How can I get every nth item from a List<T>?

Question

I'm using .NET 3.5 and would like to be able to obtain every nth item from a List. I'm not bothered as to whether it's achieved using a lambda expression or LINQ.

Edit

Looks like this question provoked quite a lot of debate (which is a good thing, right?). The main thing I've learnt is that when you think you know every way to do something (even as simple as this), think again!

Answer 1

I know it's "old school," but why not just use a for loop with stepping = n?

Answer 2

For Loop

for(int i = 0; i < list.Count; i += n)
    //Nth Item..

Answer 3

return list.Where((x, i) => i % nStep == 0);

Answer 4

Sounds like

IEnumerator<T> GetNth<T>(List<T> list, int n) {
  for (int i=0; i<list.Count; i+=n)
    yield return list[i]
}

would do the trick. I do not see the need to use Linq or a lambda expressions.

EDIT:

Make it

public static class MyListExtensions {
  public static IEnumerable<T> GetNth<T>(this List<T>, int n) {
    for (int i=0; i<list.Count; i+=n)
      yield return list[i]
  }
}

and you write in a LINQish way

from var element in MyList.GetNth(10) select element;

2nd Edit:

To make it even more LINQish

from var i in Range(0, ((myList.Length-1)/n)+1) select list[n*i];

Answer 5

You can use the Where overload which passes the index along with the element

var everyFourth = list.Where((x,i) => i % 4 == 0);

Answer 6

I'm not sure if it's possible to do with a LINQ expression, but I know that you can use the Where extension method to do it. For example to get every fifth item:

List<T> list = originalList.Where((t,i) => (i % 5) == 0).ToList();

This will get the first item and every fifth from there. If you want to start at the fifth item instead of the first, you compare with 4 instead of comparing with 0.

Answer 7

MartinStettner has the right idea. Be mindful that Linq is awesome, but division comes at a steep price. This is an example where iteration will be much cheaper then a predicate utilizing division. If linq must be used, the range example MartinStettner supplied would be best as it only requires division once.