英文:
Go update slice iterating error "does not support indexing"
问题
我正在尝试通过索引或迭代来遍历切片值并更新一个或多个值。然后我遇到了以下问题。
package mainimport "fmt"type Member struct {Name, Surname stringAge int}type Members []Memberfunc main() {members1 := Members{{"Bob", "Doe", 30},{"Jack", "Doe", 30},{"John", "Doe", 30},}members1.ModifyName()members1.ModifyAllNames1()members1.ModifyAllNames2()fmt.Println(members1)}func (members *Members) ModifyName() {(*members)[0].Name = "-"}func (members *Members) ModifyAllNames1() {for i := 0; i < len(*members); i++ {(*members)[i].Name = "-"}}func (members *Members) ModifyAllNames2() {for _, m := range *members {m.Name = "-"}}
然后我得到了以下错误：
invalid operation: members[0] (type *Members does not support indexing)invalid operation: members[i] (type *Members does not support indexing)
请注意，你需要使用(*members)来访问Members类型的切片元素，因为members是一个指向Members的指针。
英文:
I'm trying to iterate through slice values and update one or more values, accessing it by index or iterating through them. Then I have the following problem.
package mainimport "fmt"type Member struct {Name, Surname stringAge int}type Members []Memberfunc main() {members1 := Members{{"Bob", "Doe", 30},{"Jack", "Doe", 30},{"John", "Doe", 30},}members1.ModifyName()members1.ModifyAllNames1()members1.ModifyAllNames2()fmt.Println(members1)}func (members *Members) ModifyName() {*members[0].Name = "-"}func (members *Members) ModifyAllNames1() {for i := 0; i < len(*members); i++ {*members[i].Name = "-"}}func (members *Members) ModifyAllNames2() {for _, m := range *members {m.Name = "-"}}
Then I get the following errors:
invalid operation: members[0] (type *Members does not support indexing)invalid operation: members[i] (type *Members does not support indexing)
答案1
得分: 84
类型*Members不支持索引，类型Members支持。
m := make([]Members, 5)pm := &mm[:2] // 正确pm[:2] // 错误(*pm)[:2] // 正确
https://go.dev/play/p/CDaBKqB-sqM
package mainimport "fmt"type Member struct {Name, Surname stringAge int}type Members []Memberfunc main() {members1 := Members{{"Bob", "Doe", 30},{"Jack", "Doe", 30},{"John", "Doe", 30},}members1.ModifyName()members1.ModifyAllNames1()//members1.ModifyAllNames2()fmt.Println(members1)}func (members *Members) ModifyName() {(*members)[0].Name = "-"}func (members *Members) ModifyAllNames1() {for i := 0; i < len(*members); i++ {(*members)[i].Name = "-"}}func (members Members) ModifyAllNames2() {for i := range members {members[i].Name = "-"}}
英文:
Type *Members does not support indexing, type Members does.
m := make([]Members, 5)pm := &mm[:2] // goodpm[:2] // bad(*pm)[:2] // good
https://go.dev/play/p/CDaBKqB-sqM
package mainimport "fmt"type Member struct {Name, Surname stringAge int}type Members []Memberfunc main() {members1 := Members{{"Bob", "Doe", 30},{"Jack", "Doe", 30},{"John", "Doe", 30},}members1.ModifyName()members1.ModifyAllNames1()//members1.ModifyAllNames2()fmt.Println(members1)}func (members *Members) ModifyName() {(*members)[0].Name = "-"}func (members *Members) ModifyAllNames1() {for i := 0; i < len(*members); i++ {(*members)[i].Name = "-"}}func (members Members) ModifyAllNames2() {for i := range members {members[i].Name = "-"}}
答案2
得分: 6
问题出在运算符的结合性和/或优先级上，尽管我在 Golang 规范中没有找到这个特定情况的明确说明。
表达式 *members[0].Name 等同于 *(members[0].Name)，所以为了正确地进行指针解引用，你需要在需要解引用的项周围显式地加上一对括号，即 (*members)[0].Name。
英文:
> func (members *Members) ModifyName() {
> *members[0].Name = "-"
> }
>
> invalid operation: members[0] (type *Members does not support indexing)
The issue is that of operator associativity and/or precedence, although I couldn't find this specific case explicitly covered within the Golang spec.
The expression *members[0].Name appears to be equivalent to *(members[0].Name), so, to get the proper pointer deference, you gotta use a pair of parentheses explicitly around the item that must be dereferenced, as is, (*members)[0].Name.
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