Re: st: reverse lookup

[Jeph Herrin <stata@spandrel.net>]

Yes, the Mata construct is the ideal. And obviously, one must have 1-1 
mapping; this I usually check by:

 sum rate if period==1
 local rate=cond(`r(min)'==`r(max)',r(min),.)

I was thinking of writing some programs to do lookups like this, since I 
have been doing so many, and thought I'd ask first for an alternative.

thanks,
Jeph

On 1/8/2013 2:27 PM, Nick Cox wrote:
> My short answer is that yes, this is awkward, but you are working with
> the most obvious way to do it in Stata. The problem is that in general
>
> ... if <condition>
>
> is not guaranteed to identify precisely one observation. It might
> yield one, or zero or more than one.
>
> In your case you need == in your code and can use
>
> su rate if period == 1, meanonly
> local value = r(min)
>
> The misnamed -meanonly- is quieter and more efficient. If the
> condition identifies precisely one observation, then clearly r(min),
> r(mean), r(max) will be identical.
>
> The problem is discussed from a different angle in
>
> SJ-6-4  dm0025  . . . . . . . . . .  Stata tip 36: Which observations? Erratum
>          . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>          Q4/06   SJ 6(4):596                              (no commands)
>          correction of example code for Stata tip 36
>
> SJ-6-3  dm0025  . . . . . . . . . . . . . .  Stata tip 36: Which observations?
>          . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
>          Q3/06   SJ 6(3):430--432                                 (no commands)
>          tip for identifying which observations satisfy some
>          specified condition
>
> Mata is not surprisingly less awkward here:
>
> : y = 1::10
>
> : x = runiform(10,1)
>
> : x , y
>                    1             2
>       +-----------------------------+
>     1 |  .5044846558             1  |
>     2 |  .0174561641             2  |
>     3 |   .680281796             3  |
>     4 |  .9221656218             4  |
>     5 |  .1094441491             5  |
>     6 |  .7122591983             6  |
>     7 |   .765775156             7  |
>     8 |  .0226029507             8  |
>     9 |  .9540165765             9  |
>    10 |  .2686450339            10  |
>       +-----------------------------+
>
> : select(x, y :== 1)
>    .5044846558
>
> Nick
>
> On Tue, Jan 8, 2013 at 7:07 PM, Jeph Herrin <stata@spandrel.net> wrote:
>
> > I've just written the same awkward code for the untoldth time, and I'm
> > thinking there must be a better way to do it.
> >
> > The problem is to get a particular value of a variable into a local which
> > corresponds to a particular value of another variable. I think this is
> > usally call reverse lookup. For example, I might have -period- and -rate-
> > and want to store the value of -rate- which corresponds to period = 1. My
> > lazy solution is
> >
> >
> >   sum rate if period = 1
> >   local rate1 `=r(mean)'
> >
> > That is, I summarize a single observation, then put the mean in local. Is
> > there a better way to do this?
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