LINQ的两个列表之间的区别

Question

I have two lists with two fields. I don't know before hand which one is larger.

I want to achieve this:

        newList              oldList
Fruit       Qty     Diff    Fruit       Qty
--------------------------------------------
apple       3       +1      apple       2
                    -2      pear        2
peach       4       +3      peach       1
melon       5        0      melon       5
coconut     2       +2      
mango       4        0      mango       4
banana      2       -1      banana      3
lychi       1       +1      
                    -3      pineapple   3

One the left you can see newList on the right the oldList. If I compare oldList against newList I want to see every possible row and difference in Quantity between the two lists.

I wrote this, which would give me Fruit that's not contained in both lists (xor)

var difference = newList.Select(x => x.Fruit)
                        .Except(oldList.Select(x => x.Fruit))
                        .ToList()
                        .Union(oldList.Select(x => x.Fruit)
                        .Except(newList.Select(x => x.Fruit))
                        .ToList());

But I'm lost on how to combine it with Qty as well.

Answer 1

I don't see a way with LINQ that performs well and is also readable. I would prefer a mix of LINQ and loops:

var oldGrouped = oldList.ToDictionary(x => x.Fruit, x => x.Qty);
var newGrouped = newList.ToDictionary(x => x.Fruit, x => x.Qty);

var result = new List<FruitSummary>();

foreach(var oldItem in oldGrouped)
{
    var summary = new FruitSummary { OldFruit = oldItem.Key, OldQty = oldItem.Value };
    if(newGrouped.TryGetValue(oldItem.Key, out int newQuantity) && newQuantity != 0)
    {
        summary.NewFruit = oldItem.Key;
        summary.NewQty = newQuantity;
    }
    summary.Diff = oldItem.Value - newQuantity;
    newGrouped.Remove(oldItem.Key);
    result.Add(summary);
}

foreach(var newItem in newGrouped)
{
    result.Add(new FruitSummary { Diff = -newItem.Value,
                                  NewFruit = newItem.Key,
                                  NewQty = newItem.Value });
}

The class FruitSummary looks like this:

public class FruitSummary
{
    public string OldFruit { get; set; }
    public string NewFruit { get; set; }
    public int OldQty { get; set; }
    public int NewQty { get; set; }
    public int Diff { get; set; }
}

Answer 2

I assume you have a Fruit class like this:

public class Fruit
{
    public string Name {get; set;}
    public int Quantity {get; set;}
}

and you have an old and a new list like these

List<Fruit> oldList = ...
List<Fruit> newList = ...

Then this LINQ monstrum does the job, though it may not be the most performant solution (but how many kinds of fruit are there?):

var result = 
    oldList.Join(newList, 
        oldFruit => oldFruit.Name,
        newFruit => newFruit.Name,
        (oldFruit, newFruit) => new {
            Name = oldFruit.Name,
            Diff = oldFruit.Quantity - newFruit.Quantity}).
    Concat(oldList.
        Where(oldFruit => newList.All(newFruit => newFruit.Name != oldFruit.Name)).
        Select(oldFruit => new {
            Name = oldFruit.Name,
            Diff = -oldFruit.Quantity})).
    Concat(newList.Where(newFruit => oldList.All(oldFruit => oldFruit.Name != newFruit.Name)).
        Select(newFruit => new {
            Name = newFruit.Name,
            Diff = newFruit.Quantity}));

You can output the results like this:

Console.WriteLine(string.Join(Environment.NewLine, result.Select(r => $"{r.Name} {r.Diff}")));

---

Note that I come up with this because I like linq "one liners", but using a foreach seems more readable and probably even faster than this query.

Answer 3

Something like this?

var difference = newList
    .Concat(oldList.Select(x => new { x.Fruit, Qty = -x.Qty }))
    .GroupBy(x => x.Fruit)
    .Select(g => new {Fruit = g.Key, Qty = g.Sum(x => x.Qty) });

When I run foreach(var d in difference) Console.WriteLine(d); , I get:

{ Fruit = apple, Qty = 1 }
{ Fruit = peach, Qty = 3 }
{ Fruit = melon, Qty = 0 }
{ Fruit = coconut, Qty = 2 }
{ Fruit = mango, Qty = 0 }
{ Fruit = banana, Qty = -1 }
{ Fruit = lychi, Qty = 1 }
{ Fruit = pear, Qty = -2 }
{ Fruit = pineapple, Qty = -3 }