如何计算给定月份的周数？

Question

在查看了一些关于 SO 的不同建议后，我无法确定为什么下面的功能不起作用。它似乎在某些月份返回 6，而在其他月份返回 5。当改变几周或几天时，效果非常好。

例如，尝试

weeksInMonth(8, forYear 2015) 

结果为 6。

我相信我对日历上“firstWeekday”属性的作用存在误解，但尚未找到 Apple 或在线的充分解释。

已尝试过 .WeekOfMonth 和 .WeekOfYear。再次找不到确切差异的解释。

如有任何建议，我们将非常欢迎。

func weeksInMonth(month: Int, forYear year: Int) -> Int?
{
    if (month < 1 || month > 12) { return nil }

    let dateString = String(format: "%4d/%d/01", year, month)
    let dateFormatter = NSDateFormatter()
    dateFormatter.dateFormat = "yyyy/MM/dd"
    if let date = dateFormatter.dateFromString(dateString),
       let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)
    {
        calendar.firstWeekday = 2 // Monday
        let weekRange = calendar.rangeOfUnit(.WeekOfMonth, inUnit: .Month, forDate: date)
        let weeksCount = weekRange.length
        return weeksCount
    }
    else
    {
        return nil
    }
}

更新：

抱歉我的问题不清楚。我正在尝试算出一个月中有多少周包含星期一。对于 8 月，这应该是 5。

Answer 1

您的代码计算发生的周数（完整或部分） 在一个月。您显然想要的是星期一的数量 在给定的月份。与

NSDateComponents

，特别是与 您可以计算第一个

weekdayOrdinal

属性 (

weekdayOrdinal=1

) 和最后 (

weekdayOrdinal=-1

) 星期一 在一个月。然后计算周数的差异（并加一）。

在 Swift 2 中可能的实现：

func numberOfMondaysInMonth(month: Int, forYear year: Int) -> Int?
{
    let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    calendar.firstWeekday = 2 // 2 == Monday

    // First monday in month:
    let comps = NSDateComponents()
    comps.month = month
    comps.year = year
    comps.weekday = calendar.firstWeekday
    comps.weekdayOrdinal = 1
    guard let first = calendar.dateFromComponents(comps)  else {
        return nil
    }

    // Last monday in month:
    comps.weekdayOrdinal = -1
    guard let last = calendar.dateFromComponents(comps)  else {
        return nil
    }

    // Difference in weeks:
    let weeks = calendar.components(.WeekOfMonth, fromDate: first, toDate: last, options: [])
    return weeks.weekOfMonth + 1
}

注意： 负数

weekdayOrdinal

从月底开始倒数，这一点在文档中并不明显。观察到 确定特定年份阵亡将士纪念日的 NSDate 并由 Dave DeLong 确认）。

---

更新

Swift 3：

func numberOfMondaysInMonth(_ month: Int, forYear year: Int) -> Int? {
    var calendar = Calendar(identifier: .gregorian)
    calendar.firstWeekday = 2 // 2 == Monday

    // First monday in month:
    var comps = DateComponents(year: year, month: month,
                               weekday: calendar.firstWeekday, weekdayOrdinal: 1)
    guard let first = calendar.date(from: comps)  else {
        return nil
    }

    // Last monday in month:
    comps.weekdayOrdinal = -1
    guard let last = calendar.date(from: comps)  else {
        return nil
    }

    // Difference in weeks:
    let weeks = calendar.dateComponents([.weekOfMonth], from: first, to: last)
    return weeks.weekOfMonth! + 1
}

Answer 2

实际上你的问题是：某个月有多少个星期一？

我的方法是计算该月的第一个星期一，这可以通过将 CalendarUnit

WeekdayOrdinal

 设置为 1 来完成。然后获取总天数并进行一些数学计算。

斯威夫特1.2

func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
  if 1...12 ~= month {

    let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
    let components = NSDateComponents()
    components.weekday = 2 // Monday
    components.weekdayOrdinal = 1
    components.month = month
    components.year = year

    if let date = calendar.dateFromComponents(components)  {
      let firstDay = calendar.component(.CalendarUnitDay, fromDate: date)
      let days = calendar.rangeOfUnit(.CalendarUnitDay, inUnit:.CalendarUnitMonth, forDate:date).length
      return (days - firstDay) / 7 + 1
    }
  }
  return nil
}

斯威夫特2

func mondaysInMonth(month: Int, forYear year: Int) -> Int?
{
  guard 1...12 ~= month else { return nil }

  let calendar = NSCalendar(calendarIdentifier: NSCalendarIdentifierGregorian)!
  let components = NSDateComponents()
  components.weekday = 2 // Monday
  components.weekdayOrdinal = 1
  components.month = month
  components.year = year

  if let date = calendar.dateFromComponents(components)  {
    let firstDay = calendar.component(.Day, fromDate: date)
    let days = calendar.rangeOfUnit(.Day, inUnit:.Month, forDate:date).length
    return (days - firstDay) / 7 + 1
  }
  return nil
}

Answer 3

斯威夫特2

以下代码计算一个月中的周数。它不取决于该月的开始或结束日期。

func weeksInMonth(month: Int, year: Int) -> (Int)? {

  let calendar = NSCalendar.currentCalendar()

  let comps = NSDateComponents()
  comps.month = month+1
  comps.year = year
  comps.day = 0

  guard let last = calendar.dateFromComponents(comps) else {
    return nil
  }
  // Note: Could get other options as well
  let tag = calendar.components([.WeekOfMonth,.WeekOfYear,
       .YearForWeekOfYear,.Weekday,.Quarter],fromDate: last)

  return tag.weekOfMonth

}

// Example Usage:

if let numWeeks = weeksInMonth(8,year: 2015) {
  print(numWeeks) // Prints 6
}

if let numWeeks = weeksInMonth(12,year: 2015) {
  print(numWeeks) // Prints 5
}

Answer 4

斯威夫特4+：

//-- Get number of weeks from calendar
func numberOfWeeksInMonth(_ date: Date) -> Int {
     var calendar = Calendar(identifier: .gregorian)
     calendar.firstWeekday = 1
     let weekRange = calendar.range(of: .weekOfMonth,
                                    in: .month,
                                    for: date)
     return weekRange!.count
}

//-- Implementation
let weekCount = numberOfWeeksInMonth(Date)

Answer 5

斯威夫特4+：

从您的组件生成日期：

let dateComponents = DateComponents.init(year: 2019, month: 5)
let monthCurrentDayFirst = Calendar.current.date(from: dateComponents)!
let monthNextDayFirst = Calendar.current.date(byAdding: .month, value: 1, to: monthCurrentDayFirst)!
let monthCurrentDayLast = Calendar.current.date(byAdding: .day, value: -1, to: monthNextDayFirst)!

从日期检测周数：

let numberOfWeeks = Calendar.current.component(.weekOfMonth, from: monthCurrentDayLast)

Answer 6

恕我直言，最简单的解决方案是

Calendar.current.range(of: .weekOfMonth, in: .month, for: date) 无需设置任何

firstWeekday

 或使用 

NSDateComponents

.. 除了日期之外什么都没有。
仅供参考 

firstWeekday

 可能会导致意外行为，因为某些地区使用星期一作为一周的第一天，而不是星期日