swift - 如何在 Swift 中仅对数组的一部分求和？

Question

我目前正尝试在 Swift 中构建我的第一个应用程序，并希望找到一个数组的一部分相对于当月的总和。这是我的代码:

struct Hour {
    var date: String?
    var time: String?

    init(date: String?, time: String?) {
        self.date = date
        self.time = time
    }
}


let hoursData = [
    Hour(date: "Nov 29, 2015", time: "7"),
    Hour(date: "Dec 12, 2015", time: "7"),
    Hour(date: "Dec 14, 2015", time: "7"),
    Hour(date: "Dec 25, 2015", time: "7") ]

我想知道如何制作一个包含当月 time 数据总和的变量？或者任何一个月？非常感谢你们能提供的任何帮助。

Answer 1

首先我会重写你的结构，
这两个属性都可以是不可变的。

我使用 NSDate 代替日期字符串，并使用 double 来表示持续时间

struct Hour {
    let date: NSDate
    let duration: Double

    init(date: NSDate, duration: Double) {
        self.date = date
        self.duration = duration
    }
}

我创造了这样的时间

let hoursData = [
    Hour(date: { let c = NSDateComponents(); c.day = 29; c.month = 11; c.year = 2015;  return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
    Hour(date: { let c = NSDateComponents(); c.day = 12; c.month = 12; c.year = 2015;  return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
    Hour(date: { let c = NSDateComponents(); c.day = 14; c.month = 12; c.year = 2015;  return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7),
    Hour(date: { let c = NSDateComponents(); c.day = 15; c.month = 12; c.year = 2015;  return NSCalendar.currentCalendar().dateFromComponents(c)!}(), duration: 7)]

如果你对这个语法感到疑惑:我使用隐式未命名闭包来创建 NSDate 参数

现在我过滤月份和年份，并使用 reduce 方法对过滤后的对象求和

let today = NSDate()
let totalDuration = hoursData.filter{
    let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date)
    let todaysComps  = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: today)
    return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year
}.reduce(0) { (duration, hour) -> Double in
    duration + hour.duration
}

---

虽然这是一个很好的答案，但我想指出 Rob 的答案有一个小缺陷:Hour 结构隐藏了日期字符串需要某种格式的依赖项。这违反了 Dependency Inversion Principle , SOLID Principles 中的 D .但这很容易解决。

在创建 Hour 实例时只需传入日期格式化程序即可。

struct Hour {
    let date: NSDate
    let duration: Double

    init(date: NSDate, duration: Double) {
        self.date = date
        self.duration = duration
    }

    init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) {
        self.init(date: dateFormatter.dateFromString(dateString)!, duration:duration)
    }
}

let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "MMM d, y"

let hoursData = [
    Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7),
    Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7),
    Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7),
    Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7),
    Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2017", duration: 7)
]

现在使用Hour 的人可以根据需要定义格式，这可能对本地化应用程序有帮助。

过滤和减少保持不变。

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但现在我们有一个新问题:NSDateformatter 的 dateFromString() 可能返回 nil。目前我们用 ! 强制解包，但这在生产应用中可能不好。

我们应该允许适当的错误处理，通过允许便利的 init 抛出错误

enum HourError: ErrorType {
    case InvalidDate
}

struct Hour {

    let date: NSDate
    let duration: Double

    init(date: NSDate, duration: Double) {
        self.date = date
        self.duration = duration
    }

    init (dateFormatter:NSDateFormatter, dateString:String, duration:Double) throws {
        let date = dateFormatter.dateFromString(dateString)
        guard date != nil else { throw HourError.InvalidDate}
        self.init(date: date!, duration:duration)
    }
}

如果我们像这样使用它

do {
    let now = NSDate()
    let dateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "MMM d, y"

    let hoursData = [
        try Hour(dateFormatter: dateFormatter, dateString: "Nov 29, 2015", duration: 7),
        try Hour(dateFormatter: dateFormatter, dateString: "Dec 12, 2015", duration: 7),
        try Hour(dateFormatter: dateFormatter, dateString: "Dec 14, 2015", duration: 7),
        try Hour(dateFormatter: dateFormatter, dateString: "Dec 25, 2015", duration: 7),
/*⛈*/  try Hour(dateFormatter: dateFormatter, dateString: " 25, 2017", duration: 7)
    ]



    let totalDuration = hoursData.filter{
        let objectsComps = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: $0.date)
        let todaysComps  = NSCalendar.currentCalendar().components([.Month, .Year], fromDate: now)
        return objectsComps.month == todaysComps.month && objectsComps.year == todaysComps.year
        }.reduce(0) {
            $0 + $1.duration
    }
    print(totalDuration)
} catch HourError.InvalidDate{
    print("one or more datestrings must be wrong")
}

错误将被捕获。

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full code