Make a List of String array from a List of String using LINQ

Question

I have a given data (I just make it as a List here).

List<string> list1 = new List<string>();
foreach( var x in Regex.Split( "A B C D E F", " " ) )
    list1.Add( x );

Now I want to make a final List like this.

List<string[]> list2 = new List<string[]>();

So, I tried with this code (I have tried with LINQ but, no gain).

int i = 0;
string[] array1 = new string[2];
foreach( var x in list1 )
{
    if( i % 2 == 0 )
    {
        array1[0] = x;
    }
    else
    {
        array1[1] = x;
        list2.Add( array1 );
        array1 = new string[2];
    }
    i++;
}

I'd like to use LINQ for the same result. Please help.

Thanks.

(EDIT: Result should be A and B, C and D, E and F for each item in List 2)

Side note: the first list can also be obtained with the string's native split method and ToList with: List<string> list1 = "A B C D E F".Split(' ').ToList(); — Me.Name
– Me.Name, Commented Dec 18, 2016 at 9:25
@Me.Name Yes, You are right. But, that one is "given". Thanks. — Jason
– Jason, Commented Dec 18, 2016 at 9:27

Johnny · Accepted Answer · 2019-01-25 12:16:09Z

5

Try this:

List<string[]> list2 = list1
               .Select((value, index) => new { PairNum = index / 2, value })
               .GroupBy(pair => pair.PairNum)
               .Select(grp => grp.Select(g => g.value).ToArray())
               .ToList();

The steps are:

Select((value, index) => new { PairNum = index / 2, value }) => {0, 'A'} {0, 'B'} {1, 'C'} {1, 'D'} {2, 'E'} {2, 'F'}
GroupBy(pair => pair.PairNum) => {0:{'A', 'B'}}, {1:{'C', 'D'}}, {2:{'E', 'F'}}
Select(grp => grp.Select(g => g.value).ToArray()) => converts values to array because list items should be arrays.
Executes the query

edited Jan 25, 2019 at 12:16

answered Dec 18, 2016 at 9:17

Johnny

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3 Comments

Jason Over a year ago

Thanks. Looks like super difficult. I'll try it.

Johnny Over a year ago

I added a comment to explain a little bit.

Jason Over a year ago

Ahhh. I'm begining to understand it. (Not finished yet though) Thanks for the nice explanations too.

Maksim Simkin · Accepted Answer · 2016-12-18 09:28:16Z

1

list2 = list1.Where((x, i) => i % 2 == 0)
     .Zip(list1.Where((x, i) => i % 2 == 1),
           (x, y) => new[] { x, y })
           .ToList();

answered Dec 18, 2016 at 9:28

Maksim Simkin

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1 Comment

Jason Over a year ago

Thanks. It is interesting too.

Sateesh Pagolu · Accepted Answer · 2016-12-18 09:26:59Z

1

This is super easy with following trick..

var result = Enumerable.Range(0,array.Count()/2)
                       .Select(x=>array.Skip(x*2).Take(2));

edited Dec 18, 2016 at 9:26

answered Dec 18, 2016 at 9:21

Sateesh Pagolu

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Comments

Sefe · Accepted Answer · 2016-12-18 09:29:16Z

1

What you want to do is the opposite of SelectMany, for which there is no built-in way. There is also no way to also iterate in steps of two. But since your source is an array, you can access its contents by index:

string[] sourceItems = sourceStr.Split(" ");

var pairQry =
    from index in Enumerable.Range(0, sourceItems.Length - 1)
    where index % 2 == 0
    select new string[] { sourceItems[index], sourceItems[index + 1] };

List<string[]> pairs = pairQry.ToList();

answered Dec 18, 2016 at 9:29

Sefe

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1 Comment

Jason Over a year ago

Wow, I din not think that my question is not so easy one. OK. This is a good point to learn LINQ. Thanks.

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