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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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