Today I was asked a straightforward question on how would you get every 4th element from an array of numbers starting at 1 - 11

I immediately thought oh I can do this in a functional fluent manner, but then I flubbed it up because I tried to do a Skip and Take combination which is more complicated than needs to be and doesn't work correctly

Something like this

Enumerable.Range(1, 11).Skip(z => z +3).Take(1);

Problem is that Skip does not accept a function as it's input so this doesn't compile. :( :(

So you could then do it the old fashioned way of

using System;
using System.Linq;
using System.Collections.Generic;
					
public class Program
{
  public static void Main()
  {
    var items = new List<int>();
    for (var index = 1; index < 11; index = index + 3)
    {
      items.Add(index);
    }
    Console.WriteLine(string.Join(",", items));		
  }
}

Output

1,4,7,10

DotNet Fiddle Link

However I don't like this approach because the input state is intertwined with the output

So, I drove home in my car and then made dinner and sipped some tea and then got the full picture that I need to adjust my filter condition i.e. add a Where set operator to get the Nth element and so I looked at the Where documentation and the overload that lets you get the element and index is what I needed i.e. Where((element, index) => {}) and just like in JavaScript for Filter you can now do the proper list operation.

So

The generic way to represent the every Nth item in list

using System;
using System.Linq;
using System.Collections.Generic;

public class Program
{
  public static void Main()
  {
    var range = Enumerable.Range(1, 10);		
    const int skip = 3;
    Console.WriteLine(string.Join(",", range.EveryNth(skip)));		
    Console.WriteLine(string.Join(",", range.Reverse().EveryNth(skip)));
  }
}

public static class EnumerableExtensions
{
  public static IEnumerable<int> EveryNth(this IEnumerable<int> range, int skip = 1)
  {
    return (range ?? new List<int>() {}).Where((element, index) => index % skip == 0);
  }
}

Output

1,4,7,10
10,7,4,1

DotNet Fiddle Link