Finding the indices of the elements of one array in another using LINQ

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Given two arrays A and B, find the index from A of the element of B

Dim A As String() = {"a", "b", "c", "d", "e", "f", "g", "h", "i"}
Dim B As String() = {"b", "d", "e"}

For Each item As String In B
    Dim index1 As Integer = Array.IndexOf(A, item)
    Console.WriteLine("{0} {1}", index1, item)
Next

1 b
3 d
4 e

What I have tried:

My code in 'traditional' VB.NET works, but I want to convert it using LINQ.
Thanks in advance.
ATeDe

Posted 7-Feb-18 12:09pm

ATeDe

Updated 7-Feb-18 23:28pm

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Graeme_Grant · Accepted Answer · 2018-02-07T14:05:00

Solution 1

Here is one way...

VB

Dim A As String() = {"a", "b", "c", "d", "e", "f", "g", "h", "i"}
Dim B As String() = {"b", "d", "e"}

Dim c = A.Select(Function(x, i) New KeyValuePair(Of String, Integer)(x, i)) _
         .Where(Function(x) B.Contains(x.Key))

For Each item In c
    Console.WriteLine("{0} - {1}", item.Key, item.Value)
Next

Posted 7-Feb-18 14:05pm

Graeme_Grant

Comments

Maciej Los 8-Feb-18 5:28am

5ed!

Graeme_Grant 8-Feb-18 5:30am

Thanks

ATeDe 8-Feb-18 15:55pm

Thanks Graeme, nice example of LINQ application. Actually you've given me indication that KeyValuePair can be used to convert array A to listA following way:

Dim listA As List(Of KeyValuePair(Of String, Integer)) = A.Select(Function(x, i) New KeyValuePair(Of String, Integer)(x, i)).ToList()

Graeme_Grant 8-Feb-18 16:26pm

Yes... you could also use tuples...

ATeDe 8-Feb-18 17:39pm

Yeah, and instead of using Dim A As String(), listA might be created directly, and then we got very neat statement:

Dim C = listA.Where(Function(x) B.Contains(x.Key))
For Each item In C
Console.WriteLine("{0} {1}", item.Key, item.Value)
Next

Graeme_Grant 8-Feb-18 18:16pm

Yes you could. I just wanted to keep it simple for you. You could put the whole thing like this:

For Each item In (New String({"a", "b", "c", "d", "e", "f", "g", "h", "i"}) _
    .Select(Function(x, i) New KeyValuePair(Of String, Integer)(x, i)) _
    .Where(Function(x) (New String({"b", "d", "e"}).Contains(x.Key))))
    Console.WriteLine("{0} - {1}", item.Key, item.Value)
Next

But then it's hard coded and not as easy to read...

Maciej Los · Accepted Answer · 2018-02-07T23:28:00

Solution 2

As an alternative to solution#1 of Graeme_Grant[^], a "translated" (to Linq query) version of your solution is:

VB.NET

Dim A As String() = {"a", "b", "c", "d", "e", "f", "g", "h", "i"}
Dim B As String() = {"b", "d", "e"}

Dim result = A.Where(Function(q) B.Any(Function(w) q=w)). _
	Select(Function(x) New With _
		{ _
			.Index = Array.IndexOf(A, x), _
			.Value = x _
		})

I'd suggest to check the following MSDN documentation for more details:
Enumerable.Intersect(TSource) Method (IEnumerable(TSource), IEnumerable(TSource)) (System.Linq)[^]
Enumerable.Except(TSource) Method (IEnumerable(TSource), IEnumerable(TSource)) (System.Linq)[^]
Enumerable.ElementAt(TSource) Method (IEnumerable(TSource), Int32) (System.Linq)[^]

Posted 7-Feb-18 23:28pm

Maciej Los

Updated 7-Feb-18 23:34pm

v3

Comments

Graeme_Grant 8-Feb-18 5:32am

5'ed the direct translation :)

Maciej Los 8-Feb-18 5:35am

Thank you, Graeme

Richard Deeming 8-Feb-18 11:30am

That's not really a direct translation, though. :)

The original code iterates through B, finding the index in A for each item. Your version starts with A, filters it to items that exist in B, and then finds the index in A.

A real translation would be:

Dim result = B.Select(Function (x) New With
{
    .Value = x,
    .Index = Array.IndexOf(A, x)
})

Maciej Los 8-Feb-18 11:58am

OK. It's almost direct translation ;)
Cheers!
Maciej

ATeDe 8-Feb-18 15:59pm

Great stuff, Richard! Thanks a lot!