Monday, January 24, 2011

STATA: Appending in loops

Say you have a loop and you want to add the results of the previous iteration on the end of one data file.

local PBFs "David1 David2 David3"
tempfile PBFdata
foreach PBF of local PBFs {
set obs 5
gen currentPBF=`PBF'
append using `PBFdata'
save `PBFdata', replace

This is all well and good, except that code above doesn't work. It fails the first time you run the loop, because there's no `PBFdata' file at that point, only the local pointing to an empty location.

What to do? You've got some options:

-if word("`PBFs'",1) == "`PBF'" append using `PBFdata'-
// This works because it checks whether this is the first time you're running the loop or not, but who wants to type all that? Still, that word function is pretty cool, huh?

-cap append using `PBFdata'-
//This works, but if it fails for other reasons, your program will keep on going and things mess up badly.

-touch `PBFdata'-
//This is what I do. I like it so much, that I had to write the -touch- command just to make it work. Note you have to put it outside the loop (I usually put it at the very top of my do file), otherwise you'll overwrite your file each time!

To install touch, type -findit touch- to locate it in the user-contributed repositories.

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