Count number of Mondays in a given date range

Solution 1:

Try this:

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;
}

Solution 2:

Since you're using C#, if you're using C#3.0, you can use LINQ.

Assuming you have an Array/List/IQueryable etc that contains your dates as DateTime types:

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}

Added:

Not sure if you meant grouping them and counting them, but here's how to do that in LINQ as well:

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());
}

Solution 3:

It's fun to look at different algorithms for calculating day of week, and @Gabe Hollombe's pointing to WP on the subject was a great idea (and I remember implementing Zeller's Congruence in COBOL about twenty years ago), but it was rather along the line of handing someone a blueprint of a clock when all they asked what time it was.

In C#:

    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;
    }

This of course does not evaluate the end date for "Mondayness", so if this was desired, make the for loop evaluate

dt < endDate.AddDays(1.0)

Solution 4:

Here's some pseudocode:

DifferenceInDays(Start, End) / 7   // Integer division discarding remainder
+ 1 if DayOfWeek(Start) <= DayImLookingFor
+ 1 if DayOfWeek(End)   >= DayImLookingFor
- 1

Where DifferenceInDays returns End - Start in days, and DayOfWeek returns the day of the week as an integer. It doesn't really matter what mapping DayOfWeek uses, as long as it is increasing and matches up with DayImLookingFor.

Note that this algorithm assumes the date range is inclusive. If End should not be part of the range, you'll have to adjust the algorithm slightly.

Translating to C# is left as an exercise for the reader.