Active and Inactive cells after k Days?

Here we will see one interesting problem. Suppose one binary array is given of size n. Here n > 3. A true value or 1 value indicates that the active state, and 0 or false indicates inactive. Another number k is also given. We have to find active or inactive cells after k days. After every day state of ith cell will be active if the left and right cells are not same, if they are same, then it will be inactive. The left most and right most cell has no cell before and after it. So left most and right most cells are always 0.

Let us see one example to get the idea. Suppose one array is like {0, 1, 0, 1, 0, 1, 0, 1}, and the value of k = 3. So let us see how it changes day by day.

• After 1 day the array will be {1, 0, 0, 0, 0, 0, 0, 0}
• After 2 days the array will be {0, 1, 0, 0, 0, 0, 0, 0}
• After 3 days the array will be {1, 0, 1, 0, 0, 0, 0, 0}

So 2 active cells and 6 inactive cells

Algorithm

activeCellKdays(arr, n, k)

begin
   make a copy of arr into temp
   for i in range 1 to k, do
      temp[0] := 0 XOR arr[1]
      temp[n-1] := 0 XOR arr[n-2]
      for each cell i from 1 to n-2, do
         temp[i] := arr[i-1] XOR arr[i+1]
      done
      copy temp to arr for next iteration
   done
   count number of 1s as active, and number of 0s as inactive, then return the values.
end

Example

#include <iostream>
using namespace std;
void activeCellKdays(bool arr[], int n, int k) {
   bool temp[n]; //temp is holding the copy of the arr
   for (int i=0; i<n ; i++)
      temp[i] = arr[i];
   for(int i = 0; i<k; i++){
      temp[0] = 0^arr[1]; //set value for left cell
      temp[n-1] = 0^arr[n-2]; //set value for right cell
      for (int i=1; i<=n-2; i++) //for all intermediate cell if left and
         right are not same, put 1
      temp[i] = arr[i-1] ^ arr[i+1];
      for (int i=0; i<n; i++)
         arr[i] = temp[i]; //copy back the temp to arr for the next iteration
   }
   int active = 0, inactive = 0;
   for (int i=0; i<n; i++)
      if (arr[i])
         active++;
      else
         inactive++;
   cout << "Active Cells = "<< active <<", Inactive Cells = " << inactive;
}
main() {
   bool arr[] = {0, 1, 0, 1, 0, 1, 0, 1};
   int k = 3;
   int n = sizeof(arr)/sizeof(arr[0]);
   activeCellKdays(arr, n, k);
}

Output

Active Cells = 2, Inactive Cells = 6