如何基于参数对对象列表进行分类 [英] How to categorize list of objects based on a parameter

问题描述

我有一个具有此定义的对象列表:

I have a list of objects with this definition:

type MyObject struct {
    ID        int `json:"id"`
    Level     int `json:"level"`
    CityID    int `json:"city_id"`
}

我想基于 CityID 对它们进行分类，以获取列表列表，其中每个内部列表的项目都具有相同的 CityID .

I want to categorize them based on CityID in order to get a list of lists where each inner list's items have the same CityID.

例如，如果我有以下列表:

For example, if I have the following list:

[
    MyObject {ID: 1, Level: 12, CityID: 7},
    MyObject {ID: 2, Level: 15, CityID: 2},
    MyObject {ID: 3, Level: 55, CityID: 4},
    MyObject {ID: 4, Level: 88, CityID: 7},
    MyObject {ID: 5, Level: 11, CityID: 4},
    MyObject {ID: 6, Level: 5, CityID: 7},
    MyObject {ID: 7, Level: 42, CityID: 2}
]

我需要下面的输出:

[
    [MyObject {ID: 1, Level: 12, CityID: 7}, MyObject {ID: 4, Level: 88, CityID: 7}, MyObject {ID: 6, Level: 5, CityID: 7}],
    [MyObject {ID: 2, Level: 15, CityID: 2}, MyObject {ID: 7, Level: 42, CityID: 2}],
    [MyObject {ID: 3, Level: 55, CityID: 4}, MyObject {ID: 5, Level: 11, CityID: 4}]
]

我知道使用 itertools 在python中是可能的，但是我在 go 中是新手，对它的库知之甚少.有帮助吗?

I know it is possible in python using itertools, but I'm new in go and have little knowledge about its libraries. Any help?

我当前正在使用此

m := make(map[int][]MyObject)

for _, item := range myList {
    if val, ok := m[item.CityID]; ok {
        m[item.CityID] = append(val, item)
    } else {
        m[item.CityID] = []MyObject{item, }
    }
}

推荐答案

您当前的方法是可行的方法，但是可以简化，无需检查 CityID 是否已经在地图中，因为索引带有未在其中的键的地图会导致值类型的零值，对于切片而言为 nil ，您可以毫无问题地附加到 nil 切片:

Your current approach is the way to go, but it may be simplified, there is no need to check if a CityID is already in the map, because indexing a map with a key that isn't in it will result in the zero value of the value type, which is nil for slices, and you may append to a nil slice without a problem:

m := make(map[int][]MyObject)

for _, item := range myList {
    m[item.CityID] = append(m[item.CityID], item)
}

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