How to get array overlaps using LINQ?

Question

Suppose I have the following arrays:

List<int[]> numbers = new List<int[]>();
numbers.Add(new int[] { 1, 2, 3 });
numbers.Add(new int[] { 3, 4, 5 });
numbers.Add(new int[] { 5, 6, 7 });

int[] numbersToFind = new int[] { 4, 5, 6 };

I want to find which of the numbers elements contains one/more of the values in numbersToFind, is there an easy way of doing this with LINQ? That is, some code that would return a IEnumerable<int[]> containing an int[]{3,4,5} and int[]{5,6,7} in the above example.

How can this be done?

cdhowie · Accepted Answer · 2010-11-17 18:42:34Z

8

numbers.Where(array => array.Any(value => numbersToFind.Contains(value)));

answered Nov 17, 2010 at 18:42

cdhowie

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Comments

Mark Byers · Accepted Answer · 2010-11-17 18:48:33Z

6

Try this:

var query = numbers.Where(n => numbersToFind.Any(x => n.Contains(x)));

Here's an alternative approach using the method Enumerable.Intersect:

var query = numbers.Where(n => numbersToFind.Intersect(n).Any());

edited Nov 17, 2010 at 18:48

answered Nov 17, 2010 at 18:42

Mark Byers

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Comments

Lee · Accepted Answer · 2010-11-17 18:45:59Z

4

IEnumerable<int[]> matches = numbers.Where(n => numbersToFind.Intersect(n).Any());

answered Nov 17, 2010 at 18:45

Lee

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11 Comments

Mark Byers Over a year ago

+1 I just added this to my answer, but you beat me by a couple of minutes. I think this is probably better than the top voted answer. :)

cdhowie Over a year ago

Intersect() is going to create a HashMap<int> for each evaluation of the Where lambda. Therefore it will be slower, consume more memory, and create more GC churn. (At least in this limited example.)

Mark Byers Over a year ago

@cdhowie: I think it will scale better though - I haven't tested it to be sure. I'm not sure if the example in the OPs question should be understood as a representative example of the sort of query he will be doing or if it has been simplified for the purposes of explanation. I'd guess the latter.

cdhowie Over a year ago

It will not scale. For each number in each source array being considered, a brand new HashMap<int> will be created and populated with the contents of numbersToFind. My example will iterate over every element of numbersToFind until a match is found, where Intersect() is going to construct a HashMap<int> and iterate over EVERY element to build up the hash map, and finally check if it contains the value being considered. I'm sorry, but it's impossible for that to be faster under any circumstances.

cdhowie Over a year ago

If speed is a huge concern, then numbersToFind should itself be a HashMap<int>. Then my example would perform even faster.

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csauve · Accepted Answer · 2016-05-06 21:35:59Z

4

There is a LINQ function Intersect specifically to do this:

numbers.Intersect(numbersToFind)

answered May 6, 2016 at 21:35

csauve

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Comments

Homam · Accepted Answer · 2010-11-17 18:44:37Z

1

Try this:

numbers.Where(a => a.Any(s => numbersToFind.Contains(s)));

Good luck!

answered Nov 17, 2010 at 18:44

Homam

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