st: re: loops - beginner's question

[Christopher Baum <kit.baum@bc.edu>]

<>
Maik said

I have a panel dataset of firm directors. The dataset is a result of
two sets of data. The problem is, that one dataset uses a different
director identifier than the second. However, some some overlap does
exist. I would like to create a loop which replaces the missing
identifier with the once used in the year before. The loop should
check if the idendifier is missing and if so check if the firm
identifier is identical to last year. I was usind the following code:

sort firm_id year
gen temp_id=dir_id if dir_id!=.
by cusip: replace temp_id=dir_id[_n-1] if firm_id==firm_id[_n-1] &
year==year[_n-1]+1

The code only replace the first observation but I have more than one
directors in each year. I have experimenting with the following code,
which obviously does not do the job.

Can somebody please help or direct me a bit?

while condition on `local’ true {
local i=1
while `i' =. {
 if firm_id=firm_id in year=year-1
replace `i'= dir_id in year=year-1
local i=`i'+1
}


First of all, in this latter fragment, the tests for equality in your while, if and replace statements must be ==, not =.

I find your initial block of code confusing. Is firm_id identifying directors, or firms? If the latter, why is 
there also a cusip (presumably identifying firms)? Presumably for each firm (cusip) you have multiple
directors, and for each director you have, potentially, multiple years. Thus it would seem that
the first block of code should sort by -cusip firm_id year- and the by prefix should be -by cusip firm_id-. 
Then, if there are no missing years, you should be able to update temp_id by merely saying, within
that -by-, 

replace temp_id = firm_id[_n-1] if mi(firm_id) & _n > 1

assuming that the firm_id is defined in the first year for which that firm|director combination exists, that
will propagate the firm_id forward in time.

Kit

Kit Baum   |   Boston College Economics & DIW Berlin   |   http://ideas.repec.org/e/pba1.html
                              An Introduction to Stata Programming  |   http://www.stata-press.com/books/isp.html
   An Introduction to Modern Econometrics Using Stata  |   http://www.stata-press.com/books/imeus.html


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