如何在golang中创建关联地图？

Question

I'm trying to create an 'associative' map. The issue is that the superMap collects all the values of aMap. Basically even if I want to store only one instance of amap on each look I endup storing the current instance plus all the previous loops. supermap[value] = amap[value] . However the snippet below stores supermap[value] = amap. The reason is that I can't find any way to fix this. If I delete the old values of aMap after each loop they are deleted from supermap as well.

  package main

    import (
    "fmt"
)

func main() {

    aData := map[int]string{
        0: "apple",
        1: "samsung",
        2: "htc",
        3: "sony",
    }

    dir := []string{"user", "doc", "bin", "src"}

    aMap := make(map[int]string)
    superMap := make(map[string]map[int]string)

    for k, v := range dir {
        aMap[k] = aData[k]
        superMap[v] = aMap


    }
    hello(superMap)

}

func hello(superMap map[string]map[int]string) {
    fmt.Printf("superMap of user is %v 
", superMap["user"])

    cnt := len(superMap["user"])
    if cnt > 1{


    fmt.Printf("expected only one value received %v", cnt)
    }
}

Play

Answer 1

As Arjan said in the comment, you need to move the creation of aMap into the for loop. The reason is because, in the original code you posted you are dealing with one instance of aMap in memory. That means, only one map called aMap is created and when you assign another variable to the value of aMap you are assigning a reference. This means, any variable that hold a reference (to aMap) where state is mutated, will be observed in all other variables also holding the reference because they all resolve to the same object in memory.

When the aMap is moved into the for/range loop, this means that 4 individual instances of aMap will be created all with their own memory. Mutating the state of one of those aMaps will not affect the others because they are their own objects in memory. Now, if you took one of those objects and made a reference to it again with another variable then you'd end up in the same boat as the first case.

package main

import (
    "fmt"
)

func main() {

    aData := map[int]string{
        0: "apple",
        1: "samsung",
        2: "htc",
        3: "sony",
    }

    dir := []string{"user", "doc", "bin", "src"}

    //aMap := make(map[int]string) //only one map instance is created in memory
    superMap := make(map[string]map[int]string)

    for k, v := range dir {
        //multiple map instances are created in memory
        aMap := make(map[int]string)
        aMap[k] = aData[k]
        superMap[v] = aMap

    }

    hello(superMap)
}

func hello(superMap map[string]map[int]string) {
    fmt.Printf("superMap of user is %v 
", superMap["user"])
    cnt := len(superMap["user"])

    if cnt > 1 {
        fmt.Printf("expected only one value received %v", cnt)
    }
}