How to group dates by week?

Question

I am writing an Excel exporter for a bespoke application I am creating, and I have a question about LINQ grouping in C#.

Basically, this new Excel exporter class is given two dates. The class then retrieves all consignments between this date range.

As part of this exporter, I need to be able to group the dates into weeks, and get the values for that week. So for example, if I'm given 07/12/2011 and 22/12/2011 (dd/MM/yyyy format), I need to group all consignments between them ranges into weeks (each week beginning with Sunday). The ideal result using the above dates would be

Week 1: (consignments between 04/12/2011 and 10/12/2011) 
Week 2: (consignments between 11/12/2011 and 17/12/2011) 
Week 3: (consignments between 18/11/2011 and 24/12/2011)

Any ideas?

Answer 1

I noticed that the OP has week 1, week 2, etc. in the ideal output. These are not the week of the year, but the "index" of the week being displayed based on the consignment dates. Building on some of the other answers already provided, here is my solution:

void DoExample()
{
    //Load some sample data
    var range = new List<DateTime>();
    var curDate = DateTime.ParseExact("07/12/2011", "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture);
    var maxDate = DateTime.ParseExact("22/12/2011", "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture);
    while(curDate < maxDate)
    {
        range.Add(curDate);
        curDate = curDate.AddDays(1);
    }
    
    //Run the method to get the consignments
    var c = GetConsignments(range, DayOfWeek.Sunday);

    //Output to match OP's "ideal" specs
    foreach(var v in c)
    {
        Console.WriteLine($"Week {v.EntryIndex + 1} (number {v.WeekOfYear} in year): (consignments between {v.RangeStart:dd/MM/yyyy} and {v.RangeEnd:dd/MM/yyyy}). Actual date range is {v.RangeStart:dd/MM/yyyy}-{v.RangeEnd:dd/MM/yyyy} ({(v.FullWeek ? "Full" : "Partial")} week)");
    }

    //Most other answers place a lot of value on the week of the year, so this would include that.
    // Also includes the actual date range contained in the set and whether all dates in that week are present
    foreach (var v in c)
    {
        Console.WriteLine($"Week {v.EntryIndex + 1} (number {v.WeekOfYear} in year): (consignments between {v.RangeStart} and {v.RangeEnd})");
    }
}

//Note that this lets us pass in what day of the week is the start.
// Not part of OP's requirements, but provides added flexibility
public List<ConsignmentRange> GetConsignments(IEnumerable<DateTime>consignments, DayOfWeek startOfWeek=DayOfWeek.Sunday)
{
    return consignments
            .OrderBy(v => v)
            .GroupBy(v => v.AddDays(-(int)((7 - (int)startOfWeek) + (int)v.DayOfWeek) % 7))
            .Select((v, idx) => new ConsignmentRange
              {
                //These are part of the OP's requirement
                EntryIndex = idx,
                RangeStart = v.Key, // part of requirement
                RangeEnd = v.Key.AddDays(6),  // part of requirement

                //These are added as potentially useful
                WeekOfYear = CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
                             v.Key, CalendarWeekRule.FirstFourDayWeek, startOfWeek),
                FirstDate = v.Min(),
                LastDate = v.Max(),
                FullWeek = (v.Distinct().Count() == 7)
              }
            )
            .ToList();
}

We'll also need this class defined (or a subset of it depending on what data you want to include):

public class ConsignmentRange
{
    public int EntryIndex;
    public int WeekOfYear;
    public bool FullWeek;
    public DateTime FirstDate;
    public DateTime LastDate;
    public DateTime RangeStart;
    public DateTime RangeEnd;
}

Answer 2

I tried like this (and it's working :) )

@foreach (var years in _dateRange.GroupBy(y => y.Year))
{
    <p>@years.Key</p>
    foreach (var months in years.GroupBy(m => m.Month))
    {
        <p>@CultureInfo.CurrentCulture.DateTimeFormat.GetMonthName(months.Key)</p>
        foreach (var weeks in months.GroupBy(w => w.AddDays(-(int)w.DayOfWeek)))
        {
            <p>@weeks.Key.ToString("dd-MMM-yy")</p>
        }
    }
}

Answer 3

In addition to Jon's answer you can get the date of the first day in the week then group by that date.

To get the date of the first day in the week. you can use this code:

public static class DateTimeExtensions
{
    public static DateTime StartOfWeek(this DateTime dt, DayOfWeek startOfWeek)
    {
        int diff = dt.DayOfWeek - startOfWeek;
        if (diff < 0)
        {
            diff += 7;
        }
        return dt.AddDays(-1 * diff).Date;
    }
}

then you can group by the first date of the week like this:

var consignmentsByWeek = from con in consignments
                         group con by con.Datedate.StartOfWeek(DayOfWeek.Monday);

Answer 4

Hesitant though I am to disagree with as esteemed an answerer I believe the accepted answer here is wrong, and this is not fundamentally a question of projecting to a week of year value.

GetWeekOfYear(), and the concept in general, is about assigning index values to weeks within a year according to some agreed standard. It is not suitable for placing dates into groups of seven adjacent days as I believe the questioner requires.

Not only will use of GetWeekOfYear() as proposed result in groups of fewer than seven days at the end of many years, but worse still, as the various standards supported by GetWeekOfYear() will often apportion the first days of a year to the last week of the previous year, and yet the GetWeekOfYear() result contains only the integer week index with no reference to associated year, grouping by new { Year = date.Year, weekProjector(date) } or date.Year + "-" + weekProjector(date) in the questioner's year would see January 1st 2011 grouped in with Christmas Day through to New Year's Eve twelve months later that same year.

So I would argue that the original question is fundamentally one of projecting not to a week of year value but to a week of all time value, "week beginning y/m/d" you might say, so grouping need only be done by the first day of the week, i.e. (assuming you're happy to default to Sunday) simply:

group by date.AddDays(-(int)date.DayOfWeek)

Answer 5

The fundamental question here is how to project a DateTime instance into a week of year value. This can be done using by calling Calendar.GetWeekOfYear. So define the projection:

Func<DateTime, int> weekProjector = 
    d => CultureInfo.CurrentCulture.Calendar.GetWeekOfYear(
         d,
         CalendarWeekRule.FirstFourDayWeek,
         DayOfWeek.Sunday);

You can configure exactly how the "week number" is determined by tweaking the parameters in the method call. You can also decide to define the projection as e.g. an extension method if you prefer; this does not change the essence of the code. In any case, you are then ready to group by week:

var consignmentsByWeek = from con in consignments
                         group con by weekProjector(con.Date);

If you also want to constrain the output to consigments between two specific dates, just add an appropriate where clause; the grouping logic does not change.