How to flatMap List in Dart?

Question

I can map list in Dart:

[1,2,3].map((e) => e + 1)

but how can I flatMap this list? Code presented below does not work.

[1,2,3].flatMap((e) => [e, e+1])

Answer 1

expand method is equivalent to flatMap in Dart.

[1,2,3].expand((e) => [e, e+1])

What is more interesting, the returned Iterable is lazy, and calls fuction for each element every time it's iterated.

Answer 2

Coming from Swift, flatMap seems to have a little different meaning than the OP needed. This is a supplemental answer.

Given the following two dimensional list:

final list = [[1], [2, 2], [3, 3, 3], [4, 4, 4, 4]];

You can convert it into a single dimensional iterable like so:

final flattened = list.expand((element) => element);
// (1, 2, 2, 3, 3, 3, 4, 4, 4, 4)

Or to a list by appending toList:

final flattened = list.expand((element) => element).toList();
// [1, 2, 2, 3, 3, 3, 4, 4, 4, 4]