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


From   "Michael Blasnik" <michael.blasnik@verizon.net>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Re: Seeking explanation of the internal procedures of -replace-
Date   Thu, 24 May 2007 07:27:08 -0400

...
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 Kjr 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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