Collections

Maps

Insert and Lookup

A map literal {key: value, ...} builds a key-value store. Assigning to a missing key inserts it.

Program

Play the program to insert one entry and read another.

maps.dart
Replay: real traced execution (multi-file project)
void main() {
  var scores = {'Ada': 9, 'Lin': 12};
  scores['Mia'] = 6;
  print(scores['Lin']);
}

  1. scores ← {Ada: 9, Lin: 12}

    1void main() {2  var scores = {'Ada': 9, 'Lin': 12};3  scores['Mia'] = 6;
    values this step{Ada: 9, Lin: 12}scores

  2. scores ← {Ada: 9, Lin: 12, Mia: 6}

    2var scores = {'Ada': 9, 'Lin': 12};3scores['Mia'] = 6;4print(scores['Lin']);
    values this step{Ada: 9, Lin: 12} → {Ada: 9, Lin: 12, Mia: 6}scores

  3. print(scores['Lin']);

    3  scores['Mia'] = 6;4  print(scores['Lin']);5}
    output12
    values this step{Ada: 9, Lin: 12, Mia: 6}scores

Insert, Then Lookup

  1. Start with scores for Ada and Lin.
  2. Assigning scores['Mia'] = 6 inserts a new key.
  3. Looking up scores['Lin'] reads the existing value.
  4. The printed value is 12. | Key | Value after insert | | --- | --- | | Ada | 9 | | Lin | 12 | | Mia | 6 |
map literal `{'Ada': 9, ...}` is a `Map<String, int>`.
insert `scores['Mia'] = 6` inserts a new entry.
lookup `scores['Lin']` reads a value by key.

Exercise: maps.dart

Insert one new score into a map, then look up and print an existing score