What is the most elegant way to get a set of items by index from a collection?

Question

Given

IList indexes;
ICollection collection;

What is the most elegant way to extract all T in

Answer 1

Several good suggestions here already, I'll just throw in my two cents.

int counter = 0;
var x = collection
    .Where((item, index) => 
        counter < indices.Length && 
        index == indices[counter] && 
        ++counter != 0);

edit: yah, didn't think it through the first time around. the increment has to happen only when the other two conditions are satisfied..

Answer 2

Here's a faster version:

IEnumerable<T> ByIndices<T>(ICollection<T> data, IList<int> indices)
{
    int current = 0;
    foreach(var datum in data.Select((x, i) => new { Value = x, Index = i }))
    {
        if(datum.Index == indices[current])
        {
            yield return datum.Value;
            if(++current == indices.Count)
                yield break;
        }
    }
}

Answer 3

You could do it in an extension method:

static IEnumerable<T> Extract<T>(this ICollection<T> collection, IList<int> indexes)
{
   int index = 0;
   foreach(var item in collection)
   {
     if (indexes.Contains(index))
       yield item;
     index++;
   }
}

Answer 4

    public static IEnumerable<T> WhereIndexes<T>(this IEnumerable<T> collection, IEnumerable<int> indexes)
    {
        IList<T> l = new List<T>(collection);
        foreach (var index in indexes)
        {
            yield return l[index]; 
        }
    }

Answer 5

Maybe I'm missing something, but what's wrong with just:

indexes.Select( (index => values[index]))

Answer 6

I would use an extension Method

public static IEnumerable<T> Filter<T>(this IEnumerable<T> pSeq, 
                                       params int [] pIndexes)
{
      return pSeq.Where((pArg, pId) => pIndexes.Contains(pId));
}