Lists linking

Help with Snap!
1
  1. What browsers show this problem?
    Google Chrome
  2. Please share an example project (if possible).
    https://snap.berkeley.edu/project?user=biran4454&project=Script%20bug
  3. Describes the steps to reproduce this issue.
    untitled%20script%20pic
  4. What does Snap! currently do?
    Adds ['i'] to both lists.
  5. What should Snap! do instead?
    Only add ['i'] to (a) list
2

Hi @biran4454,

This is not a bug, it's the behavior of Snap! lists. Lists are mutable data structures. A list maintains its identity even when items are added, removed, or replaced.

So, when you do "Set (b) to (a)" and "a" is a list, you are not copying static a content, you are pointing to the same list element.

If you want to copy a "list content" you can use some "list reporters". For example:
copyingLists

That's all,

Joan

3

Actually, it's not just about lists, it's about how variables work in general.

It'll probably be more obvious to you with this example:

Untitled%20script%20pic(1)

If Snap! did what you suggest, then we'd end up with two copies of the sprite, which clearly doesn't make sense.

A variable is not a box where you put a unique thing, but just a name or a label you give to a value. The same value can be labelled hundreds of times, and it will always remain separate from the label.

4

A and B reference the same list.
See the Snap help dialog for "is identical to" (Operators) for more information.

5

OK, thanks everyone! That information will help a lot in future coding. Thanks!
P.S. what do I label this if it's not a bug?

6

You're welcome.

7

Hmm. I guess it could either go to Help with Snap! or Computer Science.