Re: st: Mata: Recursive structures and pointers

[Phil Schumm <pschumm@uchicago.edu>]

On May 18, 2006, at 5:36 AM, Ulrich Kohler wrote:
> we have a rather weird problem with recursive Mata structures and  
> pointers, which is even somwhat beyond the limits of what we can  
> express in English.
<snip>

> Our main problem, however, concerns the using of pointers. The  
> pointer "last" should move along the list and always point to the  
> last element of the list (it means, it should allow to set a new  
> point at the end of the list). It does not work. How can we add a  
> new element at the end of the list?

I see a few issues with the code you posted, but I'm afraid I don't  
have time right now to deconstruct and go through it.  However, if I  
understand correctly, the following will accomplish what you are  
trying to do:


struct list_element {
    real scalar    a, b
    pointer(struct list_element scalar) scalar    next
}

struct linked_list {
    pointer(struct list_element scalar) scalar    first, last
}

void add_list_element(struct linked_list scalar l, real scalar s) {
    struct list_element scalar    e
    e.a = e.b = s
    l.last->next = &e
    l.last = &e
}

struct linked_list scalar make_list(real rowvector seq) {
    struct list_element scalar    first_element
    struct linked_list scalar     result
    real scalar    i

    first_element.a = first_element.b = seq[1]
    result.first = result.last = &first_element

    for (i=2; i<=length(seq); i++) {
        add_list_element(result, i)
    }

    return(result)
}


There are, admittedly, several rough edges here, so don't take  
exactly what I've written too seriously.  Since we can't access a  
structure's contents explicitly at the interpreter, we must write a  
little function to check that the code works:


void show_list(struct linked_list scalar l) {
    struct list_element scalar    e

    e = *l.first
    while (1) {
        printf("(%f,%f)\n", e.a, e.b)
        if (e.next) {
            e = *e.next
        }
        else {
            break
        }
    }
}


Again, pretty rough code, I'm afraid.  This function just prints the  
items in the list.  For example:


: mylist = make_list((1..3))

: show_list(mylist)
(1,1)
(2,2)
(3,3)


I hope this helps.  I'm sure Bill will provide the definitive  
response at some point.


-- Phil

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