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

Answer 1

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 2

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 3

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 4

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 5

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 6

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