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

If you want to find out how many time each item presents in the, say, array, you can use Linq:

  int[] a = new int[] 
   { 2, 3, 4, 3, 4, 2, 4, 2, 4 };

  // I'd rather not used array, as you suggested, but dictionary 
  Dictionary b = a
    .GroupBy(item => item)
    .ToDictionary(item => item.Key, item => item.Count());

 ...

the outcome is

  b[2] == 3;
  b[3] == 2;
  b[4] == 4;