st: Re: Seeking explanation of the internal procedures of -replace-

["Michael Blasnik" <michael.blasnik@verizon.net>]

...
Stata works sequentially through the variable in a replace, which allows 
functions such as sum() to accumulate a running sum and allows [_n-1] constructs 
to work properly.  I don't see why you need to use clonevar and then rename and 
drop -- you could just use a temporary variable to hold the original values and 
then replace the originals with the tempvar at the end.  I wrote an ado to do 
what you want except I didn't include an [if] option which you could probably 
add without too much work.  It's short enough to include here:

program define scramble
version 9.2
syntax [varlist]
tempvar hold order rand
gen long `order'=_n
foreach var of local varlist {
gen `rand'=uniform()
gen `hold'=`var'
sort `rand'
replace `var'=`hold'[`order']
drop `rand' `hold'
}
end


You could hard code your -if- condition by changing the last -replace- line 
with:

replace `var'=`hold'[`order'] if  indexnot(`var',"?") != 0

or you could add a couple of more lines to make the program accept an -if-

Michael Blasnik


----- Original Message ----- 
From: "Jesper Kj�r Hansen" <jkh-stata@msecon.eu>
To: <statalist@hsphsun2.harvard.edu>
Sent: Wednesday, May 23, 2007 11:31 PM
Subject: st: Seeking explanation of the internal procedures of -replace-



> I am working on a simulation procedure, which randomly permutes
> all variables in a varlist before each run (optionally permutes
> groups of variables together).
>
> Given original variable dtsim20, I would have thought that the
> the two following command sequences would yield the same result,
> because my understanding of -replace- was that it works on a
> temp copy of the variable, which will not be replaced before
> the very end of the -replace- procedure.
>
> [see the full, but very basic example at the end of this post]
>
> 1. The "correct" command sequence(?)
> . clonevar _dtsim20 = dtsim20
> . replace  _dtsim20 = dtsim20[dtrank] if tag
>
> 2. Incorrect, because of [some sort of circular ref.]?
> . clonevar wrong = dtsim20
> . replace  wrong = wrong[dtrank] if tag
>
> Question:
> Could someone please explain exactly why (2) doesn't work?
>
> I guess that I will go with (1), unless someone has a more
> clever idea? The only reason I'm asking is that I now have
> to do quite a lot of clone/replace/drop/rename in my procedure.
>
> Thanks in advance.
>
>
> /Jesper K. Hansen
<snip> 
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