How to find or count the duplicate in multidimensional array in c# [closed]

Question

I am creating a 2d (3 * 3) array in c# which has to count or find the duplicate value

int[,] arr = new int[3, 3] {
   {1, 2, 6},
   {4, 1, 5},
   {6, 1, 8}
};

I expected the output has 1 and 6 are the duplicate values

For beginners only with for loop

Answer 1

You can try HashSet<int>, e.g.

   int[,] arr = new int[3, 3] {
      {1, 2, 6 }, 
      {4, 1, 5 }, 
      {6, 1, 8 }
   };

   HashSet<int> unique = new HashSet<int>();

   foreach (var item in arr)
     if (!unique.Add(item))
       Console.WriteLine(item);  // Not unique, print it out

Outcome:

  1
  6
  1

If we want to print each duplicate once, we can add another HashSet<int>:

   HashSet<int> unique = new HashSet<int>();
   HashSet<int> duplicates = new HashSet<int>();

   foreach (var item in arr)
     if (!unique.Add(item))
       duplicates.Add(item);  

   foreach (var item in duplicates)
     Console.WriteLine(item);

Finally, if you want to count duplicate occurences we can change HashSet<int> duplicates into Dictionary<int, int> duplicates:

   HashSet<int> unique = new HashSet<int>();

   Dictionary<int, int> duplicates = new Dictionary<int, int>();

   foreach (var item in arr)
     if (!unique.Add(item)) 
       if (duplicates.TryGetValue(item, out int count))
         duplicates[item] = count + 1;
       else
         duplicates[item] = 2; 

   foreach (var item in duplicates)
     Console.WriteLine($"{item.Key} appears {item.Value} times");

Edit: you can change foreach loop into nested for ones, e.g:

All duplicates

   for (int i = 0; i < arr.GetLength(0); ++i)
     for (int j = 0; j < arr.GetLength(1); ++j)
       if (!unique.Add(arr[i, j]))
         Console.WriteLine(arr[i, j]); 

Distinct duplicates

   HashSet<int> duplicates = new HashSet<int>();

   for (int i = 0; i < arr.GetLength(0); ++i)
     for (int j = 0; j < arr.GetLength(1); ++j)
       if (!unique.Add(arr[i, j]))
         if (duplicates.Add(arr[i, j]))   // print distinct duplicate only
           Console.WriteLine(arr[i, j]); 
       

Answer 2

Perhaps like this:

var arr = new int[3, 3]{{1,2,6}, {4,1,5}, {6,1,8}};
var duplicates = arr
    .Cast<int>()
    .GroupBy(n => n)
    .Where(g => g.Count() > 1)
    .Select(g => g.Key)
    .ToArray();

.Cast<int>() makes the array usable for LINQ, .GroupBy(n => n) groups numbers by their value, .Where(g => g.Count() > 1) counts the number of items in the group, .Select(g => g.Key) returns only the group keys - the original values.

Do .Where(g => g.Count() > 1).Select(g => g.Count()) to return the counts of each, or just operate on the groups as needed.

Answer 3

Not very efficient, but you could also store your counts in a dictionary, then print the keys that have a count of more than 1:

var counts = new Dictionary<int, int>();

for (int i = 0; i < arr.GetLength(0); i++)
{
    for (int j = 0; j < arr.GetLength(1); j++)
    {
        var number = arr[i, j];
        if (!counts.ContainsKey(number))
        {
            counts[number] = 0;
        }
        counts[number] += 1;
    }
}

foreach (var pair in counts)
{
    if (pair.Value > 1)
    {
        Console.WriteLine(pair.Key);
    }
}

// 1
// 6