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Re: st: reverse lookup


From   Jeph Herrin <[email protected]>
To   [email protected]
Subject   Re: st: reverse lookup
Date   Tue, 08 Jan 2013 17:58:57 -0500

String variables are a problem all their own. I usually do something like:

 encode strvar, gen(strvar_coded)
 sum strvar_coded if period==1
 local rate1= ///
    cond(`r(min)'==`r(max)',`=: label (strvar_coded) `=r(min)'',"")

which however can run into trouble if there are too many values to -strvar-.
	

On 1/8/2013 4:48 PM, Nick Cox wrote:
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 <[email protected]> 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 <[email protected]> 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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