Calculate difference from previous item with LINQ

user_444149 0
Viewed 15621

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(4),Value = 15},
            new MyObject {ID= 2,Date = DateTime.Now.AddDays(5),Value = 18}

        };

            Console.WriteLine(list);   

            Console.ReadLine();
        }
    }
}
pivot diff linq
7 Answers

Here is the refactored code with C# 7.2 using the readonly struct and the ValueTuple (also struct).

I use Zip() to create (CurrentID, PreviousID, CurrDate, PrevDate, DiffToPrev) tuple of 5 members. It is easily iterated with foreach:

foreach(var (CurrentID, PreviousID, CurrDate, PrevDate, DiffToPrev) in diffs)

The full code:

public readonly struct S
{
    public int ID { get; }
    public DateTime Date { get; }
    public int Value { get; }

    public S(S other) => this = other;

    public S(int id, DateTime date, int value)
    {
        ID = id;
        Date = date;
        Value = value;
    }

    public static void DumpDiffs(IEnumerable<S> list)
    {
        // Zip (or compare) list with offset 1 - Skip(1) - vs the original list
        // this way the items compared are i[j+1] vs i[j]
        // Note: the resulting enumeration will include list.Count-1 items
        var diffs = list.Skip(1)
                        .Zip(list, (curr, prev) => 
                                    (CurrentID: curr.ID, PreviousID: prev.ID, 
                                    CurrDate: curr.Date, PrevDate: prev.Date, 
                                    DiffToPrev: curr.Date.Day - prev.Date.Day));

        foreach(var (CurrentID, PreviousID, CurrDate, PrevDate, DiffToPrev) in diffs)
            Console.WriteLine($"Current ID: {CurrentID}, Previous ID: {PreviousID} " +
                              $"Current Date: {CurrDate}, Previous Date: {PrevDate} " +
                              $"Diff: {DiffToPrev}");
    }
}

Unit test output:

// the list:

// ID   Date
// ---------------
// 233  17-Feb-19
// 122  31-Mar-19
// 412  03-Mar-19
// 340  05-May-19
// 920  15-May-19

// CurrentID PreviousID CurrentDate PreviousDate Diff (days)
// ---------------------------------------------------------
//    122       233     31-Mar-19   17-Feb-19      14
//    412       122     03-Mar-19   31-Mar-19      -28
//    340       412     05-May-19   03-Mar-19      2
//    920       340     15-May-19   05-May-19      10

Note: the struct (especially readonly) performance is much better than that of a class.

Thanks @FelixUngman and @DavidHuxtable for their Zip() ideas!

user_11710190
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