需要帮助理解 Go 中的 `map[String]type` 行为 [英] Need help understanding `map[String]type` behaviour in Go

问题描述

请看一下这段代码:

package activity

import (
    "fmt"
    "strconv"
    "time"
)

type Activity struct {
    yearContributions map[string]weekContributions
}

type dayContributions struct {
    Date         time.Time
    Contribution int
}

type weekContributions struct {
    Notation string
    AllDays  []dayContributions
}

func (currentWeekContribution *weekContributions) addContribution(additionalDayContribution dayContributions) {
    currentWeekContribution.AllDays = append(currentWeekContribution.AllDays, additionalDayContribution)
}

func (currentActivity *Activity) restore() {
    currentActivity.yearContributions = map[string]weekContributions{
        "Sunday":    weekContributions{Notation: "S", AllDays: make([]dayContributions, 0)},
        "Monday":    weekContributions{Notation: "M", AllDays: make([]dayContributions, 0)},
        "Tuesday":   weekContributions{Notation: "T", AllDays: make([]dayContributions, 0)},
        "Wednesday": weekContributions{Notation: "W", AllDays: make([]dayContributions, 0)},
        "Thursday":  weekContributions{Notation: "T", AllDays: make([]dayContributions, 0)},
        "Friday":    weekContributions{Notation: "F", AllDays: make([]dayContributions, 0)},
        "Saturday":  weekContributions{Notation: "S", AllDays: make([]dayContributions, 0)},
    }
}

func (currentActivity *Activity) Recalibrate() {
    currentActivity.restore()

    endDate := time.Now().UTC()
    startDate := endDate.AddDate(-1, 0, 0)

    for d := startDate; endDate.After(d); d = d.AddDate(0, 0, 1) {
        stringWeekday := d.Weekday().String()
        currentWeekContribution := currentActivity.yearContributions[stringWeekday]
        currentWeekContribution.addContribution(dayContributions{Date: d, Contribution: 1})
        fmt.Println(currentWeekContribution) // prints : {T [{2017-01-26 09:39:45.711238257 +0000 UTC 1}]}
    }
    fmt.Println(currentActivity.yearContributions) // prints : map[Tuesday:{T []} Wednesday:{W []} Thursday:{T []} Friday:{F []} Saturday:{S []} Sunday:{S []} Monday:{M []}]
}

func (singleDayContribution dayContributions) ReadableDate() string {
    singleDate := singleDayContribution.Date
    return singleDate.Month().String() + " " + strconv.Itoa(singleDate.Day()) + ", " + strconv.Itoa(singleDate.Year())
}

func (currentActivity Activity) GetContributionsForYear() map[string]weekContributions {
    return currentActivity.yearContributions
}

我正在尝试将新的 dayContributions 附加到数组 weekContributions.AllDays.currentWeekContribution.AllDays = append(currentWeekContribution.AllDays, additionalDayContribution) 是 appends 到当前类型 weekContributionsAllDays 属性的一部分/代码>.

I am trying to append new dayContributions to the array weekContributions.AllDays. currentWeekContribution.AllDays = append(currentWeekContribution.AllDays, additionalDayContribution) is the part appends to the AllDays property of the current type weekContributions.

print 语句打印 weekContributions 数组和数组中的一个成员.但是下面的打印函数表示所有地图值，即 weekContributions 对象都有空数组作为 AllDays 属性.

The print statement prints the weekContributions array with a single member in the array. But the following print function says that all map values i.e. weekContributions objects have empty arrays as AllDays property.

我的目标 - 将 dayContribution 对象附加到地图 activity.yearContributions

My aim - To append dayContribution objects to values in map activity.yearContributions

我认为 -

1. append 函数按预期工作，因为它在每次迭代中附加 1 个成员
2. 不知何故，currentWeekContribution.addContribution 行 [第 48 行] 仅附加到局部变量 currentWeekContribution 并且不会更新 currentActivity 中的相应值.年贡献地图

1. The append function is working as intended as it appends 1 member on each iteration
2. Somehow, the currentWeekContribution.addContribution line [Line 48] is only appending to the local variable currentWeekContribution and doesn't update the corresponding value in currentActivity.yearContributions map

我对 golang 很陌生，我认为我没有遵循 go 方式/这在 java 或 中有效>红宝石我不知道这种行为叫什么，所以不能用谷歌搜索任何相关的东西.

I am very new to golang and I think I am not following the go way / this would have worked in java or ruby I don't know what this behaviour is called, so can't google search anything relevant.

推荐答案

有两种方法可以做到这一点.

Two way you can do this.

一，您需要使用指针 in 作为 Map 值.因此，当您附加到局部变量时，您正在通过引用进行更改.

One, You need to use pointer in as Map value. So that when you are appending in your local variable, you are changing through reference.

type Activity struct {
    yearContributions map[string]*weekContributions
}

type dayContributions struct {
    Date         time.Time
    Contribution int
}

type weekContributions struct {
    Notation string
    AllDays  []*dayContributions
}

二、当你在做以下事情时

Two, When you are doing following

currentWeekContribution := currentActivity.yearContributions[stringWeekday]

您正在分配一个局部变量.你正在附加它.现在，您需要重新分配该局部变量.喜欢

You are assigning a local variable. And you are appending to it. Now, you need to reassign that local variable. Like

currentWeekContribution := currentActivity.yearContributions[stringWeekday]
currentWeekContribution.addContribution(dayContributions{Date: d, Contribution: 1})
currentActivity.yearContributions[stringWeekday]= currentWeekContribution

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