How to find the indices of items fulfilling some condition in List of int?

Question

I have a list of int containing items { 10, 11, 23, 34, 56, 43 } and I want to find out the indices of all items which are greater than 23. These values can be any order so I do not want to sort them.

List<int> mylist = new List<int> { 10, 11, 23, 34, 56, 43 };

I am interested in indices of all items those are fulfilling the condition and not only first item which fulfill condition. so this line of code is not working for me.

int index = mylist.FindIndex( x => x > 23 );

Answer 1

var indexes = mylist.Select((v, i) => new { v, i })
                    .Where(x => x.v > 23)
                    .Select(x => x.i);

Answer 2

Linq doesn't provide such a thing directly, but you could write your own. Something like this:

public static IEnumerable<int> FindIndices<T>(this IEnumerable<T> items, Func<T, bool> predicate) 
{
    int i = 0;

    foreach (var item in items) 
    {
        if (predicate(item)) 
        {
            yield return i;
        }

        i++;
    }
}

Then something like:

foreach (int index in mylist.FindIndices( x => x > 23 ))
    ...

(This has the advantage of being more efficient than the other approaches listed above. This is only significant for LARGE sequences though!)