Find date ranges from a collection of dates C#

Question

Simple question. I have an ordered collection of dates. They are UK dates btw

01/01/10
01/02/10
01/03/10
01/04/10
02/04/10
03/04/10
04/04/10

And I want to convert this into a collection of date ranges

01/01/10 -> 01/01/10
01/02/10 -> 01/02/10
01/03/10 -> 01/03/10
01/04/10 -> 04/04/10

Just to clarify, I'm trying to convert any consecutive dates into a range. so the first 3 dates are stand alone and the last 4 get converted into a range 1st of April to 4th of April.

Now I can do this using loops but it's not very elegant. Does any one have any solutions out there that are?

Thanks

Answer 1

Given that you want to determine ranges of consecutive date ranges, I think your only option is, as you say a loop. You can do it in a single pass though, and put it in an extension method so it'll operate on any IList<DateTime>, for example:

// purely an example, chances are this will have actual, y'know logic in live
public class DateRange
{
    private List<DateTime> dates = new List<DateTime>();

    public void Add(DateTime date)
    {
        this.dates.Add(date);
    }

    public IEnumerable<DateTime> Dates
    {
        get { return this.dates; }
    }
}

public static IEnumerable<DateRange> GetRanges(this IList<DateTime> dates)
{
    List<DateRange> ranges = new List<DateRange>();
    DateRange currentRange = null;

    // this presumes a list of dates ordered by day, if not then the list will need sorting first
    for( int i = 0; i < dates.Count; ++i )
    {
        var currentDate = dates[i];
        if( i == 0 || dates[i - 1] != currentDate.AddDays(-1))
        {
            // it's either the first date or the current date isn't consecutive to the previous so a new range is needed
            currentRange = new DateRange();
            ranges.Add(currentRange);
        }

        currentRange.Add(currentDate);
    }

    return ranges;
}

You could also make it even more generic by passing in an IEnumerable<DateTime>:

public static IEnumerable<DateRange> GetRanges(this IEnumerable<DateTime> dates)
{
    List<DateRange> ranges = new List<DateRange>();
    DateRange currentRange = null;
    DateTime? previousDate = null;

    // this presumes a list of dates ordered by day, if not then the list will need sorting first
    foreach( var currentDate in dates )
    {
        if( previousDate == null || previousDate.Value != currentDate.AddDays(-1) )
        {
            // it's either the first date or the current date isn't consecutive to the previous so a new range is needed
            currentRange = new DateRange();
            ranges.Add(currentRange);
        }

        currentRange.Add(currentDate);
        previousDate = currentDate;
    }

    return ranges;
}

Answer 2

        var stringDates = new List<string> {"01/09/10", "31/08/10", "01/01/10"};

        var dates = stringDates.ConvertAll(DateTime.Parse);
        dates.Sort();

        var lastDateInSequence = new DateTime();
        var firstDateInSequence = new DateTime();  

        foreach (var range in dates.GroupBy(d => { if ((d - lastDateInSequence).TotalDays != 1)   
                                                       firstDateInSequence = d;  
                                                   lastDateInSequence = d;
                                                   return firstDateInSequence;  
        }))
        {
            var sb = new StringBuilder();
            sb.Append(range.First().ToShortDateString());
            sb.Append(" => ");
            sb.Append(range.Last().ToShortDateString());
            Console.WriteLine(sb.ToString());
        }

Answer 3

dates.Aggregate(new List<DateRange>(), (acc, dt) =>
                                       {
                                         if (acc.Count > 0 && acc.Last().d2 == dt.AddDays(-1))
                                           acc[acc.Count - 1].d2 = dt;
                                         else
                                           acc.Add(new DateRange(dt, dt));
                                         return acc;
                                       }
    );

where DateRange is a class like this:

class DateRange
{
  public DateTime d1, d2;

  public DateRange(DateTime d1, DateTime d2)
  {
    this.d1 = d1;
    this.d2 = d2;
  }
}