st: retry: copy value withing group

[frakt@bu.edu]

My appologies.  The first several lines of my previous e-mail were 
mysteriously deleted.  Hopefully this one is complete.

*Intro*: Here's a simple Stata task I solved years ago and do 
frequently, yet I have never been happy about how I do it.  I've always 
wondered if there was another way.

*Fact*: In my data there exists a variable, x, with the following 
property (known by construction or verified with an -assert-).  Within 
each group of observations identified by a unique id (call the variable 
id), x is missing for all but one observation.

*Operation*: I want to assign the single nonmissing value of x to all 
missing values of x within the group.  That is, when I'm done, x is 
nonmissing for all observations and constant within id.  This constant 
value of x (within id) remains unchanged for the original nonmissing 
values of x.

*My Solution*: Usually I accomplish this with something like:

bysort id: egen t = mean(x) //other -egen- commands work, like 
-max()-// replace x = t
drop t

*Comment*: This works because I choose an -egen- command that ignores 
missing values.  What displeases me about this is that when I review my 
code later, it is hard to tell what I'm really doing since my 
motivation is not to compute a mean or max, per se, but just to execute 
a sort of copy operation within group.  What would be nice is if there 
were a command named copy() (or something clear) that (1) verified the 
properties I require (see "Fact" above) and then (2) did the operation 
I want (see "Operation" above).  Even a more transparent way of doing 
(2) (and not (1)) would be an improvement.  I'm not skilled enough to 
easily write such a command.  Perhaps there is such a command or some 
other way to do what I want that is more transparent than my solution.  
Any suggestions?

Thanks.



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