Re: how to use recursion to find end nodes of a tree

Thankyou very much Yasir and Ross for your help and advice.
I have created a pl/pgsql version of Yasirs algorithm which
works perfectly, I am also looking into improving efficiency
by flaging leaf records. Here is my pl/pgsql solution in case
it helps anyone out:

CREATE OR REPLACE FUNCTION parentchildtest(int4)
RETURNS _int4 AS
'DECLARE
node ALIAS FOR $1;
s INTEGER[];
leaves INTEGER[];
top INTEGER;
counter INTEGER;
leaf_id INTEGER;
popped INTEGER;
child RECORD;
childCount RECORD;
BEGIN
leaf_id := 0;
top := 0;
s := ''{}'';
leaves := ''{}'';
s[top] := node;
counter := 1;


-- t a depth first search
WHILE (counter <> 0) LOOP
popped := s[top];
top := top - 1;
counter := counter - 1;

FOR child IN SELECT pc.child_id FROM parent_child AS pc
WHERE pc.parent_id = popped
LOOP
SELECT INTO childCount COUNT(*) AS count FROM parent_child AS pc
WHERE pc.parent_id = child.child_id;

--a count of zero indicates that child node has no children
IF (childCount.count = 0) THEN
leaves[leaf_id] = child.child_id;
leaf_id := leaf_id + 1;
ELSE
-- not a leaf, so add it to the stack for the next time
through
-- the loop
top := top + 1;
s[top] = child.child_id;
counter := counter + 1;
END IF;
END LOOP;
END LOOP;

RETURN leaves;
END;'
LANGUAGE 'plpgsql' VOLATILE;

-----Original Message-----
From: Yasir Malik [mailto:ymalik(at)cs(dot)stevens(dot)edu]
Sent: 10 April 2006 17:13
To: mike(at)mikeandems(dot)com
Cc: pgsql-sql(at)postgresql(dot)org
Subject: Re: [SQL] how to use recursion to find end nodes of a tree

> Hello All,
>
> I have been having a really hard time trying to come up with a
> pl/pgsql recursive function to returns the end nodes of a tree. Here
> is an example table definition:
>
> CREATE TABLE parent_child (
> parent_id integer NOT NULL,
> child_id integer NOT NULL
> );
>
> INSERT INTO parent_child (parent_id, child_id) VALUES (1, 2); INSERT
> INTO parent_child (parent_id, child_id) VALUES (1, 3); INSERT INTO
> parent_child (parent_id, child_id) VALUES (1, 4); INSERT INTO
> parent_child (parent_id, child_id) VALUES (2, 5); INSERT INTO
> parent_child (parent_id, child_id) VALUES (2, 6); INSERT INTO
> parent_child (parent_id, child_id) VALUES (4, 7); INSERT INTO
> parent_child (parent_id, child_id) VALUES (4, 8); INSERT INTO
> parent_child (parent_id, child_id) VALUES (4, 9); INSERT INTO
> parent_child (parent_id, child_id) VALUES (9, 10);
>
> This produces the following tree of data:
>
> 1
> ___|___
> | | |
> 2 3 4
> _|_ _|_
> | | | | |
> 5 6 7 8 9
> |
> 10
>
> I want to create a function that returns the terminating nodes of of
> this tree below a certain level i.e. if I input 1 to the function I
> need it to return 5,6,3,7,8,10. If I input 4 to the function I would
> get 7,8,10. I have written recursive functions which return all nodes
> on a branch of a tree but I can't think of a way to return the end
> nodes does anyone know of a solution?
>
I haven't programmed in PL/pgSQL in a while, but I'll write some pseudo
code. I think the code should be similar:

func(int node)
{
dynamic_array s;
dynamic_array leaves;
int top, count, leaf_id, popped, child;

leaf_id = top = 0;
s[top] = node;
count = 1;

// to a depth first search
while(count != 0)
{
popped = s[top];
top--;
count--;

foreach(select pc.child_id into child from parent_child pc where
pc.parent_id = popped)
{
select * from parect_child pc where parent_id = child;

// a count of zero indicates that child node has no children
if(count_of_above_query = 0)
{
leaves[leaf_id] = child;
leaf_id++;
}
else
{
// not a leaf, so add it to the stack for the next time through
// the loop
top++;
s[top] = child;
count++;
}
}
}

return leaves;
}

Regards,
Yasir