Lesson 12 · 12 of 20

Maps · Dart

Store key-value pairs in a Dart Map: a café menu with [], ??, remove, containsKey, entries and a count of orders.

Dart · Maps in DMarlon Studio: the example's code and its output in the terminal

A map (Map) links keys to values, between braces: {'coffee': 1.40, 'tea': 1.20}. Map<String, double> says the keys are strings and the values numbers with decimals. You look things up by key: menu['coffee'] gives 1.4 (Dart does not print the trailing zero of a double).

Changing a value and adding a new key look the same: menu['tea'] = 1.30 updates the tea and menu['juice'] = 2.50 adds the juice at the end.

If the key does not exist, menu['cake'] gives `null`, not an error. `??` puts a default value in its place: menu['cake'] ?? 0.0. `remove()` deletes a key with its value, `containsKey()` asks whether it exists, and `keys` and `values` give the keys and the values.

`entries` gives each pair as an object with key and value, and the for-in goes through them. padRight(8) pads the name with spaces up to 8 characters so the prices line up in a column.

A very common use is counting: for each drink ordered, counts[drink] = (counts[drink] ?? 0) + 1 adds one to what was there, or starts from zero the first time. <String, int>{} is an empty map with its types.

Code · main.dart

void main() {  // A map links keys to values: {key: value}.  Map<String, double> menu = {'coffee': 1.40, 'tea': 1.20, 'toast': 2.10};  print('A coffee costs ${menu['coffee']} euros');  // map[key] = value changes a value or adds a new key.  menu['tea'] = 1.30;  menu['juice'] = 2.50;  print(menu);  // A missing key gives null; ?? puts a default in its place.  print('Cake: ${menu['cake']}, or rather ${menu['cake'] ?? 0.0}');  // remove deletes a key with its value.  menu.remove('toast');  print('Toast? ${menu.containsKey('toast')}. Keys: ${menu.keys.toList()}');  // entries gives each key together with its value.  for (var item in menu.entries) {    print('${item.key.padRight(8)}${item.value.toStringAsFixed(2)}');  }  // Counting with a map: one key per drink.  var orders = ['coffee', 'tea', 'coffee', 'juice', 'coffee', 'tea'];  var counts = <String, int>{};  for (var drink in orders) {    counts[drink] = (counts[drink] ?? 0) + 1;  }  print('Orders: $counts');  print('Different drinks: ${counts.length}');}

Output · dart run main.dart

A coffee costs 1.4 euros
{coffee: 1.4, tea: 1.3, toast: 2.1, juice: 2.5}
Cake: null, or rather 0.0
Toast? false. Keys: [coffee, tea, juice]
coffee  1.40
tea     1.30
juice   2.50
Orders: {coffee: 3, tea: 2, juice: 1}
Different drinks: 3

Key points

A map goes between { } with key: value pairs.
m[key] = value changes or adds.
A missing key gives null; ?? supplies a default.
for (var e in m.entries) goes through e.key and e.value.
containsKey checks whether the key exists.

Exercise

Add a map with the units sold of each drink and print how much each one took in (units times price) and the total for the day.