map

Learn transformation with .map.

The map method

The map method transforms each element of a collection and produces a new collection. The original list does not change; the result is a new Iterable.

Example: var doubled = [1, 2, 3].map((n) => n * 2);. Here each number is doubled. The result is [2, 4, 6]. (n) => n * 2 is an arrow function that runs for each element.

map is lazy; to see the result you convert it with toList(). This method is the most common tool for reshaping data.

Transforming with map

The map method takes an arrow function. This function is called for each element and its return value is added to the new collection. You can also change the element type.

Example: var names = ['alex', 'mia'];. var upper = names.map((s) => s.toUpperCase());. Result: ['ALEX', 'MIA']. Here String elements are uppercased.

Another example: var lengths = names.map((s) => s.length);. This turns Strings into ints. map is flexible; the output type depends on the arrow function you provide.

Exercise Double the numbers

Call map on the list [1, 2, 3]. The arrow function should be (n) => n * 2. Convert the result to a list with toList() and print it.

Show example solution

void main() {
  var numbers = [1, 2, 3];
  var doubled = numbers.map((n) => n * 2);
  print(doubled.toList());
}

Test yourself

Does map modify the original list?
```
var a = [1, 2, 3];
var b = a.map((n) => n * 2);
```
- Yes, it mutates in place
- It only prints
- No, it returns a new Iterable
- It deletes the list
Show answer

No, it returns a new Iterable — map does not modify the original, it returns a new Iterable.

The map call

var numbers = [1, 2, 3];
var doubled = numbers.____((n) => n * 2);

Show answer

var numbers = [1, 2, 3];
var doubled = numbers.map((n) => n * 2);

Match the map parts

(n) => n * 2

Arrow function
.map(...)

Transform call
.toList()

Materialize result
[2, 4, 6]

Transformed result

Next lesson: where

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