How to get the index of an element in an IEnumerab

Question

I wrote this:

public static class EnumerableExtensions
{
    public static int IndexOf<T>(this IEnumerable<T> obj, T value)
    {
        return obj
            .Select((a, i) => (a.Equals(value)) ? i : -1)
            .Max();
    }

    public static int IndexOf<T>(this IEnumerable<T> obj, T value
           , IEqualityComparer<T> comparer)
    {
        return obj
            .Select((a, i) => (comparer.Equals(a, value)) ? i : -1)
            .Max();
    }
}

But I don't know if it already exists, does it?

Answer 1

A few years later, but this uses Linq, returns -1 if not found, doesn't create extra objects, and should short-circuit when found [as opposed to iterating over the entire IEnumerable]:

public static int IndexOf<T>(this IEnumerable<T> list, T item)
{
    return list.Select((x, index) => EqualityComparer<T>.Default.Equals(item, x)
                                     ? index
                                     : -1)
               .FirstOr(x => x != -1, -1);
}

Where 'FirstOr' is:

public static T FirstOr<T>(this IEnumerable<T> source, T alternate)
{
    return source.DefaultIfEmpty(alternate)
                 .First();
}

public static T FirstOr<T>(this IEnumerable<T> source, Func<T, bool> predicate, T alternate)
{
    return source.Where(predicate)
                 .FirstOr(alternate);
}

Answer 2

Using @Marc Gravell 's answer, I found a way to use the following method:

source.TakeWhile(x => x != value).Count();

in order to get -1 when the item cannot be found:

internal static class Utils
{

    public static int IndexOf<T>(this IEnumerable<T> enumerable, T item) => enumerable.IndexOf(item, EqualityComparer<T>.Default);

    public static int IndexOf<T>(this IEnumerable<T> enumerable, T item, EqualityComparer<T> comparer)
    {
        int index = enumerable.TakeWhile(x => comparer.Equals(x, item)).Count();
        return index == enumerable.Count() ? -1 : index;
    }
}

I guess this way could be both the fastest and the simpler. However, I've not tested performances yet.

Answer 3

A bit late in the game, i know... but this is what i recently did. It is slightly different than yours, but allows the programmer to dictate what the equality operation needs to be (predicate). Which i find very useful when dealing with different types, since i then have a generic way of doing it regardless of object type and <T> built in equality operator.

It also has a very very small memory footprint, and is very, very fast/efficient... if you care about that.

At worse, you'll just add this to your list of extensions.

Anyway... here it is.

 public static int IndexOf<T>(this IEnumerable<T> source, Func<T, bool> predicate)
 {
     int retval = -1;
     var enumerator = source.GetEnumerator();

     while (enumerator.MoveNext())
     {
         retval += 1;
         if (predicate(enumerator.Current))
         {
             IDisposable disposable = enumerator as System.IDisposable;
             if (disposable != null) disposable.Dispose();
             return retval;
         }
     }
     IDisposable disposable = enumerator as System.IDisposable;
     if (disposable != null) disposable.Dispose();
     return -1;
 }

Hopefully this helps someone.

Answer 4

The best way to catch the position is by FindIndex This function is available only for List<>

Example

int id = listMyObject.FindIndex(x => x.Id == 15); 

If you have enumerator or array use this way

int id = myEnumerator.ToList().FindIndex(x => x.Id == 15); 

or

 int id = myArray.ToList().FindIndex(x => x.Id == 15); 

Answer 5

Stumbled across this today in a search for answers and I thought I'd add my version to the list (No pun intended). It utlises the null conditional operator of c#6.0

IEnumerable<Item> collection = GetTheCollection();

var index = collection
.Select((item,idx) => new { Item = item, Index = idx })
//or .FirstOrDefault(_ =>  _.Item.Prop == something)
.FirstOrDefault(_ => _.Item == itemToFind)?.Index ?? -1;

I've done some 'racing of the old horses' (testing) and for large collections (~100,000), worst case scenario that item you want is at the end, this is 2x faster than doing ToList().FindIndex(). If the Item you want is in the middle its ~4x faster.

For smaller collections (~10,000) it seems to be only marginally faster

Heres how I tested it https://gist.github.com/insulind/16310945247fcf13ba186a45734f254e

Answer 6

This can get really cool with an extension (functioning as a proxy), for example:

collection.SelectWithIndex(); 
// vs. 
collection.Select((item, index) => item);

Which will automagically assign indexes to the collection accessible via this Index property.

Interface:

public interface IIndexable
{
    int Index { get; set; }
}

Custom extension (probably most useful for working with EF and DbContext):

public static class EnumerableXtensions
{
    public static IEnumerable<TModel> SelectWithIndex<TModel>(
        this IEnumerable<TModel> collection) where TModel : class, IIndexable
    {
        return collection.Select((item, index) =>
        {
            item.Index = index;
            return item;
        });
    }
}

public class SomeModelDTO : IIndexable
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public decimal Price { get; set; }

    public int Index { get; set; }
}

// In a method
var items = from a in db.SomeTable
            where a.Id == someValue
            select new SomeModelDTO
            {
                Id = a.Id,
                Name = a.Name,
                Price = a.Price
            };

return items.SelectWithIndex()
            .OrderBy(m => m.Name)
            .Skip(pageStart)
            .Take(pageSize)
            .ToList();