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
@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
Yes, Itcl is the relevant package. Unfortunately I'm completely clueless about incr tcl, so someone else will have to help you. — Peter Lewerin

Donal Fellows Donal Fellows · Accepted Answer · 2014-08-13T08:45:20

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)?

TCL Array: Data Not Persistent?

2 Answers