'A tour of the Dart language' page issue

[SchmadenSchmuki]

Page URL: https://dart.dev/guides/language/language-tour Page source: https://github.com/dart-lang/site-www/tree/master/src/_guides/language/language-tour.md

Description of issue:

I had the unexpected behaviour of

for (double val in doubleList)
  val *= someFactor;

not changing the values of the entries in my doubleList. After that I read on stackoverflow that primitives will be passed by value.

Maybe this could be mentioned in the language tour, especially since

Everything you can place in a variable is an object, and every object is an instance of a class. Even numbers, functions, and null are objects. All objects inherit from the Object class.

gave me the impression val will be a reference (I still don't know what is considered a primitive in dart). Maybe I didn't read the tour thoroughly enough, I surely didn't learn dart thoroughly enough, just wanted to tell you my experience, not being sure if this is the right place.

Anyway, that's all I have to say about it, so this issue can be closed or deleted (duplicate, not right info for language tour, already written in the language tour).

[kwalrath]

Thanks for the feedback. I'll think about how/where we can make this clearer.

[kwalrath]

https://stackoverflow.com/questions/22210247/list-foreach-unable-to-modify-element seems to have good answers related to this. Basically, the variable (val above) doesn't actually refer to the item in the list; it just temporarily has the same value as the item.

To apply one of lrn's explanations to your example, when you say val *= someFactor, it's like saying this:

var z = val;
z *= someFactor;

To actually change list values, you'd use a method like map(), which returns a new (lazy) Iterable. The Iterables codelab shows how to use map().

All this isn't obvious, so I definitely want to make it clearer in the language tour.