c# Comparing List (s) using Linq

Question

I want to compare two collections. I believe that I am doing it the long way (codewise). I want to find what numbers might be missing from a collection when compared to another collection. Order is unimportant.

  class Program
{
    static void Main(string[] args)
    {
       List< int> x = new List<int>() { 1 };
       List< int> y = new List<int>() { 1, 2, 3 };

        //find what numbers (if any) that x needs to  have in order to have  an identical list as y (order not important)

        List<int> missingNumbers = new List<int>();
        foreach (var number in y)
        {
            if (!x.Contains(number))
            {
                missingNumbers.Add(number);
            }
        }
        foreach (var missingNumber in missingNumbers)
        {
            x.Add(missingNumber);
        }
    }
}

That doesn't make much sense - you can just do `x = new List(); x.AddRange(y)` - surely there's more to it that just making x have the same items as y? — Charleh, Feb 10 '16 at 19:27
@PaulStanley That sounds like you just need `x = y.ToList()`, but I assume you want to keep values in x that are not in y, correct? — juharr, Feb 10 '16 at 19:27
I want to know what numbers are missing. Then add them to x .Obviously in a complex object int could be a key but the process would be the same. I have two Lists to start with not one. — Paul Stanley, Feb 10 '16 at 19:29
Refer to the top answer on this question: http://stackoverflow.com/questions/12795882/quickest-way-to-compare-two-list — georgecb, Feb 10 '16 at 19:37

CodeNotFound · Answer 1 · 2016-02-10T19:34:53.323

4

Just use the Union extension method as follow:

// x will contain 1, 2, 3. 
// No ducplicate will be added 
// and the missing numbers 2 and 3 are added.
x = x.Union(y).ToList();

edited Feb 10 '16 at 19:34

answered Feb 10 '16 at 19:25

CodeNotFound

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This doesn't answer the OP's question which was "find the numbers in x that are missing from y" – Charleh Feb 10 '16 at 19:25
@Charleh the OP doesn't know what he wants `I want x to have the same items as y. order not important.` – Thomas Ayoub Feb 10 '16 at 19:26
1

"I want to know what numbers are missing. Then add them to x." by the OP – CodeNotFound Feb 10 '16 at 19:29
1

Yeah you were right in the first place, clarification by the OP always helps :) – Charleh Feb 10 '16 at 19:32

score 2 · Accepted Answer · answered Feb 10 '16 at 19:44

If you want to directly combine the lists, a .Union() would certainly work. If you just want to find the values that are missing from one list to another, do an .Except(), e.g.

List<int> x = new List<int>() { 1 };
List<int> y = new List<int>() { 1, 2, 3 };
var result = y.Except(x).ToList();

Where result will return { 2, 3 }. If you then wanted to add result to x, simply do an x.AddRange(result).

Thomas Ayoub · Answer 3 · 2016-02-10T19:50:50.403

1

This does the trick:

x.AddRange(y.Where(num => !x.Contains(num)));

edited Feb 10 '16 at 19:50

answered Feb 10 '16 at 19:30

Thomas Ayoub

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1

What's the point of the `ToList`? Also there seems to be a missing `)`. – juharr Feb 10 '16 at 19:33
2

This won't remove duplicates – Anthony Chu Feb 10 '16 at 19:35

c# Comparing List (s) using Linq

3 Answers3