Change.dart

import 'dart:io';

void main() {
//  printSameFormat(getBills(10.00, 90.00));
  while (true) {
    print("Enter price: ");
    double price = double.tryParse(stdin.readLineSync());
    print("Enter tender: ");
    double tender = double.tryParse(stdin.readLineSync());
    try {
      printSameFormat(getBills(price, tender));
    } catch (e) {
      print(e.toString());
    }
  }
}

/// The available bills formatted the same way as the price and the tender.
const List<double> bills = [100.00, 50.00, 20.00, 10.00, 5.00, 1.00, 0.25, 0.10, 0.05, 0.01];

/// Returns the left over bills from a price and tender.
///
/// Throws a [Exception] if price or tender is null
/// Throws a [Exception] if price is greater than tender.
Map<double, int> getBills(double price, double tender) {
  if (price == null || tender == null) {
    throw ("Invalid price/tender, please try again");
  }
  double cashBack = tender - price;
  if (cashBack < 0) {
    throw ("Thats not enough money! I still need \$" + (cashBack * -1).toStringAsFixed(2) + " more!");
  }
  Map<double, int> ret = new Map();
  bills.forEach((bill) {
    int amount;
    if ((amount = (cashBack ~/ bill)) > 0) {
      cashBack = cashBack % bill;
    }
    ret.putIfAbsent(bill, () {
      return amount;
    });
  });
  return ret;
}

/// Prints the bills and their amounts following the challenges format.
void printSameFormat(Map<double, int> billAmounts) {
  billAmounts.forEach((bill, amount) {
    if (amount == 1) {
      print("$amount bill of \$$bill.");
    } else if (amount > 1) {
      print("$amount bills of \$$bill.");
    }
  });
}