Find the position of a List Item in a List based upon specifice criteria using LINQ

Question

Something similar to linq find in which position is my object in List, except his accepted answer is evaluating at an object level.

Say I have

public interface IFoo{
    string text {get;set;}
    int number {get;set;}
}

and

public class Foo : IFoo{
    public string text {get;set;}
    public int number {get;set;}
}

Now I have a list collection of IFoo like so:

public IList<IFoo> Foos;

I want to write a method that brings back the index(es) of Foos based upon a property value and/ or values (for example, text or number in this case) so I can do something like:

var fooIndexes = Foos.GetIndexes( f => f.text == "foo" && f.number == 8 );

How would I write that?

score 4 · Accepted Answer · edited Jun 04 '12 at 19:52

4

You could use something like:

public static IEnumerable<int> GetIndices<T>(this IEnumerable<T> items, Func<T, bool> predicate)
{
    return items.Select( (item, index) => new { Item = item, Index = index })
                     .Where(p => predicate(p.Item))
                     .Select(p => p.Index);
}

edited Jun 04 '12 at 19:52

hunter

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answered Jun 04 '12 at 19:46

Reed Copsey

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score 1 · Answer 2 · answered Jun 04 '12 at 19:57

Here’s the non-LINQ implementation:

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

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

       i++;
    }
}

In this case, this would probably be more efficient since it avoids the anonymous type initialization.

Find the position of a List Item in a List based upon specifice criteria using LINQ

2 Answers2