Question

I have a list of objects. Each object has an integer quantity and a DateTime variable which contains a month and year value. I’d like to traverse the list and pad the list by adding missing months (with quantity 0) so that all consecutive months are represented in the list. What would be the best way to accomplish this?

Example:
Original List

{ Jan10, 3 }, { Feb10, 4 }, { Apr10, 2 }, { May10, 2 }, { Aug10, 3 }, { Sep10, -3 }, { Oct10, 6 }, { Nov10, 3 }, { Dec10, 7 }, { Feb11, 3 }

New List

{ Jan10, 3 }, { Feb10, 4 }, {Mar10, 0}, { Apr10, 2 }, { May10, 2 }, { Jun10, 0 }, { Jul10, 0 } { Aug10, 3 }, { Sep10, -3 }, { Oct10, 6 }, { Nov10, 3 }, { Dec10, 7 }, { Jan11, 0 }, { Feb11, 3 }

Answer 1

One possible algorithm is to keep track of the previous and current months. If the difference between previous and current is 1 month, append current to the result. If the difference is more than one month, add the missing months first, then afterwards copy the current month.

Foo prev = months.First();
List<Foo> result = new List<Foo> { prev };
foreach (Foo foo in months.Skip(1))
{
    DateTime month = prev.Month;
    while (true)
    {
        month = month.AddMonths(1);
        if (month >= foo.Month)
        {
            break;
        }
        result.Add(new Foo { Month = month, Count = 0 });
    }
    result.Add(foo);
    prev = foo;
}

Results:

01-01-2010 00:00:00: 3
01-02-2010 00:00:00: 4
01-03-2010 00:00:00: 0
01-04-2010 00:00:00: 2
01-05-2010 00:00:00: 2
01-06-2010 00:00:00: 0
01-07-2010 00:00:00: 0
01-08-2010 00:00:00: 3
01-09-2010 00:00:00: -3
01-10-2010 00:00:00: 6
01-11-2010 00:00:00: 3
01-12-2010 00:00:00: 7
01-01-2011 00:00:00: 0
01-02-2011 00:00:00: 3

Other code needed to make this compile:

class Foo
{
    public DateTime Month { get; set; }
    public int Count { get; set; }
}

List<Foo> months = new List<Foo>
{
    new Foo{ Month = new DateTime(2010, 1, 1), Count = 3 },
    new Foo{ Month = new DateTime(2010, 2, 1), Count = 4 },
    new Foo{ Month = new DateTime(2010, 4, 1), Count = 2 },
    new Foo{ Month = new DateTime(2010, 5, 1), Count = 2 },
    new Foo{ Month = new DateTime(2010, 8, 1), Count = 3 },
    new Foo{ Month = new DateTime(2010, 9, 1), Count = -3 },
    new Foo{ Month = new DateTime(2010, 10, 1), Count = 6 },
    new Foo{ Month = new DateTime(2010, 11, 1), Count = 3 },
    new Foo{ Month = new DateTime(2010, 12, 1), Count = 7 },
    new Foo{ Month = new DateTime(2011, 2, 1), Count = 3 }
};

Note: For simplicity I haven’t handled the case where the original list is empty but you should do this in production code.