Fill in the gaps (within ID)

*(Q) 

Assume the data looks like this (a CH_ID that ends in 00 is actually
the parent, whose information I am trying to impute into the
children):

PAR_ID      CH_ID       VAR1     VAR2 
X001        X00100     2          1
X001        X00102     .          .
X001        X00103     .          .
X002        X00200     1          .
X002        X00201     .          .
X003        X00300     3          2
X003        X00302     .          .
X003        X00303     .          .

I would like to know if anyone has a script (or modification of the
one above) that fill in all the missings for each X00's child with
the- but NOT run into the next person.  That is, not fill in the
missing under VAR2 for X002, because that is a new person and the "1"
under VAR2 for X001 is not applicable to that person.

The corrected dataset would look like:

PAR_ID      CH_ID       VAR1     VAR2 

X001        X00100     2         1
X001        X00102     2         1
X001        X00103     2         1
X002        X00200     1         .
X002        X00201     1         .
X003        X00300     3         2
X003        X00302     3         2
X003        X00303     3         2



* (A) Posted to newsgroup by Ray on 2002/02/18.

DATA LIST LIST /PAR_ID (A4)    CH_ID(A6)    VAR1      VAR2 (2F8.0).
BEGIN DATA
X001        X00100     2          1
X001        X00102     .          .
X001        X00103     .          .
X002        X00200     1          .
X002        X00201     .          .
X003        X00300     3          2
X003        X00302     .          .
X003        X00303     .          .
END DATA.
LIST.


SET ERRORS=no.
DO IF $CASENUM>1.
DO IF par_id = LAG(par_id).
DO REPEAT v=ch_id TO var2.
* The next 2 lines are a nice trick: 
* one the 2 line results in a (non consequential) error 
* but the other gives the correct answer.
IF LEN(RTRIM(v))=0 v=LAG(v).
IF MISSING(v) v=LAG(v).
END REPEAT PRINT.
END IF.
END IF.
SET ERRORS=yes.
EXECUTE.
LIST.