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Re: st: RE: Re: Storing output from a loop in only one variable


From   Dani Tilley <tilleydani@yahoo.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Re: Storing output from a loop in only one variable
Date   Thu, 8 Jul 2010 12:21:17 -0700 (PDT)

Thanks so much. This is very clear and helpful.



----- Original Message ----
From: Nick Cox <n.j.cox@durham.ac.uk>
To: statalist@hsphsun2.harvard.edu
Sent: Thu, July 8, 2010 12:30:22 PM
Subject: st: RE: Re: Storing output from a loop in only one variable

A fairly general recipe is 

gen output = . 
local i = 1 

foreach ... { 
    ... 
    replace output = ... in `i++' 
      ... 
} 

su output 

The presumption is that you have enough observations for this to work. 

The `i++' uses the stored value of local macro i, then bumps it up. 

You asked why does 

... in t 

not work, when -t- is a numeric variable. 

-replace- expects to see a literal number or numeric range as the
argument of -in-. 

... in `=t' 

would work as a contraction of `=t[1]' but using local macros as above
is greatly preferable. It would work because the evaluation of `=t'
takes place before -replace- sees the rest of the command line. 

On loops, see for example my namesake's articles

SJ-3-2  pr0009  . . . . . . . . . . . . . Speaking Stata:  Problems with
lists
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q2/03   SJ 3(2):185--202                                 (no
commands)
        discusses ways of working through lists held in macros

SJ-2-2  pr0005  . . . . . .  Speaking Stata:  How to face lists with
fortitude
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q2/02   SJ 2(2):202--222                                 (no
commands)
        demonstrates the usefulness of for, foreach, forvalues, and
        local macros for interactive (non programming) tasks

Nick 
n.j.cox@durham.ac.uk 

Dani Tilley

Hi Eric,

Thanks for your reply. Unfortunately, this doesn't give me what I'm
looking for. 
With the correction you pointed below, I get an x variable that only has
the 
output from the first loop on ALL its entries. I'm rather looking for a
variable 
that has -a- entries (the output from each loop on each line) and -N-a-
missing 
values. 


Incidentally, could anyone tell me why I'm getting this error:

gen x=.
gen int t=2
replace x=123 in t
't' invalid obs no
r(198);

what is a valid obs no apart from actual integers plugged in? If I can
figure 
out a way to generate a variable that would not trigger this error, I
can then 
replace var=var+i and solve the problem I have.

Eric Booth 

Sorry, that should be "x", not "`x'" in the line:


> replace x = `a' if !mi(`a') & mi(`x')


From: Dani Tilley <tilleydani@yahoo.com>

I'm trying to store certain output in a new variable while I loop
through 
different categories. At the end I want to average what I've stored and
perhaps 
do some other manipulations. Right now, my code looks like this:

foreach a in {range} {
...
gen x`a'={output} in 1
...
}

su x*, mean
di r(mean)
drop x*

So I create `a' variables that have the desired output as their 1st
observations 

and N-1 missing points. I then average across these variables and drop
them. 
While this gets the job done, I feel it's very clumsy. I'm looking to
create a 
single variable, x, that will hold all `a' outputs. Something like

scalar i=1
foreach a in {range} {
...
gen x={output} in i
i++ //or i=i+1
}

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