What's the fastest way to divide an integer by 3?

Question

int x = n / 3;  // <-- make this faster

// for instance

int a = n * 3; // <-- normal integer multiplication

int b = (n << 1) + n; // <-- potentiall         


        

Answer 1

For really large integer division (e.g. numbers bigger than 64bit) you can represent your number as an int[] and perform division quite fast by taking two digits at a time and divide them by 3. The remainder will be part of the next two digits and so forth.

eg. 11004 / 3 you say

11/3 = 3, remaineder = 2 (from 11-3*3)

20/3 = 6, remainder = 2 (from 20-6*3)

20/3 = 6, remainder = 2 (from 20-6*3)

24/3 = 8, remainder = 0

hence the result 3668

internal static List Div3(int[] a)
{
  int remainder = 0;
  var res = new List();
  for (int i = 0; i < a.Length; i++)
  {
    var val = remainder + a[i];
    var div = val/3;

    remainder = 10*(val%3);
    if (div > 9)
    {
      res.Add(div/10);
      res.Add(div%10);
    }
    else
      res.Add(div);
  }
  if (res[0] == 0) res.RemoveAt(0);
  return res;
}