Is there a Linq method to tell that A is the subset of B?

Question

Let's I have 2 arrays string[] A = { "a", "b", "c" } and string[] B = { "a", "b", "c", "d", "e" }.

Is there any method that can tell directly that A is a subset of B? Except will just remove distinct elements of B, but won't tell whether it's completely a subset of B.

Thank you for helping.

score 12 · Answer 1 · answered Oct 10 '14 at 21:30

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you can do this,simply:

A.All(B.Contains);

And you may wanna check the Length to make sure they are not the same size and A is a subset

bool isSubset = A.All(B.Contains) && A.Length < B.Length;

answered Oct 10 '14 at 21:30

Selman Genç

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AFAIK, any set is a subset of itself. – Mephy Oct 10 '14 at 21:32
@Mephy yes, the above check would only be needed for a *proper* subset. – DeCaf Oct 10 '14 at 21:34
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This can be _very_ inefficient depending on the size of your sets. – Cory Nelson Oct 10 '14 at 21:38
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@CoryNelson and least readable of posted answers imo. – brz Oct 10 '14 at 21:40
@brz It's readable to me... I read it as, 'Does B Contain All of A'. In fact, all the words of that sentence are in his statement except 'Does' and 'of'. :) – Rufus L Oct 10 '14 at 22:08
I suppose for more efficiency, you could compare the lengths first. But they can be equal and still return true. – Rufus L Oct 10 '14 at 22:16
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Another nagging: the `Length`-check is pointless since even a longer array can be a subset of a shorter if there are duplicates: `{"A", "A"}` is a subset of `{"A"}`. – Tim Schmelter Oct 10 '14 at 22:25

Tim Schmelter · Answer 2 · 2014-10-16T09:37:54.053

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There's even a method for this purpose. You could use HastSet<T>.IsSubsetOf:

var aSet = new HashSet<string>(A);
bool isASubsetOfB = aSet.IsSubsetOf(B);

edited Oct 16 '14 at 09:37

answered Oct 10 '14 at 21:34

Tim Schmelter

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This is not LINQ though. But perhaps it is more efficient than using linq. – DeCaf Oct 10 '14 at 21:35
@DeCaf: why using LINQ if there's already a readable and efficient method available? – Tim Schmelter Oct 10 '14 at 21:37
No reason other than that the question asked if there was a Linq method. – DeCaf Oct 10 '14 at 21:38
@DeCaf: i guess OP just thought that there's nothing buil-in. I also like LINQ very much because it can increase readability but sometimes available methods are even more readable and more efficient. Basically LINQ are also just methods sitting in a class in the `System.Linq` namespace. – Tim Schmelter Oct 10 '14 at 21:41
This will likely be very slightly more efficient than mine. `Except` just builds a hash set behind the scenes. – Cory Nelson Oct 10 '14 at 22:27
@CoryNelson: you're right. I thought that yours doesn't need to load the `HashSet` completely but it also creates a `HashSet` from the second sequence(as opposed to mine which uses the first). http://codeblog.jonskeet.uk/2010/12/30/reimplementing-linq-to-objects-part-17-except/ – Tim Schmelter Oct 10 '14 at 22:38

Cory Nelson · Answer 3 · 2014-10-10T22:01:55.630

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This should work

bool isSubSet = !A.Except(B).Any();

edited Oct 10 '14 at 22:01

answered Oct 10 '14 at 21:37

Cory Nelson

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Should probably negate it for readability, no? A ⊆ B is false for the example given when using this code, and that seems confusing. – Michael Oct 10 '14 at 21:44
In that case, make it an extension method like `IsSubsetOf()`. – Cory Nelson Oct 10 '14 at 21:49
Yes, you have to add `!` otherwise your checking if it's **not** a subset so not what it's suppoed to do. – Tim Schmelter Oct 10 '14 at 21:54
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@CoryNelson Maybe, but my point is that the variables are backwards. This code is checking something along the lines of "Is A a superset of B" – Michael Oct 10 '14 at 21:55
Ahah, now I see what you mean. Fixed. – Cory Nelson Oct 10 '14 at 22:02

score 2 · Answer 4 · answered Oct 10 '14 at 21:31

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You can combine SequenceEquals and Intersect:

var isASubsetOfB = B.Intersect(A).SequenceEqual(A)

answered Oct 10 '14 at 21:31

brz

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Make sure you actually care about order when using this. – Michael Oct 10 '14 at 21:39

Is there a Linq method to tell that A is the subset of B?

4 Answers4