How do I solve this in Swift? - 600: Fatal error: Index out of range

I am new to Swift and coding in general.
I am trying to get a list of busy periods from multiple events that occur in one day but keep getting "index out of range."

What is wrong here? (I am sorry my code is very messy)

`func busyPeriod() -> [[String:Date]]{

    var busylist: [[String: Date]] = []
    var eventlist: [[String: Date]] = 
      [["start": 2023-02-16 09:00:00, "end": 2023-02-16 10:00:00], 
      ["start": 2023-02-16 10:00:00, "end": 2023-02-16 10:15:00]],
      ["start": 2023-02-16 13:00:00, "end": 2023-02-16 14:00:00]]

    if eventlist.count == 1{
       return eventlist
    }

    for i in eventlist.indices{
        if i == 0 {
            busylist += [["start": eventlist[0]["start"]!, "end": eventlist[0]["end"]!]]
    } else {
        //This part comes out as Thread 1: Fatal Error: Index out of range
        if busylist[-1]["start"]! <= eventlist[i]["start"]!, eventlist[i]["start"]! <= busylist[-1]["end"]! { 
            busylist[-1]["start"] = min(busylist[-1]["start"]!, eventlist[i]["start"]!)
            busylist[-1]["end"] = max(busylist[-1]["end"]!, eventlist[i]["end"]!)
    } else {
        busylist += [["start": eventlist[i]["start"]!, "end": eventlist[i]["end"]!]]
    }}
    return busylist
}

What I expect as an outcome:

    busylist = [
        ["start": 2023-02-16 09:00:00, "end": 2023-02-16 10:15:00],
        ["start": 2023-02-16 13:00:00, "end": 2023-02-16 14:00:00]]

</details>


# 答案1
**得分**: 0

某些编程语言，例如Python，允许您使用负索引从数组的末尾开始计数。因此，在Python中，`busylist[-1]` 会给您数组 `busylist` 中的最后一个元素（如果 `busylist` 不为空）。

Swift 不支持这种行为。

此外，您的代码过于复杂，因为您使用了 `[String: Date]` 而不是一个特定领域的类型来存储时间跨度。

我会更像以下方式编写您的函数，尽管为了避免日期解析的复杂性，我只是使用 `String` 来保存日期（因此违反了我自己使用良好类型的建议😅）：

```swift
import Foundation

struct TimeSpan: CustomStringConvertible {
    var start: String
    var end: String
    
    var description: String {
        "\(start) - \(end)"
    }
}

func busyPeriods(for events: [TimeSpan]) -> [TimeSpan] {
    guard var last = events.first else {
        return []
    }
    
    var answer: [TimeSpan] = []
    
    for event in events.dropFirst() {
        if event.start <= last.end {
            last.start = min(last.start, event.start)
            last.end = max(last.end, event.end)
        } else {
            answer.append(last)
            last = event
        }
    }
    
    answer.append(last)
    return answer
}

print(busyPeriods(for: [
    .init(start: "2023-02-16 09:00:00", end: "2023-02-16 10:00:00"), 
    .init(start: "2023-02-16 10:00:00", end: "2023-02-16 10:15:00"),
    .init(start: "2023-02-16 13:00:00", end: "2023-02-16 14:00:00"),
]))

输出：

[2023-02-16 09:00:00 - 2023-02-16 10:15:00, 2023-02-16 13:00:00 - 2023-02-16 14:00:00]

请注意，您的函数（以及我的重写）对输入做出了一些假设：

• 事件按照开始时间排序。

• 一个事件最多可以与一个之前的事件重叠。

如果违反了这些假设中的任何一个，函数将返回错误的答案。

import Foundation

struct TimeSpan: CustomStringConvertible {
    var start: String
    var end: String
    
    var description: String {
        "\(start) - \(end)"
    }
}

func busyPeriods(for events: [TimeSpan]) -> [TimeSpan] {
    guard var last = events.first else {
        return []
    }
    
    var answer: [TimeSpan] = []
    
    for event in events.dropFirst() {
        if event.start <= last.end {
            last.start = min(last.start, event.start)
            last.end = max(last.end, event.end)
        } else {
            answer.append(last)
            last = event
        }
    }
    
    answer.append(last)
    return answer
}

print(busyPeriods(for: [
    .init(start: "2023-02-16 09:00:00", end: "2023-02-16 10:00:00"), 
    .init(start: "2023-02-16 10:00:00", end: "2023-02-16 10:15:00"),
    .init(start: "2023-02-16 13:00:00", end: "2023-02-16 14:00:00"),
]))

[2023-02-16 09:00:00 - 2023-02-16 10:15:00, 2023-02-16 13:00:00 - 2023-02-16 14:00:00]