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Dart Maps

A map is a collection that stores values under keys. A key finds its value again later.

A map of scores

The map literal sits on one line with a key and a value in each pair. Print shows both pairs in the order they were written.

void main() {
  var scores = {'Ana': 90, 'Bo': 75};
  print(scores);
}

Reading a value by key

Each print reads one value through its key. Ana gives 90 and Bo gives 75.

void main() {
  var scores = {'Ana': 90, 'Bo': 75};
  print(scores['Ana']);
  print(scores['Bo']);
}

A missing key with a fallback

The map holds no Cy key, so the first read gives null. The ?? fallback swaps that null for 0 in the second read.

void main() {
  var scores = {'Ana': 90, 'Bo': 75};
  print(scores['Cy']);
  print(scores['Cy'] ?? 0);
}

Adding and changing entries

The Cy line adds a new entry at the end of the map. The Bo line changes the value that Bo already held.

void main() {
  var scores = {'Ana': 90, 'Bo': 75};
  scores['Cy'] = 60;
  scores['Bo'] = 80;
  print(scores);
}

Example

void main() {
  var prices = {'pen': 2, 'book': 5};
  print(prices['pen']);
  prices['ruler'] = 3;
  print(prices);
  print(prices.length);
}

Key points

  • A map stores pairs of a key and a value.
  • Each value in a map sits behind its own key.
  • Use a key inside square brackets to read its value.
  • A missing key reads as null.
  • Use = with a key to add a new entry or change an old one.
  • The length of a map counts its entries.

In this example, prices starts with a pen at 2 and a book at 5. The first print reads the pen value through its key and shows 2. The next line adds a ruler at 3. The second print shows all three entries in the order they were added. The last print shows the length 3.

Note

Print shows a map with curly braces and a colon after each key.

Try it Yourself

Edit the code, then press Run.

Running is simulated · DartPad

Exercise

What does a missing key read as in a map?