如何在两个日期之间获取每个星期五的日期？

Question

I currently use the following code to return an array of dates for every single day between two dates, including today's date and the last date itself. This works great.

• However, how would I go about modifying what I'm already doing in order to do the same exact thing otherwise, but instead return an array of the date of every Friday between the dates? For example, if the function was called on Wed Oct 23rd 2019 to return every Friday until November 10th, the first date would be Fri the 25th, Nov 1st, and then Nov 8th.
• How would I do the same thing as above but for the 1st of every month? If I called the function on Wed Oct 23rd 2019 to return the first of every month until December 16th. The array should have Nov 1st and Dec 1st in it.

func dates(for date: String) -> [String] {
    // first get the endDate
    guard var endDate = Formatter.date.date(from: date) else { return [] }
    // for calendrical calculations you should use noon time
    endDate = Calendar.current.date(bySettingHour: 12, minute: 0, second: 0, of: endDate)!
    // lets get todays noon time to start
    var date = Calendar.current.date(bySettingHour: 12, minute: 0, second: 0, of: Date())!
    var dates: [String] = []
    // while date less than or equal to end date
    while date <= endDate {
        // add the formatted date to the array
        dates.append( Formatter.date.string(from: date))
        // increment the date by one day
        date = Calendar.current.date(byAdding: .day, value: 1, to: date)!
    }

    return dates
}

Answer 1

You just need to add a weekday parameter to your method and check if the weekday of the date inside the loop before adding it to your array:

extension Formatter {
    static let date = DateFormatter()
}

---

func dates(for date: String, weekday: Int? = nil) -> [String] {
    Formatter.date.locale = Locale(identifier: "en_US_POSIX")
    Formatter.date.dateFormat = "yyyy-MM-dd"
    // first get the endDate
    guard var endDate = Formatter.date.date(from: date) else { return [] }
    // for calendrical calculations you should use noon time
    endDate = Calendar.current.date(bySettingHour: 12, minute: 0, second: 0, of: endDate)!
    // lets get todays noon time to start
    var date = Calendar.current.date(bySettingHour: 12, minute: 0, second: 0, of: Date())!
    var dates: [String] = []
    // while date less than or equal to end date
    while date <= endDate {
        if weekday == nil {
            dates.append(Formatter.date.string(from: date))
            date = Calendar.current.date(byAdding: .day, value: 1, to: date)!
        } else if let weekday = weekday, Calendar.current.component(.weekday, from: date) == weekday {
            // add the formatted date to the array
            dates.append(Formatter.date.string(from: date))
            date = Calendar.current.date(byAdding: .weekOfYear, value: 1, to: date)!
        } else {
            date = Calendar.current.date(byAdding: .day, value: 1, to: date)!
        }
    }
    return dates
}

---

dates(for: "2019-12-25")  // ["2019-10-23", "2019-10-24", "2019-10-25", "2019-10-26", "2019-10-27", "2019-10-28", "2019-10-29", "2019-10-30", "2019-10-31", "2019-11-01", "2019-11-02", "2019-11-03", "2019-11-04", "2019-11-05", "2019-11-06", "2019-11-07", "2019-11-08", "2019-11-09", "2019-11-10", "2019-11-11", "2019-11-12", "2019-11-13", "2019-11-14", "2019-11-15", "2019-11-16", "2019-11-17", "2019-11-18", "2019-11-19", "2019-11-20", "2019-11-21", "2019-11-22", "2019-11-23", "2019-11-24", "2019-11-25", "2019-11-26", "2019-11-27", "2019-11-28", "2019-11-29", "2019-11-30", "2019-12-01", "2019-12-02", "2019-12-03", "2019-12-04", "2019-12-05", "2019-12-06", "2019-12-07", "2019-12-08", "2019-12-09", "2019-12-10", "2019-12-11", "2019-12-12", "2019-12-13", "2019-12-14", "2019-12-15", "2019-12-16", "2019-12-17", "2019-12-18", "2019-12-19", "2019-12-20", "2019-12-21", "2019-12-22", "2019-12-23", "2019-12-24", "2019-12-25"]
dates(for: "2019-12-25", weekday: 6) // ["2019-10-25", "2019-11-01", "2019-11-08", "2019-11-15", "2019-11-22", "2019-11-29", "2019-12-06", "2019-12-13", "2019-12-20"]

---

func firstDayOfTheMonth(until date: String) -> [String] {
    Formatter.date.locale = Locale(identifier: "en_US_POSIX")
    Formatter.date.dateFormat = "yyyy-MM-dd"
    guard let endDate = Formatter.date.date(from: date) else { return [] }
    var date = Date()
    var dates: [String] = []
    // while date less than or equal to end date
    while let firstDayOfTheMonth = Calendar.current.nextDate(after: date, matching: .init(day: 1), matchingPolicy: .nextTime), firstDayOfTheMonth <= endDate {
        dates.append(Formatter.date.string(from: firstDayOfTheMonth))
        date = firstDayOfTheMonth
    }
    return dates
}

---

firstDayOfTheMonth(until: "2019-12-25")  // ["2019-11-01", "2019-12-01"]

Answer 2

You can get the weekday for any day of the week with:

let weekDayIndex = Calendar.current.component(.weekday, from: Date())

Friday happens to be day 5. You can get the name of any day with:

print(Calendar.current.weekdaySymbols[weekDayIndex])

So just loop over all of your dates and filter out anything where the weekday is not 5 and you have your answer:

func fridays(in dates: [Date]) {
  dates.filter { Calendar.current.component(.weekday, from: $0) == 5 }
}