Cacher is the code snippet organizer for pro developers

/*
  There are 1000 lockers in a high school with 1000 students.  The
  problem begins with the first student opening all 1000 lockers; next
  the second student closes lockers 2,4,6,8,10 and so on to locker 1000;
  the third student changes the state (opens lockers closed, closes
  lockers open) on lockers 3,6,9,12,15 and so on; the fourth student
  changes the state of lockers 4,8,12,16 and so on. This goes on until
  every student has had a turn.

  How many lockers will be open at the end? What is the formula?
 */
import 'dart:io';

int numberOfLockers = 1000;

// Lockers start all open because of the first student
List<String> lockers = new List.filled(numberOfLockers + 1, 'open');

main() {

  for (var student = 2; student <= numberOfLockers; student++) {

    // For the student, open the locker if closed, or close if open
    for (var locker = student; locker <= numberOfLockers; locker += student) {
      lockers[locker] = lockers[locker] == 'open' ? 'closed' : 'open';
    }

  }

  // Count of lockers remaining open
  int total = 0;

  // List the lockers that remain open
  for (var locker = 1; locker <= numberOfLockers; locker ++ ) {
    if (lockers[locker] == 'open') {
      stdout.write("${locker} ");
      total += 1;
    }
  }

  print("\nThere are ${total} lockers remaining open.");
}

// 1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 400 441 484 529 576 625 676 729 784 841 900 961 
// There are 31 lockers remaining open.