Calculate difference from previous item with LINQ

Question

I'm trying to prepare data for a graph using LINQ.

The problem that i cant solve is how to calculate the "difference to previous.

the result I expect is

ID= 1, Date= Now, DiffToPrev= 0;

ID= 1, Date= Now+1, DiffToPrev= 3;

ID= 1, Date= Now+2, DiffToPrev= 7;

ID= 1, Date= Now+3, DiffToPrev= -6;

etc...

Can You help me create such a query ?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    public class MyObject
    {
        public int ID { get; set; }
        public DateTime Date { get; set; }
        public int Value { get; set; }
    }

    class Program
    {
        static void Main()
        {
              var list = new List<MyObject>
              {
                new MyObject {ID= 1,Date = DateTime.Now,Value = 5},
                new MyObject {ID= 1,Date = DateTime.Now.AddDays(1),Value = 8},
                new MyObject {ID= 1,Date = DateTime.Now.AddDays(2),Value = 15},
                new MyObject {ID= 1,Date = DateTime.Now.AddDays(3),Value = 9},
                new MyObject {ID= 1,Date = DateTime.Now.AddDays(4),Value = 12},
                new MyObject {ID= 1,Date = DateTime.Now.AddDays(5),Value = 25},
                new MyObject {ID= 2,Date = DateTime.Now,Value = 10},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(1),Value = 7},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(2),Value = 19},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(3),Value = 12},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(3),Value = 15},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(4),Value = 18},
                new MyObject {ID= 2,Date = DateTime.Now.AddDays(5),Value = 19}
            };

            Console.WriteLine(list);   

            Console.ReadLine();
        }
    }
}

Answer 1

In C#4 you can use the Zip method in order to process two items at a time. Like this:

        var list1 = list.Take(list.Count() - 1);
        var list2 = list.Skip(1);
        var diff = list1.Zip(list2, (item1, item2) => ...);

Answer 2

One option (for LINQ to Objects) would be to create your own LINQ operator:

// I don't like this name :(
public static IEnumerable<TResult> SelectWithPrevious<TSource, TResult>
    (this IEnumerable<TSource> source,
     Func<TSource, TSource, TResult> projection)
{
    using (var iterator = source.GetEnumerator())
    {
        if (!iterator.MoveNext())
        {
             yield break;
        }
        TSource previous = iterator.Current;
        while (iterator.MoveNext())
        {
            yield return projection(previous, iterator.Current);
            previous = iterator.Current;
        }
    }
}

This enables you to perform your projection using only a single pass of the source sequence, which is always a bonus (imagine running it over a large log file).

Note that it will project a sequence of length n into a sequence of length n-1 - you may want to prepend a "dummy" first element, for example. (Or change the method to include one.)

Here's an example of how you'd use it:

var query = list.SelectWithPrevious((prev, cur) =>
     new { ID = cur.ID, Date = cur.Date, DateDiff = (cur.Date - prev.Date).Days);

Note that this will include the final result of one ID with the first result of the next ID... you may wish to group your sequence by ID first.

Answer 3

Use index to get previous object:

var LinqList = list.Select((myObject, index) => new {ID = myObject.ID, Date = myObject.Date, Value = myObject.Value, DiffToPrev = (index > 0 ? myObject.Value - list[index - 1].Value : 0)});