Combine Two List and Get Another New List of Object

Question

I've the following data set that has to be combined and get a new list of object:

var enumerable1 = new[]
{
    new {Id = "A", Value = 1.0},
    new {Id = "B", Value = 2.0},
    new {Id = "C", Value = 3.0},
    new {Id = "D", Value = 4.0},
    new {Id = "E", Value = 5.0},
};

var enumerable2 = new[]
{
    new {Id = "A", Value = 6.0},
    new {Id = "A", Value = 16.0},
    new {Id = "A", Value = 20.0},
    new {Id = "NOT PRESENT", Value = 542.23},
    new {Id = "C", Value = 7.0},
    new {Id = "D", Value = 8.0},
    new {Id = "E", Value = 9.0},
    new {Id = "E", Value = 10.0},
};

Expected Output:

A - 43
B - 2
C - 3
D - 4
E - 24

For single set of data, I used the following and the data set as follows:

var result = enumerable1.Join(enumerable2, arg => arg.Id, arg => arg.Id,
(first, second) => new {Id = first.Id, Value1 = first.Value, Value2 = second.Value});

Data Set:

var enumerable1 = new[]
{
    new {Id = "A", Value = 1.0},
    new {Id = "B", Value = 2.0},
    new {Id = "C", Value = 3.0},
    new {Id = "D", Value = 4.0},
    new {Id = "E", Value = 5.0},
};

var enumerable2 = new[]
{
    new {Id = "A", Value = 6.0},
    new {Id = "NOT PRESENT", Value = 542.23},
    new {Id = "C", Value = 7.0},
    new {Id = "D", Value = 8.0},
    new {Id = "E", Value = 9.0},
};

But for the repeated values in list 2, how can I merge and sum all repeated values with list 1?

Guru Stron · Accepted Answer · 2023-02-01 19:38:42Z

1

One approach is to filter out missing from the second collection, concatenate and use GroupBy:

var unique = enumerable1.Select(e => e.Id).ToHashSet(); 
var list = enumerable1
    .Concat(enumerable2.Where(e => unique.Contains(e.Id)))
    .GroupBy(e => e.Id)
    .Select(g => new { Id = g.Key, Value = g.Sum(e => e.Value) })
    .ToList();

A for join - I would recommend to switch to query syntax to perform outer join (otherwise B will be missing). Something along this lines:

var list1 = (from e1 in enumerable1
        join e2 in enumerable2 on e1.Id equals e2.Id into j
        from outer in j.DefaultIfEmpty()
        group new { e1.Id, e1.Value, Value2 = outer?.Value ?? 0 } 
             by new { e1.Id, e1.Value } // light hack for easier result calculation 
        into gr
        select new { gr.Key.Id, Value = gr.Key.Value + gr.Sum(e => e.Value2) }
    )
    .ToList();

Both approaches rely on the uniqueness of the Id in the first collection.

edited Feb 1, 2023 at 19:38

answered Feb 1, 2023 at 19:24

Guru Stron

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2 Comments

Soner from The Ottoman Empire Over a year ago

Why do you need to make use of ToHashSet and prefer Concat over Join ?

Guru Stron Over a year ago

@SonerfromTheOttomanEmpire so I don't need to figure out the left outer join and I find it more intuitive.

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