arrays - 快速获取 double 组的总和

Question

我有一个字符串数字数组，我正在从核心数据中获取这些数字并将其转换为 double 值。完成此操作后，我想得到他们的总和，但出现错误。我试过这样:

override func viewDidLoad() {
    super.viewDidLoad()

        //CoreData
        let appDelegate = UIApplication.sharedApplication().delegate as AppDelegate
        let managedContext : NSManagedObjectContext = appDelegate.managedObjectContext!
        var fetchRequest = NSFetchRequest(entityName: "Log")
        fetchRequest.returnsObjectsAsFaults = false;
        var results: NSArray = managedContext.executeFetchRequest(fetchRequest, error: nil)!

        if (results.count > 0) {
            for res in results {
                var totalWorkTimeInHoursString = res.valueForKey("totalWorkTimeInHoursString")  as String
                //get double value of strings array
                var totalWorkTimeInHoursNSString = NSString(string: totalWorkTimeInHoursString)
                var totalWorkTimeInHoursNSStringDoubleValue = totalWorkTimeInHoursNSString.doubleValue
                lastLogHoursWorked.text = "\(totalWorkTimeInHoursString) hours"
                totalHoursWorkedSum.text = "\(totalWorkTimeInHoursNSStringDoubleValue)"

                let sum = totalWorkTimeInHoursNSStringDoubleValue.reduce(0,+)
                    //throws an error saying 'Double' does not have a member named 'reduce'

                println(totalWorkTimeInHoursNSStringDoubleValue)
                    //lists the array of doubles in console successfully
                println(sum)
                    //checking to see if 'sum' works properly

        }
        }else {
            println("zero results returned, potential error")
        }
}

我处理得当吗？我错过了什么？

Answer 1

查看您的代码，我推断 results 是一个字典数组，每个字典都有一个 totalWorkTimeInHoursString 键，其值是 double 字的字符串表示形式.

我会使用一些函数式方法来解决它，使用 filter、map 和 reduce:

if results.count > 0 {
    var res = results as [NSDictionary]

    var result = res.filter { $0["totalWorkTimeInHoursString"] is String }
        .map { ($0["totalWorkTimeInHoursString"] as NSString).doubleValue }
        .reduce (0, +)
}

代码的作用:

• 将results数组转换为NSDictionary数组
• 通过删除所有不具有 totalWorkTimeInHoursString 键或其对应值不是字符串的元素(字典)来过滤数组
• 通过提取与 totalWorkTimeInHoursString 键对应的值，将数组的每个元素转换为 double，转换为 NSString，然后转换为 Double
• 应用reduce 方法，传递+ 运算符对Double 的结果数组求和

这是我在 Playground 上测试我的解决方案的数据:

var results: NSArray = [
    ["totalWorkTimeInHoursString": "10.0"],
    ["totalWorkTimeInHoursString": "5.0"]
]