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# Re: st: reverse lookup

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: reverse lookup Date Tue, 8 Jan 2013 21:48:57 +0000

```Here's a sketch. (Also, what about string variables?)

program vallookup
version 8.2
syntax varname(numeric) [if] [in] [, local(str) scalar(str) ]

marksample touse, strok
qui count if `touse'
if r(N) == 0 error 2000

capture confirm numeric variable `varlist'

su `varlist' if `touse', meanonly
if r(min) != r(max) {
di as err "specification not satisfied by single value"
exit 498
}

di r(min)
if "`local'" != "" {
c_local `local' = r(min)
}
if "`scalar'" != "" {
scalar `scalar' = r(min)
}
end

On Tue, Jan 8, 2013 at 9:33 PM, Jeph Herrin <stata@spandrel.net> wrote:
> 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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```