Efficient way to remove duplicate strings from a string array in C#

Question

I would like to know an efficient method to remove duplicate items from a string array in C#.

For example,

string[] a = { "abc", "xyz","abc", "def", "ghi", "asdf", "ghi","xd", "abc" };

will become,

string[] a = { "abc", "xyz","def", "ghi", "asdf", "xd" };

How to fill the gaps after removing the duplicate entries? Is there a way to do this without using an extra array for storing the elements?

Method which I used:

1) Sorted the array

2) Replaced the duplicate entries with null

3) Copied NOT null string to a new array.

But looking for an optimized way to doing the same.

EDIT: I am using .NET 2.0 and VS 2005

Ohad Schneider · Accepted Answer · 2011-04-11 07:59:34Z

16

You can use a HashSet:

string[] a = { "abc", "xyz","abc", "def", "ghi", "asdf", "ghi","xd", "abc" };
var b = new HashSet<string>(a);

answered Apr 11, 2011 at 7:59

Ohad Schneider

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SyncMaster Over a year ago

Can I use var in VS 2005 with .Net 2.0 ?

Guffa · Accepted Answer · 2011-04-11 08:01:37Z

10

You can't resize an array in .NET, so whatever way you use to remove the duplicates, you have to create a new array for the result.

You can use a HashSet<string> to easily remove the duplicates:

a = new HashSet<string>(a).ToArray();

The hash set will add the items from the array to itself, and automatically discard the duplicates. As the hash set uses hash codes to check for existing items, this will be somewhat faster than sorting the items, however the result is of course not sorted.

answered Apr 11, 2011 at 8:01

Guffa

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2 Comments

SyncMaster Over a year ago

I installed .Net 3.5 update. But I get the error, "'System.Collections.Generic.HashSet<string>' does not contain a definition for 'ToArray'".

Guffa Over a year ago

@pragadheesh: The ToArray is not a method in the class itself, it's an extension method. You need using System.Linq; at the top of the file to get the extension methods.

archil · Accepted Answer · 2011-04-11 07:59:01Z

7

Take a look at IEnumerable.Distinct() Method

answered Apr 11, 2011 at 7:59

archil

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Achinth Gurkhi · Accepted Answer · 2011-04-11 09:12:17Z

6

If using .NET 3.0 you can use LINQ:

using System;
using System.Linq;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] a = { "abc", "xyz", "abc", "def", "ghi", "asdf", "ghi", "xd", "abc" };
            string[] b = a.Distinct().ToArray();
            foreach (string s in b)
                Console.WriteLine(s);
            Console.ReadLine();
        }
    }
}

answered Apr 11, 2011 at 9:12

Achinth Gurkhi

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