How to make variables that are only visible to a custom block

[mark4sisb]

It would help if you could explain a bit more about what you are doing.

Here are some examples of how to use the things suggested in this thread:

Script variables can be used to store information to be used later in the block's definition. Here is an example:

This saves the state of the pen and the location of the sprite and restores it after running.

Block Variables:
Block variables are unique because they are shared across every time the same instance of a block is called, but not every call on that block. This is more easily demonstrated like this:

Do you see what is going on here? Each time you see the block "counter", there is a different variable for that block.
This means that block variables are not very helpful when it comes to recursion:
I just whipped up a (probably not useful) recursive algorithm using block variables:

As you can see, this reports powers of two, skipping ahead the number of specified powers (here we skip 2, 8, 32, 128, and 512):

However, because the definition of the block shares the same instance of the recursion, another script can interfere/be interfered with:

By contrast, running the same script without first running the original example provides the intended result:

Well, actually, I just realized that it doesn't give the intended result, that my block doesn't actually work, but you get the picture of the issue with block variables in recursive algorithms. I give up on trying to figure out a recursive algorithm that might want block vars.

Linked Lists:

@qw23

When you SET a variable (e.g. “var”) TO another variable (“original”) the value is copied. If you then change the value of one of the variables, the other is not affected. So far, so good:

If, however, “original” is a list, and you SET “var” TO “original”, the two variables become like one: - any change to one’s value will affect the other, as demonstrated by the following script:

The cause is that a variable to which a list is assigned does not actually hold the list’s value, but a reference to the list’s address in the computer memory. SET () TO () copies the address of the list from one variable to the other. So REPLACE ITEM (2) OF (original) WITH (thing) means: replace the 2nd item of the list whose address is kept by “original” with “thing”. And REPORT (var) means: report (the contents of) the list whose address is kept by “var”. Since both variables keep the same address, both blocks refer to the same list.

If you don’t want this (and often you don’t) you could make a full (or “deep”) copy of the original list. ID OF () creates a completely new and independent copy of the original list: