LINQ indexOf a particular entry

Question

I have an MVC3 C#.Net web app. I have the below string array.

    public static string[] HeaderNamesWbs = new[]                                        {                                           WBS_NUMBER,                                           BOE_TITLE,                                           SOW_DESCRIPTION,                                           HARRIS_WIN_THEME,                                           COST_BOGEY                                        }; 

I want to find the Index of a given entry when in another loop. I thought the list would have an IndexOf. I can't find it. Any ideas?

Answer 1

Well you can use Array.IndexOf:

int index = Array.IndexOf(HeaderNamesWbs, someValue); 

Or just declare HeaderNamesWbs as an IList<string> instead - which can still be an array if you want:

public static IList<string> HeaderNamesWbs = new[] { ... }; 

Note that I'd discourage you from exposing an array as public static, even public static readonly. You should consider ReadOnlyCollection:

public static readonly ReadOnlyCollection<string> HeaderNamesWbs =     new List<string> { ... }.AsReadOnly(); 

If you ever want this for IEnumerable<T>, you could use:

var indexOf = collection.Select((value, index) => new { value, index })                         .Where(pair => pair.value == targetValue)                         .Select(pair => pair.index + 1)                         .FirstOrDefault() - 1; 

(The +1 and -1 are so that it will return -1 for "missing" rather than 0.)

Answer 2

I'm late to the thread here. But I wanted to share my solution to this. Jon's is awesome, but I prefer simple lambdas for everything.

You can extend LINQ itself to get what you want. It's fairly simple to do. This will allow you to use syntax like:

// Gets the index of the customer with the Id of 16. var index = Customers.IndexOf(cust => cust.Id == 16); 

This is likely not part of LINQ by default because it requires enumeration. It's not just another deferred selector/predicate.

Also, please note that this returns the first index only. If you want indexes (plural), you should return an IEnumerable<int> and yield return index inside the method. And of course don't return -1. That would be useful where you are not filtering by a primary key.

  public static int IndexOf<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate) {            var index = 0;      foreach (var item in source) {         if (predicate.Invoke(item)) {            return index;         }         index++;      }       return -1;   }