Hello, Dart: Variables, Types and Strings

What you'll build

A small "trip budget" program: it stores costs, computes totals and per-person shares, and prints a neat summary with string interpolation.

The idea

Dart is a typed, object-oriented language from Google, best known as the language of Flutter (iOS, Android, web and desktop apps from one codebase). It also runs servers and command-line tools, and compiles to native code and to JavaScript or WebAssembly.

Every program starts at main(). Run a file with dart run main.dart; create a project with dart create my_app.

Variables:

• var name = 'Ana';: type inferred (String), can be reassigned.
• final total = 42;: assigned once (at runtime).
• const pi = 3.14159;: a compile-time constant.
• Explicit types work too: int count = 0; double price = 9.99; bool done = false; String city = 'Lisbon';

Strings use single or double quotes, '$name' and '${expression}' for interpolation, and triple quotes for multi-line text. num covers both int and double.

The code

const currency = '€';

void main() {
  final travellers = ['Ana', 'Ben', 'Cleo'];
  var nights = 4;
  final nightlyRate = 96.50;
  final flights = 3 * 129.99;

  nights += 1; // var can change; final and const can't
  final hotel = nights * nightlyRate;
  final total = hotel + flights;
  final perPerson = total / travellers.length;

  print('Trip for ${travellers.length}: ${travellers.join(', ')}');
  print('Hotel ($nights nights): $currency${hotel.toStringAsFixed(2)}');
  print('Flights: $currency${flights.toStringAsFixed(2)}');
  print('Total: $currency${total.toStringAsFixed(2)}, '
      'per person: $currency${perPerson.toStringAsFixed(2)}');

  final summary = '''
Checklist:
  - passports
  - ${nights} nights booked''';
  print(summary);

  print('types: ${nights.runtimeType}, ${total.runtimeType}, ${travellers.runtimeType}');
  print('int division: ${7 ~/ 2}, remainder: ${7 % 2}, double division: ${7 / 2}');
}

Trip for 3: Ana, Ben, Cleo
Hotel (5 nights): €482.50
Flights: €389.97
Total: €872.47, per person: €290.82
Checklist:
  - passports
  - 5 nights booked
types: int, double, List<String>
int division: 3, remainder: 1, double division: 3.5

Common mistakes

• Using var for everything. Prefer final for values that don't change; it documents intent and prevents bugs.
• const vs final. const needs a value known at compile time; final can be computed at runtime.
• Integer division surprise. / always returns a double in Dart; use ~/ for integer division.
• Forgetting braces in interpolation. '$user.name' interpolates user then adds ".name"; write '${user.name}'.

Key points

• main() is the entry point; dart run executes a file or project.
• var (reassignable), final (set once), const (compile-time constant).
• Core types: int, double, num, String, bool, List, Map, Set.
• $name and ${expr} interpolation; ''' for multi-line strings.

Further reading: Dart language tour: variables, Built-in types.