Map iteration
- Dart course
- Beginner
- Lesson 63/116
- 5 min
Learn to iterate over Maps with for-in and forEach.
Iterating a Map
Several ways exist to walk a Map. The first is forEach: ages.forEach((k, v) => print('$k: $v'));. It yields the key and the value in order.
The second is for-in over entries: for (var e in ages.entries) { print(e.key); }. Each e is a MapEntry with .key and .value fields.
To walk only keys or values, use ages.keys or ages.values. A Map keeps the insertion order, so the iteration order is predictable.
entries and keys
entries yields each pair as a MapEntry with .key and .value fields. keys returns only the keys, values only the values.
For example for (var k in ages.keys) print(k); writes each key on a new line. for (var v in ages.values) print(v); writes the values.
When the order of work matters, entries is handier; if you only care about one side, keys or values is a shortcut.
Practice Print the ages
Create a Map<String, int> with two people. Use forEach to print each pair as $key: $value.
Starter code
void main() {
var ages = <String, int>{};
// add two pairs
// forEach to print
}Sample solution
void main() {
var ages = <String, int>{};
ages['Ayşe'] = 28;
ages['Mert'] = 31;
ages.forEach((k, v) => print('$k: $v'));
}Expected output
Ayşe: 28 Mert: 31
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the iteration ways
| Term | Meaning |
|---|---|
forEach |
Callback per pair |
entries |
List of MapEntry |
keys |
Only keys |
values |
Only values |
2 What is the forEach signature?
- (k, v) => ... Correct answer
- (k) => ...
- (v) => ...
- (e) => ...
Explanation: Map.forEach takes two parameters: key and value.
3 The for-in loop
void main() {
var ages = <String, int>{'Ayşe': 28};
for (var e in ages.____) {
print(e.key);
}
}void main() {
var ages = <String, int>{'Ayşe': 28};
for (var e in ages.entries) {
print(e.key);
}
}Explanation: entries yields each pair as a MapEntry.