Goのrangeループで変数に異なるポインタを割り当てる

Problem

I’d like to count up, but not working below:

package main import ( "fmt" ) type obj struct { key string counter int } func goodcase(objs []obj) map[string]*obj { fmt.Println("*** good case ***") grpMap := make(map[string]*obj) for i, _ := range objs { v := objs[i] // important if grpMap[v.key] != nil { grpMap[v.key].counter += 1 } else { print(&v) fmt.Println(v) grpMap[v.key] = &v } } return grpMap } func badcase(objs []obj) map[string]*obj { fmt.Println("*** bad case ***") grpMap := make(map[string]*obj) for _, v := range objs { if grpMap[v.key] != nil { grpMap[v.key].counter += 1 } else { print(&v) fmt.Println(v) grpMap[v.key] = &v } } return grpMap } func printMap(grpMap map[string]*obj) { for k, v := range grpMap { fmt.Println(k, v) } } func main() { objs := []obj{ obj{"a", 1}, obj{"b", 1}, obj{"a", 1}, obj{"c", 1}, obj{"c", 1}, } fmt.Println("objs: ", objs) printMap(badcase(objs)) printMap(goodcase(objs)) }

Output:

objs: [{a 1} {b 1} {a 1} {c 1} {c 1}] *** bad case *** 0xc0000100c0{a 1} 0xc0000100c0{b 1} 0xc0000100c0{c 1} b &{c 2} c &{c 2} a &{c 2} ...

How

Look at the goodcase function. Important is v := objs[i], what you allocate different pointer.

Output of example code in Problem section:

*** good case *** 0xc000010168{a 1} 0xc000010198{b 1} 0xc0000101e0{c 1} a &{a 2} b &{b 1} c &{c 2}

References

• Golang range and pointers