st: New method to avoid looping over each observation and across a variable list

[Jeremy Page <miahpage@gmail.com>]

Hello Everybody,

I would like some advice about how to change some code that currently
loops across a long list of variables for each observation in my data
set.

My data set has one record per person and there are monthly
occurrences of variables that have a suffix of yymm (two digit year
and two digit month) to record monthly information.  I also have a and
a string variable which contains the year and month an event
(eventyymm) also given as "yymm". I would like produce a new variable
which gives the information in var0201-var0312 during the month of the
event (eventyymm).

The example below contains an example data set and my current code.
The code produces the correct result but in my actual data set I have
millions of observations and about 15 years of yymm variables to loop
over. My current method will take an extremely long time to process.

Best,
Jeremy

******begin example***********
clear all
input str5 id str4 eventyymm ///
      var0201 var0202 var0203 ///
      var0204 var0205 var0206 ///
      var0207 var0208 var0209 ///
      var0210 var0211 var0212 ///
      var0301 var0302 var0303 ///
      var0304 var0305 var0306 ///
      var0307 var0308 var0309 ///
      var0310 var0311 var0312
      A 0203 0 0 0 0 0 0 1 1 1 1 1 1 ///
                 1 1 . . . 0 0 0 0 2 2 2
      B 0301 . . . . . 0 0 0 0 0 0 0 ///
                 0 0 0 0 9 9 9 1 1 1 1 1
      C 0210 1 1 1 1 1 1 1 1 1 1 1 1 ///
                 1 1 1 3 3 3 3 3 3 3 3 3
      D 0212 0 0 0 0 0 0 . . . . . . ///
                 . 9 0 0 0 1 1 1 1 1 1 1
      E 0310 3 3 3 3 3 3 3 3 3 3 3 3 ///
                 3 3 3 3 3 3 3 3 3 3 3 3
end

***generate variable to match with variable name
gen varyymm_string = "var" + eventyymm

***generate empty variable to populate in the loop
gen var_value = .

***loop over observations
foreach i of num 1(1)5 {
   ***loop across variables
   foreach x of varlist var0201 - var0312 {
      replace var_value = `x' if varyymm_string == `"`x'"' & _n == `i'
   } /* close loop across variables */
} /* close loop over observations */

******end example***********
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