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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