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Re: st: Mata: dropping structures from memory

From (William Gould, Stata)
Subject   Re: st: Mata: dropping structures from memory
Date   Tue, 13 Jun 2006 12:15:53 -0500

Zurab Sajaia <> writes, 

> I have a recursive problem involving linked list of structures in Mata, but
> after some number of chains (number is not fixed, that's why I decided to
> use list instead of a matrix) I don't need an earlier members of the list
> and they can be dropped to free some memory. If I don't drop them, my list
> is getting too long and even makes Stata to crash.
> I was wondering if there was some way of clearing structures like other
> objects in Stata.

I assume Zurab is using structure pointers.  If not, I do not understand, 
and Zurab will have to provide more information.

So let's imagine we have a structure like this:

        struct node {
                real scalar            info
                pointer(struct node)   left
                pointer(struct node)   right

-struct node- is useful for creating binary trees.

So now imagine I'm inside a program, and variable -x- is a -struct node-.
Further assume that variable x is in the middle of the tree, so that 
x.left != NULL and x.right != NULL.  

Assume x.right points to a large branch of the tree.  I now want to clear that
branch, and free all the nested structures.  Just to be clear, *(x.right) is a
structure, and (x.right)->left might be another structure, and
(x.right)->right might be another, and if so, ((x.right)->left))->left might
be another structure, and ((x.right)->left))->right, and so on, and so on.

To chop of the tree at x.right, and free all memory associated with the 
branch, I code

                x.right = NULL

That is all there is no it!

Memory management is handled automatically by Mata. Mata will go through 
the following logic:

        I, Mata, have been asked to assign NULL to x.right.  That is is
        NULL is really not important, and I'll deal with that later.
        First, however, I must deal with the fact that I have been asked 
        to assign something to x.right.

        x.right might already contain something.  I had better look.
        Yes, I see it does.  All right, let's drop and release the memory 
        associated with x.right.

Then Mata continues, 

        I, Mata, have been asked to release the memory of *(x.right).
        *(x.right) is a structure, so to free it, I must free all the 
        elements of *(x.right).  So I think I will tell myself to 
        free the first element of *(x.right), then free the second, 
        and so on.

Then Mata continues 

        I, mata, have been asked to free (*(x.right)).left.  
        I see that *(*(x.right).left) is a structure, so to free it, I must
        free all the elements [...]

and Mata continues like that until there is no more -- until it runs into 
pointers containing NULL.

I mention all of that because, you might think that coding 

                x.right = NULL

doesn't do very much except assign NULL to x.right.  In fact, Mata is 
contantly keeping track of what is in use and what isn't, and freeing 
anything it can.

-- Bill
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