Find indices of particular items in the list using linq

Question

I have a list of integers from 1 to 20. I want the indices of items which are greater than 10 using linq. Is it possible to do with linq?

Thanks in advance

Answer 1

Use the overload of Select which includes the index:

var highIndexes = list.Select((value, index) => new { value, index })
                      .Where(z => z.value > 10)
                      .Select(z => z.index);

The steps in turn:

• Project the sequence of values into a sequence of value/index pairs
• Filter to only include pairs where the value is greater than 10
• Project the result to a sequence of indexes

Answer 2

    public static List<int> FindIndexAll(this List<int> src, Predicate<int> value)
    {
        List<int> res = new List<int>();
        var idx = src.FindIndex(x=>x>10);           
        if (idx!=-1) {
        res.Add(idx);
         while (true)
         {
            idx = src.FindIndex(idx+1, x => x > 10);
            if (idx == -1)
                break;
            res.Add(idx);
         }
        }
        return res;
    }

Usage

        List<int>  test= new List<int>() {1,10,5,2334,34,45,4,4,11};
        var t = test.FindIndexAll(x => x > 10);