < 100; j++ {            cMap[fmt.Sprintf("%d", j)] = j //fatal error: concurrent map writes            }        }()    }     time.Sleep(3 * time.Second)

Output Result:
m = map[value:0]
M1 = map[value:0]
m = map[value:1]
M1 = Map[value:1]
We found that when modifying the m1,m also changed, it seems to be a reference, but in fact map is also transmitted value, its principle and array of slices similar. The map maintains a pointer that points to the true map storage space. We can describe the map as the following structure:

Type Map[key]value struct{
Impl *map_k_v
}
Type Map_k_v struct{
//......
}
In fact, like map and Slice,channel, there is a pointer to the real storage space inside, so even if the parameter is a copy of the value (value), it all points to the same piece of storage space.

Go Language---Map