Dart basics: why Dart and Flutter, setup, variables and null safety, strings, control flow, functions and collections

flutter doctor
dart create hello_dart   # a console Dart project
cd hello_dart && dart run

void main() {
  print('Habari, Kenya!');
  print('Learning Dart for Flutter apps.');
}

void main() {
  var school = 'Alliance High';     // type inferred: String
  String town = 'Kikuyu';           // explicit type
  int students = 45;
  double fee = 12500.50;
  bool paid = true;

  final createdAt = DateTime.now();       // set once at runtime, can't change
  const vat = 0.16;                       // compile-time constant

  students = students + 1;                // var/typed variables can change
  print('$school in $town has $students students, fee KSh $fee, paid: $paid');
  print('VAT on 1000 is ${1000 * vat}');
  print('Record created at ${createdAt.year}');
}

void main() {
  String name = 'Achieng';
  String? nickname;                       // may be null
  print(nickname ?? 'No nickname');       // ?? gives a default when null

  int? discount = getDiscount('VIP');
  if (discount != null) {
    print('Discount: $discount%');        // inside the check, discount is treated as int
  }

  print(nickname?.length);                // ?. returns null instead of crashing
  nickname = 'Chichi';
  print(nickname!.toUpperCase());         // ! asserts it's not null (crashes if it is: use carefully)
  print(name.length);
}

int? getDiscount(String customerType) => customerType == 'VIP' ? 10 : null;

void main() {
  var first = 'Wanjiku';
  var last = 'Kamau';
  var full = '$first $last';                      // interpolation
  print('Name: $full (${full.length} characters)');
  print(full.toUpperCase());
  print(full.contains('Kamau'));
  print(full.split(' ').first);
  print('0712345678'.replaceFirst('0', '254'));
  print('KSh ${1500.5.toStringAsFixed(2)}');
  print(int.parse('1500') + 500);
  print(int.tryParse('abc') ?? 0);                // safe conversion
  var multi = '''
Line one
Line two''';
  print(multi);
}

void main() {
  var a = 17, b = 5;
  print('${a + b} ${a - b} ${a * b} ${a / b} ${a ~/ b} ${a % b}');   // / gives double, ~/ integer division

  var mark = 72;
  String grade;
  if (mark >= 80) {
    grade = 'A';
  } else if (mark >= 65) {
    grade = 'B';
  } else if (mark >= 50) {
    grade = 'C';
  } else {
    grade = 'D/E';
  }
  print('Mark $mark -> $grade');

  var status = 'shipped';
  var label = switch (status) {             // switch expression (Dart 3)
    'pending' => 'Waiting for payment',
    'paid' || 'processing' => 'Preparing your order',
    'shipped' => 'On the way',
    _ => 'Unknown',
  };
  print(label);

  for (var i = 1; i <= 3; i++) {
    print('7 x $i = ${7 * i}');
  }

  var towns = ['Nairobi', 'Mombasa', 'Kisumu'];
  for (var town in towns) {
    print(town);
  }

  var balance = 0.0;
  var months = 0;
  while (balance < 50000) {
    balance = balance * 1.01 + 4500;
    months++;
  }
  print('Months to save 50,000: $months');
}

double vat(double amount, [double rate = 0.16]) => amount * rate;   // optional positional parameter

String greet({required String name, String title = ''}) {           // named parameters
  return title.isEmpty ? 'Hello, $name' : 'Hello, $title $name';
}

double applyAll(double price, List<double Function(double)> rules) {
  var result = price;
  for (final rule in rules) {
    result = rule(result);
  }
  return result;
}

void main() {
  print(vat(1000));
  print(vat(1000, 0.08));
  print(greet(name: 'Otieno'));
  print(greet(name: 'Akinyi', title: 'Dr.'));

  double tenPercentOff(double p) => p * 0.9;
  final addDelivery = (double p) => p + 250;        // anonymous function
  print(applyAll(5000, [tenPercentOff, addDelivery]));
}

void main() {
  // List: ordered
  var marks = <int>[67, 82, 45, 90];
  marks.add(58);
  print('Count ${marks.length}, first ${marks.first}, last ${marks.last}');
  print('Passed: ${marks.where((m) => m >= 50).toList()}');
  print('With bonus: ${marks.map((m) => m + 5).toList()}');
  print('Total: ${marks.reduce((a, b) => a + b)}');
  marks.sort();
  print('Sorted: $marks');

  // Set: unique values
  var receipts = <String>{'QJK1', 'QJK2'};
  print('Added QJK1 again? ${receipts.add('QJK1')}');   // false: already present

  // Map: key -> value
  var stock = <String, int>{'unga': 40, 'sugar': 25};
  stock['rice'] = 30;
  stock['unga'] = stock['unga']! - 5;
  stock.forEach((item, qty) => print('$item: $qty'));
  print('Has salt? ${stock.containsKey('salt')}');

  // Collection if/for and spread (used a lot in Flutter widget lists)
  var isAdmin = true;
  var menu = ['Home', 'Courses', if (isAdmin) 'Admin', for (var t in ['Help', 'Logout']) t];
  print(menu);
  var all = [...marks, 100];
  print(all);
}