How do I check if my array has repeated values inside it?

Question

So here is my array.

double[] testArray = new double[10];
// will generate a random numbers from 1-20, too lazy to write the code

I want to make a search loop to check if any values are being repeated. How do I do that?

I would prefer not to use any special built-in methods since this is a small array.

Answer 1

You could do this with a little Linq:

if (testArray.Length != testArray.Distinct().Count())
{
    Console.WriteLine("Contains duplicates");
}

The Distinct extension method removes any duplicates, and Count gets the size of the result set. If they differ at all, then there are some duplicates in the list.

Alternatively, here's more complicated query, but it may be a bit more efficient:

if (testArray.GroupBy(x => x).Any(g => g.Count() > 1))
{
    Console.WriteLine("Contains duplicates");
}

The GroupBy method will group any identical elements together, and Any return true if any of the groups has more than one element.

Both of the above solutions work by utilizing a HashSet<T>, but you can use one directly like this:

if (!testArray.All(new HashSet<double>().Add))
{
    Console.WriteLine("Contains duplicates");
}

Or if you prefer a solution that doesn't rely on Linq at all:

var hashSet = new HashSet<double>();
foreach(var x in testArray) 
{
    if (!hashSet.Add(x)) 
    {
        Console.WriteLine("Contains duplicates");
        break;
    }
}

Answer 2

With (OP) 10 random doubles quite fast. The chance of a repeat: ~0.000002 %.

static bool repeat(double[] a)
{
    return
        a[0] == a[1] || a[0] == a[2] || a[0] == a[3] || a[0] == a[4] ||
        a[0] == a[5] || a[0] == a[6] || a[0] == a[7] || a[0] == a[8] ||
        a[0] == a[9] || a[1] == a[2] || a[1] == a[3] || a[1] == a[4] ||
        a[1] == a[5] || a[1] == a[6] || a[1] == a[7] || a[1] == a[8] ||
        a[1] == a[9] || a[2] == a[3] || a[2] == a[4] || a[2] == a[5] ||
        a[2] == a[6] || a[2] == a[7] || a[2] == a[8] || a[2] == a[9] ||
        a[3] == a[4] || a[3] == a[5] || a[3] == a[6] || a[3] == a[7] ||
        a[3] == a[8] || a[3] == a[9] || a[4] == a[5] || a[4] == a[6] ||
        a[4] == a[7] || a[4] == a[8] || a[4] == a[9] || a[5] == a[6] ||
        a[5] == a[7] || a[5] == a[8] || a[5] == a[9] || a[6] == a[7] ||
        a[6] == a[8] || a[6] == a[9] || a[7] == a[8] || a[7] == a[9] ||
        a[8] == a[9];
}

More general, with 10 numbers ~2 times slower than above,
but ~7 times faster than the hashset approach.

static bool repeat(double[] a)
{
    int k = a.Length - 1;
    if (k < 70)
    {
        double aj;
        for (int i = 0, j; i < k; )
        {
            for (aj = a[k--], j = k; j >= i; j--)
                if (aj == a[j]) return true;
            for (aj = a[i++], j = i; j <= k; j++)
                if (aj == a[j]) return true;
        }
        return false;
    }
    var h = new HashSet<double>();
    while (k >= 0) if (!h.Add(a[k--])) return false;
    return true;
}

Two lines (slow with a repeat ;)

static bool repeat(double[] a)
{ return (new HashSet<double>(a).Count < a.Length); }

Answer 3

take look at my implementation its generic and efficient

public static bool HasDuplicates<T>(IList<T> items)
    {
        Dictionary<T, bool> map = new Dictionary<T, bool>();
        for (int i = 0; i < items.Count; i++)
        {
            if (map.ContainsKey(items[i]))
            {
                return true; // has duplicates
            }
            map.Add(items[i], true);
        }
        return false; // no duplicates
    }

here are some calls

string[] strings = new[] { "1", "2", "3" };
Utility.HasDuplicates(strings)// this will return false

int[] items=new []{1,2,3,1};
Utility.HasDuplicates(items)// this will return true

Answer 4

use a hashset to add members to, then check if there a previous occurrence of current member

public bool ContainsDuplicate(double[] nums) 
{
            int size = nums.Length;
            HashSet<double> set1 = new HashSet<double>();

            for (int i = 0; i < size; i++)
            {
                if (set1.Contains(nums[i]))
                {
                    return true;
                }
                else
                {
                    set1.Add(nums[i]);
                }
            }
            return false;
}

Answer 5

Use this:

bool CheckUniqueness(double[] values)
{
    var uniqueValues = new HashSet<double>();
    foreach (double d in values)
    {
        if(uniqueValues.Contains(d))
        {
            return false;
        }
        uniqueValues.Add(d);
    }
    return true;
}

Answer 6

We must initialize j from i on the first loop and add one(i+1) because we want to compare first loop value with the next value of same array.

int[] arr = new int[]{1,2,3,1,4,2,5,4};

//create one loop for arr values
for (int i = 0;  i < arr.Length; i++)
{
    //create nested loop for compare current values with actual value of arr
    for (int j = i+1; j < arr.Length; j++)
    {

        //and here we put our condition
        if (arr[i] == arr[j])
        {
            Console.WriteLine(arr[i]);
        }
    }
}

Answer 7

Generic extension method :

public static bool HasDuplicate<T>(this IEnumerable<T> source, IEqualityComparer<T> comparer)
{
    if (source == null)
        throw new ArgumentException(nameof(source));

    HashSet<T> set = new HashSet<T>(comparer);
    foreach (var item in source)
        if (!set.Add(item))
            return true;

    return false;
}

Answer 8

        int[] nums = new int[] { 1, 2, 3, 4, 5};



        Console.WriteLine(AnyDuplicate(nums));
    }
    /// <summary>
    /// Returns true if there is at least a duplicate in the array.
    /// </summary>
    /// <returns></returns>
    static bool AnyDuplicate(int[] numbers)
    {            
        for (int i = 0; i < numbers.Length; i++)
        {
            for (int j = i + 1; j < numbers.Length; j++)
            {
                if (numbers[i] == numbers[j])
                {
                    return true; 
                }
            }
        }
        return false;