CODEWITHSUNDEEP
dart
nham
nham

Reputation: 175

How do I use the result of map() on a list in Dart?

I have the following code:

scaleV (xs, c) => [c * xs[0], c * xs[1], c * xs[2]];

double phi = (1 + sqrt(5))/2;
List<List<double>> v = [[ 0.0, -1.0, phi],
                        [-phi,  0.0, 1.0],
                        [-1.0,  phi, 0.0],
                        [ 1.0,  phi, 0.0],
                        [ phi,  0.0, 1.0]];

To negate each vector, I would expect I could use map(), which is a common functional construct:

var w = v.map((x) => scaleV(x, -1.0));

w[0], w[1], etc. are apparently not directly usable. In the browser console I get "TypeError: w.$index is not a function" when trying to use w[0]. Also "w is List" is false.

I expect this has something to do with the documentation for the map() method:

Returns a lazy Iterable where each element e of this is replaced by the result of f(e).

But I'm not completely sure I understand this. Using List.from on the above doesn't seem to work either.

How do I use map() ?

Upvotes: 0

Views: 2037

Answers (1)

Fox32
Fox32

Reputation: 13560

As you mentioned, map() returns an iterable. You can iterate over it, for example using forEach, but you can not access elements by their index. You can call toList to create a list of it. 

var w = v.map((x) => scaleV(x, -1.0)).toList();

assert(w is List);

But it should also work with the List.from constructor. I'm not sure why isn't working, do you get an exception?

var w = new List.from(v.map((x) => scaleV(x, -1.0)));

assert(w is List);

Upvotes: 1

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