st: Replace current values matching certain condition using values from other observations?

[Chris Yang <bhpchg@gmail.com>]

Dear All,

I have a dataset that resembles the following structure:

group seq var1 var2 var3
1       1     3      2     3
1       2     1      1     2
1       3     2      2     4
2       1     3      2     1
2       2     3      3     3
3       1     3      2     1
...

Now, within each group, for all the observations whose seq > 1, I want
to replace the values for var1 and var2 with those of the observation
whose seq == 1. For example, for group 1 above, after the
replacements, it would look like this:

group seq var1 var2 var3
1       1     3     2      3
1       2     3     2      2
1       3     3     2      4

If there is no seq > 1 in a given group, then no replacement is needed.

Intuitively, it seems that I need a looping structure to go through
all the observations one by one. And at each step i will check the seq
variable. If it is greater than 1, then look up the values for var1
and var2 from the observation *within the same group* whose seq == 1,
and use them to update the current observation. The question is that
how do I do such look-ups in a loop?

As always, is there a better/more efficient way of doing it? Your
thoughts and suggestions are appreciated.

Best regards,

Chris
*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/faqs/resources/statalist-faq/
*   http://www.ats.ucla.edu/stat/stata/