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Re: st: Looping across observations (forwards and backwards)


From   Pedro Nakashima <nakashimapedro@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Looping across observations (forwards and backwards)
Date   Tue, 8 Nov 2011 19:47:18 -0200

Thanks Nick, but it didn't work.

Below I put a larger sample , a code that worked (for this small small
sample) and, at the end, a description of what I want to do.

clear
input v269 v270 v271 ordem novaordem sinal
         1        1986          10          96         -96           .
         1        1988          50         148        -148           .
         1        1986         100         187        -187           .
         1        1986         100         513        -513           .
         1        1985          20         743        -743           .
         1        1985          40         944        -944           .
         1        1985          40         945        -945           .
         1        1988         100         954        -954           .
         2        1985          40         966        -966           1
         1        1986          40         971        -971           .
         1        1986          40         992        -992           .
         2        1985          20        1001       -1001           1
         0        1985          20        1019       -1019           .
         2        1985          20        1026       -1026          -1
         0        1985          40        1032       -1032           .
         1        1986         100        1034       -1034           .
         0        1985          40        1035       -1035           .
         0        1985          40        1045       -1045           .
         2        1986          10        1053       -1053           1
         0        1986          40        1054       -1054           .
         2        1986         100        1056       -1056           1
         2        1986          40        1062       -1062          -1
         2        1985          20        1064       -1064          -1
         2        1985          40        1065       -1065          -1
         1        1986          45        1068       -1068           .
         2        1986          45        1070       -1070           1
         2        1986         100        1074       -1074           1
         2        1988          10        1079       -1079           0
         2        1988         100        1081       -1081           1
         2        1988          50        1088       -1088           1
         0        1988          50        1091       -1091           .
         0        1988          50        1093       -1093           .
         2        1988          70        1094       -1094           0
         0        1988          50        1098       -1098           .
         2        1988          50        1099       -1099          -1
         0        1988          10        1102       -1102           .
         2        1988          10        1103       -1103          -1
         0        1988          50        1104       -1104           .
         2        1988          10        1105       -1105          -1
         2        1988          10        1107       -1107          -1
         2        1988          10        1110       -1110          -1
         0        1988          50        1113       -1113           .
         2        1988          50        1115       -1115          -1
         2        1988          10        1116       -1116          -1
         2        1988          10        1118       -1118          -1
         0        1988          10        1119       -1119           .
         2        1988          10        1120       -1120          -1
         0        1986          40        1124       -1124           .
         2        1986          10        1127       -1127           1
         2        1986          10        1131       -1131           1
         2        1986          10        1135       -1135           1
end
sort time
capture drop orde* sina*
gen ordem = _n
gen ordemnova = -_n
sort ordemnova
gen sinal2=.

forvalues i=1/`=_N' {
	if v269[`i']==2 {
		local pr = v270[`i']
		local qt = v271[`i']
		local j=`i'+1
		while ((v269[`j']==2) | (v270[`j']!=`pr' | v271[`j']!=`qt')) & (`j'<=`=_N') {
			local ++j
		}
		if v269[`j']==0 {
			local ordem = -1
		}
		else if v269[`j']==1 {
			local ordem = 1
		}
		else {
			 local ordem = 0
		}
		quietly replace sinal2 = `ordem' in `i'
	}
}
sort ordem

Description:
1) The variable "sinal2" replicates de desired "sinal"
2) The first entry of v269 in which v269==2 has the pair v270=185 e v271=40.
I want to put one of the 3 numbers (-1, 1 or 1) in the variable "sinal".
What decides which one is the entry in v269 in other observation: the
one that has the same values (v270==185 and v271==40).
3) To do that, I search backwards(in observations) for the pair
v270==185 and v271==40, skiping observations that, even though they
have the same pair v270, v271, have also v269==2. To conclude, I want
to see the first observation that I find when looking backwards,
starting from a observation in which v269==2, that have either v269==0
or v269==1
4) For the first case in which v269==2 occurs, the looping go
backwards 2 observations (2 observations before we have v269==1,
v270==185 and v271==40). Seeing this v269==1, I store the value +1 in
the local macro "ordem" and then put it in variable sinal.
     For the second case in which v269==2 occurs, the looping go
backwards 7 observations .
     For the third case, the looping go backwards 2 observations.
And so on..

The problem is that when running this code in a dta-file that has
920,000 lines, time goes by and it seems the task will never end. And
I think it's not normal.

I wonder if a code without loopings, as you did first, would be able
to do what I described, given that It's  perfect possible 1) that we
can have consecutive observations v269==2 and, 2) the number of times
the macro j is increased can overlap among v269==2 observations.

I would thank if one could think with me of this problem. Also it
might be usefull for other people..

Best,
Pedro.

2011/10/4 Nick Cox <njcoxstata@gmail.com>:
> I have looked at this again. I am still not sure what you are trying
> to do here, but this reproduces your first example:
>
> 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 long order = _n
> gen start = v_269 == 2
> gen block = sum(start)
> bysort block (order) : ///
>        gen match = sum(v_270 == v_270[1] | v_271 == v_271[1])
> by block : ///
>        replace match = sum(cond(inlist(v_269, 1, 0), v_269  * (match == 1),.))
> by block : replace match = match[_N]
> by block : gen sinalt = cond(match == 1, 1, cond(match == 0, -1, .)) if block
>
>
>
>
> On Tue, Oct 4, 2011 at 3:32 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
>> 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[5]=1.5 and qt[5]=100. Once that observation was found, go backwards
>>>>> through observations looking for the first observation j in which
>>>>> pr[j]==pr[5] & qt[j]==qt[5]. In the example, j=2.
>>>>> 3) Replace sinalt[5]=`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:
>>>>> in not found
>>>>> r(111);
>>>>
>>>
>>
>> *
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>>
>
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