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st: Bug in -use- or -if- ?


From   Sergiy Radyakin <serjradyakin@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   st: Bug in -use- or -if- ?
Date   Wed, 4 Feb 2009 18:20:43 -0500

Dear All,

in a different thread Dan Blanchette asked about cooperation of -in-
and -if-. I have asked myself a slightly different question whether
specifying if-conditions can always substitute for in-conditions: e.g.
 instead of "in #A/#B" one can type "if inrange(_n,#A,#B)".

There seems to be a bug in -use- that get's confused by such a
condition. My colleague has suggested that this might happen because
Stata will qualify _n according to the current dataset in memory, but
qualify if- for the dataset during the load. I was able to come up
with an example where it get's confused unconditionally on the current
dataset. It seems that the conditon "larger" is not evaluated properly
in this case.

	*** bug with use ... if F(_n)
	*** N(auto.dta)=74
	
	sysuse auto, clear
	local fullauto `r(fn)'
	
	use `"`fullauto'"' in 1/37, clear
	count
	assert (_N==37)
	
	use `"`fullauto'"' in 38/74, clear
	count
	assert (_N==37)
	
	use `"`fullauto'"' if _n<=37, clear
	count
	assert (_N==37)
	
	use `"`fullauto'"' if _n>37, clear
	count
	assert (_N==37)

It is hard to understand what Stata will think of _n while loading
data, but it is definitely not the observation number.
Strangely the condition inrange(_n,1,20) loads 20 (twenty)
observations, but inrange(_n,2,20) loads 0 (zero).

So if you ever try to work with large datasets in smaller portions,
slice them with an in-condition, not an if-condition!

Stata MP for Windows, v10.1.551 born 02 Feb 2009, (currently latest.
This recent update brings some very welcomed changes: thank you!)

Best regards, Sergiy Radyakin
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