Simplify conventional foreach nested loop using linq & lambda expression

Question

(See my code snippet below) I want to find all items of coll1 that matched to items of coll2 (number of items of coll2 <= number of items of coll1) and put the query result in coll3. How to achieve it using linq and lambda expression? Surely, I can simply copy coll2 to coll3 :-) but that is not my goal. I want to know the ways using linq and lambda to replace such conventional logic construct. Thank you in advance.

        var coll1 = new List<int>() { 1, 2, 3, 4, 5 };
        var coll2 = new List<int>() { 2, 4 };
        var coll3 = new List<int>();
        foreach ( var selected in coll2 )
        {
            foreach ( var item in coll1 )
            {
                if ( selected == item )
                {
                    coll3.Add(item);
                }
            }
        }

Anirudha · Accepted Answer · 2013-07-26 09:47:08Z

14

You can use Intersect

coll1.Intersect(coll2);

But this wont work as expected(see King King's comment)..You can instead do this

coll2.Where(x=>coll1.Any(y=>x==y));

edited Jul 26, 2013 at 9:47

answered Jul 26, 2013 at 9:29

Anirudha

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4 Comments

Matthew Watson Over a year ago

This seems the best solution to me. +1

King King Over a year ago

this won't work as expected: if col2 = {1,2,3,4}, col1 = {2,2,4}. This will return {2,4} while the result should be {2,2,4}?

Anirudha Over a year ago

@KingKing nice observation king..thanks to point it out..corrected the ans..:)

MagB Over a year ago

@Anirudh: it works for me like this: coll3 = coll2.Where(x => coll1.Any(y => x == y)).ToList(); .. +1 still.

Andrei · Accepted Answer · 2013-07-26 09:28:12Z

6

coll3 = coll1.Where(i => coll2.Contains(i)).ToList();

Update. A little bit simpler, as suggested in comments:

coll3 = coll1.Where(coll2.Contains).ToList();

answered Jul 26, 2013 at 9:28

Andrei

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1 Comment

Matthew Watson Over a year ago

You can simplify that to: var coll3 = coll1.Where(coll2.Contains).ToList();

AlexH · Accepted Answer · 2013-07-26 09:28:50Z

2

As a first step, you can use a where clause to make your code more readable :

    var coll1 = new List<int>() { 1, 2, 3, 4, 5 };
    var coll2 = new List<int>() { 2, 4 };
    var coll3 = new List<int>();
    foreach (var selected in coll2)
    {
        coll3.AddRange(coll1.Where(item => selected == item));
    }

answered Jul 26, 2013 at 9:28

AlexH

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Comments

It'sNotALie. · Accepted Answer · 2013-07-26 09:40:45Z

2

Use Intersect: http://msdn.microsoft.com/en-us/library/bb460136.aspx

var coll3 = coll1.Intersect(coll2)

edited Jul 26, 2013 at 9:40

It'sNotALie.

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answered Jul 26, 2013 at 9:30

Bart

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1 Comment

MagB Over a year ago

it doesn't work as expected (see comment of King King to Anirudh's answer).

Matthew Watson · Accepted Answer · 2013-07-26 09:27:37Z

1

You could do this; not sure if it's more readable tho!

var coll3 = (from selected in coll2
                from item in coll1
                where selected == item
                select item).ToList();

answered Jul 26, 2013 at 9:27

Matthew Watson

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3 Comments

Mechanical Object Over a year ago

This exactly what resharper proposes too.

Matthew Watson Over a year ago

I wonder about the downvote. This code is correct, so why? Although it looks like some numpty has just downvoted all the answers here...

MagB Over a year ago

@Matthew Watson: who downvoted? strange. I am sure I give u +1 :-)

Recipe · Accepted Answer · 2013-07-26 09:35:25Z

0

If you do not want to directly assign the result to your list, you could add 'ForEach' to your Linq statement:

coll1.Where(i => coll2.Contains(i)).ToList().ForEach(i => coll3.Add(i));

You might want to check for Distinct though

answered Jul 26, 2013 at 9:35

Recipe

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