st: RE: RE: RE: Weights vs subscripts

 From "Nick Cox" To 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
>
>
> 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
> 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
> of subscripts
> ------------------------------
>
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```