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c#searchlistduplicates
Kevin Jensen
Kevin Jensen

Reputation: 1428

What is the most elegant way to find index of duplicate items in C# List

I've got a List<string> that contains duplicates and I need to find the indexes of each.

What is the most elegant, efficient way other than looping through all the items. I'm on .NET 4.0 so LINQ is an option. I've done tons of searching and connect find anything.

Sample data:

var data = new List<string>{"fname", "lname", "home", "home", "company"}();

I need to get the indexes of "home".

Upvotes: 11

Views: 12754

Answers (4)

Faisal Hasnain
Faisal Hasnain

Reputation: 485

I myself needed to find and remove the duplicates from list of strings. I first searched the indexes of duplicate items and then filtered the list in functional way using LINQ, without mutating the original list:

public static IEnumerable<string> RemoveDuplicates(IEnumerable<string> items)
{
    var duplicateIndexes =  items.Select((item, index) => new { item, index })
                            .GroupBy(g => g.item)
                            .Where(g => g.Count() > 1)
                            .SelectMany(g => g.Skip(1), (g, item) => item.index);
    return items.Where((item, index) => !duplicateIndexes.Contains(index));
}

Upvotes: 0

Adriaan Stander
Adriaan Stander

Reputation: 166476

How about something like this

var data = new List<string>{"fname", "lname", "home", "home", "company"};

            var duplicates = data
                            .Select((x, index) => new { Text = x, index})
                            .Where( x => (  data
                                            .GroupBy(i => i)
                                            .Where(g => g.Count() > 1)
                                            .Select(g => g.Key).ToList()
                                          ).Contains(x.Text));

Upvotes: 0

Guffa
Guffa

Reputation: 700562

You can create an object from each item containing it's index, then group on the value and filter out the groups containing more than one object. Now you have a grouping list with objects containing the text and their original index:

var duplicates = data
  .Select((t,i) => new { Index = i, Text = t })
  .GroupBy(g => g.Text)
  .Where(g => g.Count() > 1);

Upvotes: 24

Kirill Osenkov
Kirill Osenkov

Reputation: 8986

using System;
using System.Collections.Generic;

class Program
{
    static void Main(string[] args)
    {
        var data = new List<string> { "fname", "lname", "home", "home", "company" };
        foreach (var duplicate in FindDuplicates(data))
        {
            Console.WriteLine("Duplicate: {0} at index {1}", duplicate.Item1, duplicate.Item2);
        }
    }

    public static IEnumerable<Tuple<T, int>> FindDuplicates<T>(IEnumerable<T> data)
    {
        var hashSet = new HashSet<T>();
        int index = 0;
        foreach (var item in data)
        {
            if (hashSet.Contains(item))
            {
                yield return Tuple.Create(item, index);
            }
            else
            {
                hashSet.Add(item);
            }
            index++;
        }
    }
}

Upvotes: 3

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