给定一个日期范围，我需要知道该范围内有多少个星期一（或星期二、星期三等）。
我目前在C#工作。
试试这个：
static int CountDays(DayOfWeek day, DateTime start, DateTime end)
{
TimeSpan ts = end - start; // Total duration
int count = (int)Math.Floor(ts.TotalDays / 7); // Number of whole weeks
int remainder = (int)(ts.TotalDays % 7); // Number of remaining days
int sinceLastDay = (int)(end.DayOfWeek - day); // Number of days since last [day]
if (sinceLastDay < 0) sinceLastDay += 7; // Adjust for negative days since last [day]
// If the days in excess of an even week are greater than or equal to the number days since the last [day], then count this one, too.
if (remainder >= sinceLastDay) count++;
return count;
}
由于使用C#，如果使用C#3.0，则可以使用LINQ。
假设您有一个数组/List/IQueryable等，其中包含DateTime类型的日期：
DateTime[] dates = { new DateTime(2008,10,6), new DateTime(2008,10,7)}; //etc....
var mondays = dates.Where(d => d.DayOfWeek == DayOfWeek.Monday); // = {10/6/2008}
添加：
不确定你是否打算对它们进行分组和计数，但以下是如何在LINQ中做到这一点：
var datesgrouped = from d in dates
group d by d.DayOfWeek into grouped
select new { WeekDay = grouped.Key, Days = grouped };
foreach (var g in datesgrouped)
{
Console.Write (String.Format("{0} : {1}", g.WeekDay,g.Days.Count());
}
研究不同的算法来计算一周中的哪一天很有趣，@Gabe Hollombe在这个问题上指出WP是一个很好的主意（我记得大约20年前用COBOL实现了Zeller的同余），但当人们问到现在是什么时候时，这相当于给他们一个时钟的蓝图。
在C#中：
private int CountMondays(DateTime startDate, DateTime endDate)
{
int mondayCount = 0;
for (DateTime dt = startDate; dt < endDate; dt = dt.AddDays(1.0))
{
if (dt.DayOfWeek == DayOfWeek.Monday)
{
mondayCount++;
}
}
return mondayCount;
}
这当然不评估"；Mondayness"；，因此，如果需要的话，让for循环评估
dt < endDate.AddDays(1.0)
这里有一些伪代码：
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayOfWeek(Start) <= DayImLookingFor
+ 1 if DayOfWeek(End) >= DayImLookingFor
- 1
其中，DifferenceInDays以天为单位返回End - Start，DayOfWeek以整数形式返回一周中的某一天。DayOfWeek使用什么映射并不重要，只要它在增加并与DayImLookingFor匹配即可。
请注意，此算法假定日期范围包含在内。如果End不应该是范围的一部分，您将不得不稍微调整算法。
翻译成C#是留给读者的练习。
是否有任何特定的语言以及日期格式？
如果日期表示为天数，那么两个值加一（天）并除以7之间的差就是大部分答案。如果两个结束日期都是有问题的日期，请添加一个。
编辑：将"模7"更正为"除以7"-谢谢。这就是整数除法。
如果你想在两个日期之间获得特定的星期，你可以试试这个
public List<DateTime> GetSelectedDaysInPeriod(DateTime startDate, DateTime endDate, List<DayOfWeek> daysToCheck)
{
var selectedDates = new List<DateTime>();
if (startDate >= endDate)
return selectedDates; //No days to return
if (daysToCheck == null || daysToCheck.Count == 0)
return selectedDates; //No days to select
try
{
//Get the total number of days between the two dates
var totalDays = (int)endDate.Subtract(startDate).TotalDays;
//So.. we're creating a list of all dates between the two dates:
var allDatesQry = from d in Enumerable.Range(1, totalDays)
select new DateTime(
startDate.AddDays(d).Year,
startDate.AddDays(d).Month,
startDate.AddDays(d).Day);
//And extracting those weekdays we explicitly wanted to return
var selectedDatesQry = from d in allDatesQry
where daysToCheck.Contains(d.DayOfWeek)
select d;
//Copying the IEnumerable to a List
selectedDates = selectedDatesQry.ToList();
}
catch (Exception ex)
{
//Log error
//...
//And re-throw
throw;
}
return selectedDates;
}
添加尽可能小的数字，使第一天成为星期一。减去尽可能小的数字，使最后一天成为星期一。计算天数差，除以7。
将日期转换为儒略日数字，然后做一点数学运算。由于周一是零模式7，你可以这样计算：
JD1=JulianDayOf(the_first_date)
JD2=JulianDayOf(the_second_date)
Round JD1 up to nearest multiple of 7
Round JD2 up to nearest multiple of 7
d = JD2-JD1
nMondays = (JD2-JD1+7)/7 # integer divide
我今天也有同样的需求。我从cjm函数开始，因为我不理解JonB功能，而且Cyberherbox的函数不是线性的。
我必须纠正
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayOfWeek(Start) <= DayImLookingFor
+ 1 if DayOfWeek(End) >= DayImLookingFor
- 1
至
DifferenceInDays(Start, End) / 7 // Integer division discarding remainder
+ 1 if DayImLookingFor is between Start.Day and End.Day
使用返回true的between函数，如果从开始的一天开始，我们在endDay之前的第一天ImLookingFor见面。
我通过计算从startDay到另外两天的天数来完成between函数：
private int CountDays(DateTime start, DateTime end, DayOfWeek selectedDay)
{
if (start.Date > end.Date)
{
return 0;
}
int totalDays = (int)end.Date.Subtract(start.Date).TotalDays;
DayOfWeek startDay = start.DayOfWeek;
DayOfWeek endDay = end.DayOfWeek;
///look if endDay appears before or after the selectedDay when we start from startDay.
int startToEnd = (int)endDay - (int)startDay;
if (startToEnd < 0)
{
startToEnd += 7;
}
int startToSelected = (int)selectedDay - (int)startDay;
if (startToSelected < 0)
{
startToSelected += 7;
}
bool isSelectedBetweenStartAndEnd = startToEnd >= startToSelected;
if (isSelectedBetweenStartAndEnd)
{
return totalDays / 7 + 1;
}
else
{
return totalDays / 7;
}
}
这将返回一组整数，显示一周中每天在日期范围内发生的次数
int[] CountDays(DateTime firstDate, DateTime lastDate)
{
var totalDays = lastDate.Date.Subtract(firstDate.Date).TotalDays + 1;
var weeks = (int)Math.Floor(totalDays / 7);
var result = Enumerable.Repeat<int>(weeks, 7).ToArray();
if (totalDays % 7 != 0)
{
int firstDayOfWeek = (int)firstDate.DayOfWeek;
int lastDayOfWeek = (int)lastDate.DayOfWeek;
if (lastDayOfWeek < firstDayOfWeek)
lastDayOfWeek += 7;
for (int dayOfWeek = firstDayOfWeek; dayOfWeek <= lastDayOfWeek; dayOfWeek++)
result[dayOfWeek % 7]++;
}
return result;
}
或者是一个轻微的变化，可以执行FirstDate.TotalDaysOfWeeks（SecondDate）并返回Dictionary
public static Dictionary<DayOfWeek, int> TotalDaysOfWeeks(this DateTime firstDate, DateTime lastDate)
{
var totalDays = lastDate.Date.Subtract(firstDate.Date).TotalDays + 1;
var weeks = (int)Math.Floor(totalDays / 7);
var resultArray = Enumerable.Repeat<int>(weeks, 7).ToArray();
if (totalDays % 7 != 0)
{
int firstDayOfWeek = (int)firstDate.DayOfWeek;
int lastDayOfWeek = (int)lastDate.DayOfWeek;
if (lastDayOfWeek < firstDayOfWeek)
lastDayOfWeek += 7;
for (int dayOfWeek = firstDayOfWeek; dayOfWeek <= lastDayOfWeek; dayOfWeek++)
resultArray[dayOfWeek % 7]++;
}
var result = new Dictionary<DayOfWeek, int>();
for (int dayOfWeek = 0; dayOfWeek < 7; dayOfWeek++)
result[(DayOfWeek)dayOfWeek] = resultArray[dayOfWeek];
return result;
}
一位修改后的代码在这里工作并由我测试
private int CountDays(DayOfWeek day, DateTime startDate, DateTime endDate)
{
int dayCount = 0;
for (DateTime dt = startDate; dt < endDate; dt = dt.AddDays(1.0))
{
if (dt.DayOfWeek == day)
{
dayCount++;
}
}
return dayCount;
}
示例：
int Days = CountDays(DayOfWeek.Friday, Convert.ToDateTime("2019-07-04"),
Convert.ToDateTime("2019-07-27")).ToString();
我在一份报告中遇到了类似的问题。我需要两次约会之间的工作日数。我本可以循环浏览日期并计数，但我的离散数学训练不允许。下面是我用VBA编写的一个函数，用于获取两个日期之间的工作日数。我相信.net也有类似的WeekDay功能。
1
2 ' WorkDays
3 ' returns the number of working days between two dates
4 Public Function WorkDays(ByVal dtBegin As Date, ByVal dtEnd As Date) As Long
5
6 Dim dtFirstSunday As Date
7 Dim dtLastSaturday As Date
8 Dim lngWorkDays As Long
9
10 ' get first sunday in range
11 dtFirstSunday = dtBegin + ((8 - Weekday(dtBegin)) Mod 7)
12
13 ' get last saturday in range
14 dtLastSaturday = dtEnd - (Weekday(dtEnd) Mod 7)
15
16 ' get work days between first sunday and last saturday
17 lngWorkDays = (((dtLastSaturday - dtFirstSunday) + 1) / 7) * 5
18
19 ' if first sunday is not begin date
20 If dtFirstSunday <> dtBegin Then
21
22 ' assume first sunday is after begin date
23 ' add workdays from begin date to first sunday
24 lngWorkDays = lngWorkDays + (7 - Weekday(dtBegin))
25
26 End If
27
28 ' if last saturday is not end date
29 If dtLastSaturday <> dtEnd Then
30
31 ' assume last saturday is before end date
32 ' add workdays from last saturday to end date
33 lngWorkDays = lngWorkDays + (Weekday(dtEnd) - 1)
34
35 End If
36
37 ' return working days
38 WorkDays = lngWorkDays
39
40 End Function
private System.Int32 CountDaysOfWeek(System.DayOfWeek dayOfWeek, System.DateTime date1, System.DateTime date2)
{
System.DateTime EndDate;
System.DateTime StartDate;
if (date1 > date2)
{
StartDate = date2;
EndDate = date1;
}
else
{
StartDate = date1;
EndDate = date2;
}
while (StartDate.DayOfWeek != dayOfWeek)
StartDate = StartDate.AddDays(1);
return EndDate.Subtract(StartDate).Days / 7 + 1;
}
四年后，我想我应该运行一个测试：
[TestMethod]
public void ShouldFindFridaysInTimeSpan()
{
//reference: http://stackoverflow.com/questions/248273/count-number-of-mondays-in-a-given-date-range
var spanOfSixtyDays = new TimeSpan(60, 0, 0, 0);
var setOfDates = new List<DateTime>(spanOfSixtyDays.Days);
var now = DateTime.Now;
for(int i = 0; i < spanOfSixtyDays.Days; i++)
{
setOfDates.Add(now.AddDays(i));
}
Assert.IsTrue(setOfDates.Count == 60,
"The expected number of days is not here.");
var fridays = setOfDates.Where(i => i.DayOfWeek == DayOfWeek.Friday);
Assert.IsTrue(fridays.Count() > 0,
"The expected Friday days are not here.");
Assert.IsTrue(fridays.First() == setOfDates.First(i => i.DayOfWeek == DayOfWeek.Friday),
"The expected first Friday day is not here.");
Assert.IsTrue(fridays.Last() == setOfDates.Last(i => i.DayOfWeek == DayOfWeek.Friday),
"The expected last Friday day is not here.");
}
我对TimeSpan的使用有点过头了——实际上我想直接查询TimeSpan。