TCL Array: Data Not Persistent?

Question

I'm going through a crash course in TCL and I'm running into a problem with arrays in TCL. I have two classes, say A and B. In class B I have a method that updates a local array. The format for the array is something like this:

filterData(1) = 23904890234009
filterData(2) = 28974501002990
filterData(3) = 69398018930453

... and it stops there. Only 3 indices. In class A, I instantiate a B object and run a method on it to update the local array. The method inside class B looks like this:

method addData {} {
    lappend filterData($type) $data
}

The $type variable is a number 1-3, and the $data variable is a string of numbers. Whenever I run this method and print the arrays contents, it has nothing in it, like it's a new array. What's strange is I have other local variables (lists, strings) in class B that I do the same operation to that are persistent unlike this array which seems to be resetting itself. Any ideas as to how I may be handling this incorrectly? If more info is needed, I can provide.

The use of `method` suggests that this is one of the object-oriented extensions of Tcl. Regular Tcl uses dynamic scope which means that a variable inside a `proc` is not the same as a variable with the same name outside it. To be able to tell what's what for you we need to know what kind of OO extension you use. — Peter Lewerin, Aug 12 '14 at 19:03
@Hoodiecrow The packages included at the top of the code are Tk, Itcl, img::bmp, udp, and configfile. Does that answer your question? I believe Itcl is what you're interested in. — Scott James Walter, Aug 12 '14 at 19:16
Yes, Itcl is the relevant package. Unfortunately I'm completely clueless about incr tcl, so someone else will have to help you. — Peter Lewerin, Aug 12 '14 at 20:23

score 2 · Answer 1 · answered Aug 13 '14 at 08:45

In Itcl, the behaviour with this sort of thing depends critically on your variable declarations. Here's an example with a simple variable declaration

% package req Itcl 4
4.0b7
% itcl::class Foo {
    variable filterData
    method addData {type data} {
        lappend filterData($type) $data
    }
}
% Foo a
a
% a addData 1 2
2
% a addData 1 3
2 3
% Foo b
b
% b addData 1 4
4
% a addData 1 5
2 3 5

Notice how a and b don't share their filterData array? It's an instance variable. For a class variable that is shared between many instances, you instead use common to declare them:

% package req Itcl 4
4.0b7
% itcl::class Foo {
    common filterData
    method addData {type data} {
        lappend filterData($type) $data
    }
}
% Foo a
a
% a addData 1 2
2
% a addData 1 3
2 3
% Foo b
b
% b addData 1 4
2 3 4
% a addData 1 5
2 3 4 5

See how I changed one word in the declaration and got the shared variable (shared array really)?

Interesting. That begs the question then that why would someone need to declare two different variables if they share the same data? — Scott James Walter, Aug 13 '14 at 15:32

score 1 · Accepted Answer · answered Aug 13 '14 at 15:35

I ended up figuring out the problem yesterday. The problem was with my array declaration. Before I had:

array set filterData {}

...and only that at the top of my code. I then changed it to:

variable filterData
array set filterData {}

and the variable was then saving to the class object I had instantiated of it on subsequent calls to methods belonging to that class. It was a silly mistake on my part.

TCL Array: Data Not Persistent?

2 Answers2