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# RE: st: Looping across observations (forwards and backwards)

 From Nick Cox To "'statalist@hsphsun2.harvard.edu'" Subject RE: st: Looping across observations (forwards and backwards) Date Tue, 4 Oct 2011 15:32:48 +0100

```I don't fully understand what you are trying to do here, but

local ++j

need not stop before

v_270[`j']==v_270[`i'] | v_271[`j']==v_271[`i']

and perhaps that is not guaranteed for all values of 2.

so perhaps you need another condition to stop it, say that the next value of v_269 is 2.

I think you need another approach. Evidently blocks start with some key values and then you count something within blocks. A few fragmentary suggestions

gen start = v269 == 2
gen block = sum(start)
egen start_v269 = total(start * v269), by(block)
egen start_v270 = total(start * v270), by(block)
egen start_v271 = total(start * v271), by(block)

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

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Pedro Nakashima
Sent: 03 October 2011 20:39
To: statalist@hsphsun2.harvard.edu
Subject: Re: st: Looping across observations (forwards and backwards)

Thanks, Nick

When I applied you tip to the code:

clear all
input v_269 v_270 v_271 desired_sinalt
0 1.4 100 .
1 1.5 100 .
0 1.5 95 .
0 1.4 100 .
2 1.5 100 1
1 1.7 98 .
0 1.2 99 .
2 1.5 95 -1
0 1.8 101 .
end
gen order = _n
gen neworder=-_n
sort neworder
gen sinalt=.
set trace on
forvalues i=1/`=_N' {
if v_269[`i']==2{
local j=`i'+1
while (v_270[`j']!=v_270[`i'] | v_271[`j']!=v_271[`i']) {
local ++j
}
if v_270[`j']==v_270[`i'] | v_271[`j']==v_271[`i'] {
if v_269[`j']==1{
local sinal=1
}
else if  v_269[`j']==0 {
local sinal=-1
}
else {
local sinal=.
}
}
replace sinalt=`sinal' in `i'
}
}
set trace off
sort order

,, it worked,

But if I replace the third observation as follows:
replace v_269 = 2 in 3
replace v_271 = 100 in 3

The looping never ends..

Also, It's important to say that if the criterion matches v_269 and
v_271 in observation number 3 (where v_269==2), as in the above
example, I want to ignore it.

Thanks in advance for the help.

Best regards
Pedro Nakashima.

2011/9/24 Nick Cox <njcoxstata@gmail.com>:
> A different comment is that it is much easier to go forwards in Stata
> than backwards. So, reversing the whole dataset, and defining spells
> "started" in a certain way might be easier. When all is done you
> reverse it again.
>
> Reversing is easy
>
> gen neworder = -_n
> sort neworder
>
> On Sat, Sep 24, 2011 at 4:07 PM, Nick Cox <njcoxstata@gmail.com> wrote:
>> When your program gets to
>>
>>      replace sinalt=`sinal' in `i'
>>
>> evidently `sinal' is undefined so Stata sees
>>
>>      replace sinalt= in `i'
>>
>> It tries first to interpret -in- as the name of a variable or scalar,
>> fails, and aborts with error.
>>
>> Perhaps when you coded
>>
>>  if cod[j]==1 {
>>
>> you meant
>>
>>  if cod[`j']==1 {
>>
>> On Sat, Sep 24, 2011 at 3:28 PM, pedromfn <nakashimapedro@gmail.com> wrote:
>>
>>> My database looks like:
>>>
>>> obs cod pr qt sinalt
>>> 1 1 1.4 100 .
>>> 2 2 1.5 100 .
>>> 3 1 1.5 95 .
>>> 4 1 1.4 100 .
>>> 5 3 1.5 100 .
>>>
>>> and I want to replace observations of sinalt in which cod==3, according to
>>> the following rule:
>>> 1) Go across observations looking for observations in which cod=3
>>> 2) In the above example, the first observation is observation 5, in which
>>> pr=1.5 and qt=100. Once that observation was found, go backwards
>>> through observations looking for the first observation j in which
>>> pr[j]==pr & qt[j]==qt. In the example, j=2.
>>> 3) Replace sinalt=`sinal' , where the macro sinal is defined as:
>>>     if cod[j]==1, store in the local sinal the value 1
>>>     if cod[j]==2, store in the local sinal the value -1
>>> 4) Once last replace was done, look for the next observation in which cod==3
>>> and do the same thing.
>>>
>>> I wrote the following do-file, but it didn't work:
>>>
>>> forvalues i=1/`=_N' {
>>>        if cod[`i']==3{
>>>                local j=`i'-1
>>>                if pr[`j']==pr[`i'] & qt[`j']==qt[`i'] {
>>>                        if cod[j]==1 {
>>>                                local sinal 1
>>>                        }
>>>                        else if cod[`j']==2 {
>>>                                local sinal -1
>>>                        }
>>>                        else {
>>>                                local sinal
>>>                        }
>>>                }
>>>                else {
>>>                        while pr[`j']!=pr[`i'] | qt[`j']!=qt[`i'] {
>>>                                local --j
>>>                        }
>>>                }
>>>        replace sinalt=`sinal' in `i'
>>>        }
>>> }
>>>
>>> ERROR: