Get remaining days in date range

Question

I wonder if there are some possible ways to simplify my code.

startDate = '04/02/2014'
endDate = '04/06/2014'

mondayTag = 0
tuesdayTag = 0
wednesdayTag = 1
thursdayTag = 0
fridayTag = 1
saturdayTag = 0
sundayTag = 0

public int GetRemainingDays(DateTime startDate, DateTime endDate, bool mondayTag, bool tuesdayTag, bool wednesdayTag, bool thursdayTag, bool fridayTag, bool saturdayTag, bool sundayTag)
{
    int i = 0;
    for (DateTime day = startDate.AddDays(1); day.Date <= endDate.Date; day = day.AddDays(1))
    {
        if (day.DayOfWeek == DayOfWeek.Monday && mondayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Tuesday && tuesdayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Wednesday && wednesdayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Thursday && thursdayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Friday && fridayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Saturday && saturdayTag == true)
        {
            i = i + 1;
        }
        else if (day.DayOfWeek == DayOfWeek.Sunday && sundayTag == true)
        {
            i = i + 1;
        }
    }
    return i > 0 ? i : 0;
}

Answer 1

Using System.Linq library the method can be rewritten like this:

public static int GetRemainingDays(DateTime startDate, DateTime endDate, ISet<DayOfWeek> includedDays)
{
    return Enumerable.Range(0, Int32.MaxValue)
        .Select(n => startDate.AddDays(n+1))
        .TakeWhile(date => date <= endDate)
        .Count(date => includedDays.Contains(date.DayOfWeek));
}

Which can then be more easily called like this:

GetRemainingDays(startDate, endDate, 
    new HashSet<DayOfWeek>{DayOfWeek.Wednesday, DayOfWeek.Friday});

Note: You should test this (and the original version) against off-by-one errors.

Answer 2

Interface

I can see this method being useful for, say, counting the number of times a class will meet if it is scheduled for Mondays, Wednesdays, and Fridays from 2014-01-01 to 2014-06-01.

I would expect startDate to be included in the count; it surprises me that the startDate is not counted. It's not obvious whether endDate should be included or not. Either way, you need to document how the two dates at each end are treated.

This method is a pure function, and should probably be made static.

There is a maxim among programmers that there are either zero, one, or many of something. If you ever need to handle three or seven of something, you should generalize to handle many. By that principle, the method signature should be

public static int CountOccurrences(DateTime startDate, DateTime endDate, DayOfWeek[] days)

One way to implement that is to sum the results of calling a helper method

public static int CountOccurrences(DateTime startDate, DateTime endDate, DayOfWeek day)

… for each of the days.

Implementation

The return statement should be just return i; since i will only be negative in case of overflow.

Your implementation doesn't scale very well if the date range is large. You should find a way to count the number of whole weeks within the range, then handle the partial weeks at each end.

Answer 3

Since DayOfWeek is a an Enum with 0 = Sunday through 6 = Saturday.

Instead of passing Boolean tags, you can pass an array of integers where index = 0 is for Sunday, with value 1 for true and 0 for false.

        var dayOfWeeksTags = new int[7];

then you can compute something like

        i += dayOfWeeksTags[(int) day.DayOfWeek];

Your function becomes something like this.

    public int GetRemainingDays(DateTime startDate, DateTime endDate, int[] dayOfWeekTags)
    {
        int i = 0;

        for (DateTime day = startDate.AddDays(1); day.Date <= endDate.Date; day = day.AddDays(1))
           i += dayOfWeekTags[(int) day.DayOfWeek];

        return i;
    }