Get index of lines that match my linq

Question

I have a linq statement and I would like to know if it is possible to get indicies of lines that match my statement? Here it is:

var result = list3.Where(middle          


        

Answer 1

Also note that if you want to search for the indicies of items matching a predicate very often, it could be worth writing a very simple extension method:

public static class IEnumerableExt
{
    public static IEnumerable<int> FindIndices<T>(this IEnumerable<T> self, Predicate<T> predicate)
    {
        int i = 0;

        foreach (var element in self)
        {
            if (predicate(element))
                yield return i;

            ++i;
        }
    }
}

Which you would call like this:

var result = list3.FindIndices(x => list4.Any(xx => xx == x.middle.Middle.category1));

Answer 2

do you mean this?

var result = list3.Where(middle => list4.Any(x => x == middle.Middle.category1))
           .Select(obj => new { obj, dt = DateTime.ParseExact(obj.LeftColumn, dateFormat, CultureInfo.InvariantCulture) })
           .Where(x => x.dt >= czas11 && x.dt <= czas22)
           .Select((x,index) =>new{ x.obj,Index=index}).ToList();

Answer 3

You can use the overload of Select (or Where) which projects also the index of the element:

var result = list3.Select((middle, index) => new{ middle, index })
    .Where(x => list4.Any(xx => xx == x.middle.Middle.category1))
    .Select(x => new { x.middle, x.index, dt = DateTime.ParseExact(x.middle.LeftColumn, dateFormat, CultureInfo.InvariantCulture) })
    .Where(x => x.dt >= czas11 && x.dt <= czas22)
    .Select(x => x.index)
    .ToList();

Side-note: consider to change your variable names to be more meaningful. That is unreadable.