q17.6.cpp

#include <iostream>

using namespace std;

void print_number_twos(int n) {
  int current_multiple = 10, totalCount = 0;
  int q, rem;

  while (current_multiple < n * 10) {
    q = n / current_multiple;
    rem = n % current_multiple;

    totalCount += q * (current_multiple / 10);

    if (rem >= 3 * (current_multiple / 10))
      totalCount += (current_multiple / 10);
    else if ((2 * current_multiple / 10) <= rem)
      totalCount += (rem - (2 * current_multiple / 10) + 1);

    current_multiple *= 10;
  }

  cout << "total number of 2's in numbers from 0 to " << n << " : "
       << totalCount << endl;

  return;
}

int main(void) {
  int n;

  while (true) {
    cout << "enter value of n to count number of 2's from 0 to n or enter 0 to "
            "exit: ";
    cin >> n;
    if (n == 0)
      break;
    print_number_twos(n);
  }

  return 0;
}