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st: RE: RE: RE: Weights vs subscripts


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: RE: Weights vs subscripts
Date   Wed, 4 Oct 2006 17:05:34 +0100

replace X = 5 in 5 if y == z 

Nick 
n.j.cox@durham.ac.uk 

> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu]On Behalf Of Fan, Shihe
> Sent: 04 October 2006 16:41
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: RE: Weights vs subscripts
> 
> 
> Thanks for your and others' reply. Being a new user, it is always good
> to learn the subtle details of the program. Now suppose that I have a
> vector variable, and want to replace the value of some 
> elements based on
> certain conditions, how should the command be given. Say:
> 
> X[5] = 5 if y == z
> 
> Thank you
> 
> Shihe
>   
> 
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
> Sent: Wednesday, October 04, 2006 3:35 AM
> To: statalist@hsphsun2.harvard.edu
> Subject: st: RE: Weights vs subscripts
> 
> You pretty well answered your own question. 
> 
> Stata permits weights in some commands, although not in -replace-. 
> 
> Or rather Stata permits subscripts in -replace-, by virtue of their
> being part of an expression, appearing to the RHS of an = sign. In an
> example like 
> 
> . gen d = c
> 
> . replace d[1] = 1
> weights not allowed
> r(101);
> 
> Stata's reasoning is, or is equivalent to, 
> 
> (a) "replace d" I understand. 
> 
> (b) but what is that "[1]"? 
> 
> (c) it is to the left of the = sign, so it is _not_ part of the
> expression 
> 
> (d) so the user must intend it as some specification of weights
> 
> (e) but weights are not allowed in -replace-. 
> 
> There remains the question of what you are trying to do. I think Alex
> and Michael misread your question, as your code is looping over a
> varlist. 
> 
> foreach i of varlist x {
> 	replace y=y[`i'-1]+ z[`i']
> }
> 
> As the varlist contains a single variable, the loop is redundant here,
> so this boils down to 
> 
> replace y = y[x - 1] + z[x] 
> 
> The implication is that x contains
> observation numbers, in effect pointers. Is that right? 
> It is a way of getting some subtle effects, or you might be confused. 
> 
> Nick
> n.j.cox@durham.ac.uk 
> 
> Michael Blasnik
> -------------------------------
> Actually, the better (faster) way to do this is 
> 
> replace y=y[`i'-1] +z[`i'] in `i'
> -------------------------------
> 
> Alex Ogan
> -------------------------------
> Somebody more knowledgeable will probably explain exactly why 
> you can't
> use subscripts on the left side like that.
> 
> But I can suggest a workaround:
> 
> replace y=y[`i'-1] +z[`i'] if _n==`i'
> ------------------------------
> 
> 
> Shihe Fan
> ------------------------------
> Could any one explain to me why it is OK to write the code in the
> following way
>  
> gen y=0
>  
> foreach i of varlist x {
> 	replace y=y[`i'-1]+ z[`i']
> }
>  
> but not OK in this way
> 
> replace y[`i']=y[`i'-1] +z[`i']
>  
> the program always treat the [`i'] on the left side as 
> weights, instead
> of subscripts
> ------------------------------
> 
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