英文:
practical way to set and sequentially increment a value inside a map?
问题
package mainimport ("fmt")var store = map[string]int{}func breadArrived(num int) {if breadTotal, ok := store["bread"]; ok {breadTotal += num} else {store["bread"] = num}fmt.Printf("%v\n", store)}func main() {breadArrived(1)breadArrived(2)breadArrived(3)}
上面的代码忽略了 += 运算符，所以 store["bread"] 总是等于 1。我猜我在这里漏掉了类似于 "按引用传递" 的东西。此外，有没有更方便的方法来做到这一点？
英文:
<!-- language: go -->
package mainimport ("fmt")var store = map[string]int{}func breadArrived(num int) {if breadTotal, ok := store["bread"]; ok {breadTotal += num} else {store["bread"] = num}fmt.Printf("%v\n", store)}func main() {breadArrived(1)breadArrived(2)breadArrived(3)}
Code above ignores the += operator, so store["bread"] always equals to 1. I assume I'm missing something like "passing by reference" here. Also, is there any more convenient way of doing this?
答案1
得分: 19
你只是增加了breadTotal这个局部变量的值，而没有增加store映射中的值。应该是这样的：
store["bread"] = breadTotal + num
另外，你也可以简单地这样做：
store["bread"] += num
另外，由于对映射进行索引会返回映射中尚未存在的键的零值（int的零值是0，表示没有面包），所以if语句是完全不必要的。你可以简单地这样做：
func breadArrived(num int) {store["bread"] += numfmt.Printf("%v\n", store)}
输出结果（在Go Playground上尝试）：
map[bread:1]map[bread:3]map[bread:6]
英文:
You're only incrementing the breadTotal local variable, and not the value in the store map. It should be:
store["bread"] = breadTotal + num
Also you can simply do:
store["bread"] += num
Also since indexing a map returns the zero value of the value type for keys which are not yet in the map (zero value for int is 0 – properly telling no bread yet), that if is completely unnecessary. You can simply do:
func breadArrived(num int) {store["bread"] += numfmt.Printf("%v\n", store)}
Output (try it on the Go Playground):
map[bread:1]map[bread:3]map[bread:6]
答案2
得分: 3
breadTotal变量只是map整数值的一个副本，你需要将其引用为store["bread"] += num来增加map整数值。
package mainimport ("fmt")var store = map[string]int{}func breadArrived(num int) {store["bread"] += numfmt.Printf("%v\n", store)}func main() {breadArrived(1)breadArrived(2)breadArrived(3)}
顺便说一下，第一次增加map整数的操作有效是因为Go语言将所有变量初始化为默认值，在整数的情况下，默认值是0。
英文:
The breadTotal variable is only a copy of the map integer value, you need to reference it as store["bread"] += num to increment the map integer value.
https://play.golang.org/p/LQzrbSZudH
package mainimport ("fmt")var store = map[string]int{}func breadArrived(num int) {store["bread"] += numfmt.Printf("%v\n", store)}func main() {breadArrived(1)breadArrived(2)breadArrived(3)}
By the way, the first incrementing of the map integer works because go initializes everything to a default value, in the case of integers the default value is 0.
答案3
得分: 0
简单的增量操作，类似于C/C++风格：
package mainimport ("fmt")var store = map[string]int{}func main() {store["some_key"] = 0if _, found := store["some_key"]; found {fmt.Println(store["some_key"])store["some_key"]++fmt.Println(store["some_key"])}}
点击此处查看代码。
英文:
Simple increment like C\C++ style:
package mainimport ("fmt")var store = map[string]int{}func main() {store["some_key"] = 0if _, found:= store["some_key"]; found{fmt.Println(store["some_key"])store["some_key"]++fmt.Println(store["some_key"])}}
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