How to count occurences of an int in an array?

Question

My array is A = {2, 3, 4, 3, 4, 2, 4, 2, 4}

I need an array B that stock at the index i the number of occurences of i in the a

Answer 1

Your task can be easily accomplished using a Dictionary.

Here is the code :

 Dictionary ItemCount = new Dictionary();

 int[] items = { 2, 3, 4, 3, 4, 2, 4, 2, 4 };

 foreach (int item in items)
 {
    if (ItemCount.ContainsKey(item))
    {
         ItemCount[item]++;
    }
    else {
        ItemCount.Add(item,1);
    }
 }

  Console.WriteLine("A|B");
  foreach (KeyValuePair res in ItemCount)
  {
      Console.WriteLine(res.Key +"|"+res.Value);
  }

output :

A  |   B
2  |   3
3  |   3
4  |   4

Note : I think this might be too advance for you , but it's an easy way

Without Dictionary, (A primitive approach)

     int[] A = { 2, 3, 4, 3, 4, 2, 4, 2, 4 };
     List B = new List(); // <= We need this to check already counted numbers in array

     int temp = 0; // <= A temporary variable to get a count per a specific elemet
     int count = 0; // < = Will hold number of elements we have already counted 

     Console.WriteLine("A|B");

     for (int i = 0; i < A.Length; i++)
     {
         temp = 0;

         // Check for a fresh number 
         if (!B.Contains(A[i]))
         {
             B.Add(A[i]);
             // For each element we try to count the number of occurrence 
             for (int j = 0; j < A.Length; j++)
             {
                 // Current element i matched with a element in array; counts increased 
                 if (A[i] == A[j])
                 {
                        temp++; // < = Local count
                        count++; // <= Kind of the global count of elements we have passed
                 }
             }

                Console.WriteLine(A[i] + "|" + temp); 
            }

           // We need to do this only for unique elements; when we have counted all elements in Array A we are done
           if (count >= A.Length)
           {
                break;
           }

      }