Delete unique values of an Array using LINQ?

Question

I want to store the duplicated values of an Array, or what is the same I want to delete the Unique names.

For example if I have an Array with this content:

{a, b, c, c, c}

I want to store this in other array:

{c, c, c}

I know how to perform the operation using a For loop but I want to improve the code using LINQ extensions (if is possibly).

So, the original array just keeps {a, b}, not even a single c? Also, is the input sorted? What's the performance requirements? (Because if performance is any factor, I suggest a loop + Array.Resize) — sehe
– sehe, Commented Sep 7, 2013 at 21:53
@sehe, really no matter if the first array keeps the C or not while the new array contains the duplicated value 1 time, yes the array is sorted, like I've said I don't preffer to use For having he usefull LINQ extensions to improve it, thanks for comment. — ElektroStudios
– ElektroStudios, Commented Sep 7, 2013 at 22:33

sehe · Accepted Answer · 2013-09-07 22:58:24Z

2

This does what you want, but will reorder the elements in your original collection.

var query = yourArray.GroupBy(x=>x)
     .Where(x=>x.Count() > 1)
     .SelectMany(x=>x)
     .ToArray();

To get the difference between the two, you can then use Except, doing:

var exceptResult = yourArray.Except(query);

edited Sep 7, 2013 at 22:58

sehe

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answered Sep 7, 2013 at 22:00

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3 Comments

sehe Over a year ago

Yes, this is what I had in mind for "non-performant". Yet, several helpless performance kittens are weeping silently in a corner. Really, with the input sorted, a simple one-pass algorithm would be so much more performant, and arguably more clear...

Save Over a year ago

@sehe Well I agree that this problem doesn't really requires LINQ. But since he asked for a LINQ solution, I provided it :) It should be enough to have the performance remark in the comments, to remember everybody that's not the ideal way to do it

ElektroStudios Over a year ago

I've tried something like array1.except(array1.distinct) but does not worked, I comment that 'cause if you can have a better performance idea using linq, you guys consider that in an Array which can have around 100 entries (NO MORE) using @Save code can be a too much negative point of performance?

Alex Siepman · Accepted Answer · 2013-09-08 09:48:56Z

(In C#) this a much faster implementation then doing a GroupBy/Where/SelectMany (and a Count() that enumerates). But I must agree that it is more code ;-)

var array = new[] { 1, 2, 3, 3, 3 };
var valueCounter = new ValueCounter<int>(array);
var query = valueCounter.Where(p => p.Value > 1)
    .SelectMany(p => Enumerable.Repeat(p.Key, p.Value)).ToArray();

Using this ValueCounter class:

public class ValueCounter<T> : IEnumerable<KeyValuePair<T, int>>
{
    private readonly IEqualityComparer<T> _comparer;
    private readonly Dictionary<T, int> _valueCounter;
    private int _nullCount = 0;

    public ValueCounter(IEnumerable<T> values, IEqualityComparer<T> comparer)
    {
        _comparer = comparer ?? EqualityComparer<T>.Default;
        _valueCounter = new Dictionary<T, int>(_comparer);
        if (values != null)
        {
            foreach (var value in values)
            {
                Add(value);
            }
        }
    }

    public ValueCounter(IEqualityComparer<T> comparer)
        : this(null, comparer)
    {
    }

    public ValueCounter(IEnumerable<T> values)
        : this(values, null)
    {
    }

    public ValueCounter()
        : this(null, null)
    {
    }

    public void Add(T value)
    {
        if (value == null)
        {
            _nullCount++;
        }
        else
        {
            int count;
            if (_valueCounter.TryGetValue(value, out count))
            {
                _valueCounter[value] = count + 1;
            }
            else
            {
                _valueCounter.Add(value, 1);
            }
        }
    }

    /// <summary>
    /// Removes a value 
    /// </summary>
    /// <param name="value">The value that needs to be removed</param>
    /// <returns>True if a value was removed</returns>
    public bool Remove(T value)
    {
        if (value == null)
        {
            if (_nullCount > 0)
            {
                _nullCount--;
                return true;
            }
        }
        else
        {
            int count;
            if (_valueCounter.TryGetValue(value, out count))
            {
                if (count == 1)
                {
                    _valueCounter.Remove(value);
                }
                else
                {
                    _valueCounter[value] = count - 1;
                }
                return true;
            }
        }
        return false;
    }

    public int GetCount(T value)
    {
        int result;
        _valueCounter.TryGetValue(value, out result);
        return result;
    }

    public IEnumerator<KeyValuePair<T, int>> GetEnumerator()
    {
        return _valueCounter.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

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