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# RE: st: Looping Enquiry

 From George Bouliotis <[email protected]> To "[email protected]" <[email protected]> Subject RE: st: Looping Enquiry Date Thu, 29 Sep 2011 17:49:39 +0100

```Dear Robert, Nick, Valerie and Richard

Thank you for your response to my question and your fruitful comments. Yes, my problem is programming the running-product estimation.

I have already been through different approaches and ideas (e.g. egen with product function or sums) but nothing worked correctly. The "lagged" approach seems promising but the tricky point is when the (sub)products should be 1

In an attempt to make things more clear, I provide a more clear dataset here:

time   group   indic   RESULT   step1   step2   step3  step4
30       1     0    -.958      47      48   .9791667  1-2*(47/48) =-0.958
57       0     0    -.916      46      47   .9787234  1-2*((46/47)*(47/48)) =-0.916
58       0     0    -.874      45      46   .9782609  1-2*((45/46)*(46/47)*(47/48)) =-0.874
67       0     0    -.834      44      45   .9777778  1-2*((44/45)*(45/46)*(46/47)*(47/48)) =-0.834
74       0     0    -.792      43      44   .9772727  1-2*((43/44)*(44/45)*(45/46)*(46/47)*(47/48)) =-0.792
79       0     0     -.75      42      43   .9767442  1-2*((42/43)*(43/44)*(44/45)*(45/46)*(46/47)*(47/48)) =-0.750
79       1     1     .125      42      43   .9767442  1-1*((42/43)*(43/44)*(44/45)*(45/46)*(46/47)*(47/48)) =0.125
82       1     1     .125      42      43   .9767442  1-1*((42/43)*(43/44)*(44/45)*(45/46)*(46/47)*(47/48)) =0.125
89       0     0    -.708      41      42   .9761904  1-2*((41/42)*(42/43)*(43/44)*(44/45)*(45/46)*(46/47)*(47/48)) =0.708
95       1     0    -.662      40      41   .9756098  and so on...
98       0     0    -.678      39      40       .975
101       0     0    -.574      38      39    .974359
104       0     0    -.532      37      38   .9736842
110       0     0    -.448      36      37    .972973
118       0     0    -.444      35      36   .9722222

The task is to generate correctly the "step4" variable which varies conditional upon the value of the dummy variable "indic". If indic is 0 then ste4 is calculated as 1-2*(products) and if indic is 0 then step for is estimated as 1-1*(products). The variable (column) RESULT simply illustrates what the correct values should be for the variable "step4".

Thank you very much
George

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Robert Picard
Sent: 29 September 2011 17:40
To: [email protected]
Subject: Re: st: Looping Enquiry

It looks like George is stuck on calculating a running product,
similar to the sum() function. This can easily be done without a loop:

gen rp = _n
replace rp = rp * rp[_n-1] if _n > 1

Since George is trying to replicate the RESULT variable and he's not
using the time and group variables, I suspect that Richard's solution
is not what George is looking for. While I can't quite figure out
everything, here's an attempt that get close:

*----------- begin example -------------
clear
input    time   group   var  RESULT
30       1        0  -0.958
57       0        0  -0.916
58       0        0  -0.834
67       0        0  -0.874
74       0        0  -0.792
79       0        0  -0.750
79       1        1   0.125
82       1        1   0.125
89       0        0  -0.706
95       1        0  -0.662
98       0        0  -0.678
101       0        0  -0.574
104       0        0  -0.532
110       0        0  -0.448
118       0        0  -0.444
end

* setup, per OP
egen ssize=seq() if var==0,from(47) to(1)
replace ssize=ssize[_n-1] if ssize==.
gen  product= (ssize/(ssize+1))

* runing product that does not change when var == 1
gen double rprod = cond(var,1,product)
replace rprod = rprod * rprod[_n-1] if _n > 1

* replicate RESULT
gen score = 1 - cond(var,1,2) * rprod
format %9.3g score
list, clean noobs
*------------ end example --------------

On Thu, Sep 29, 2011 at 10:24 AM, Nick Cox <[email protected]> wrote:
> This looks a neat solution, assuming that Richard has correctly understood the question.
>
> I'd add a thought that no harm would be done by putting the result of -generate- into a -double-. These numbers don't look problematic, but a little worry about loss of precision would do no harm.
>
> Nick
> [email protected]
>
> Richard Herron
>
> If I understand the question, I think you can do this without a loop.
> If you sort on group and time, then you can create a sequential time
> index, use -tsset-, and use lag operators to generate your product.
> Here's my attempt, please let me know if I got your question wrong.
>
> * begin code
> clear
> input    time   group   var  RESULT
>          30       1        0  -0.958
>          57       0        0  -0.916
>          58       0        0  -0.834
>          67       0        0  -0.874
>          74       0        0  -0.792
>          79       0        0  -0.750
>          79       1        1   0.125
>          82       1        1   0.125
>          89       0        0  -0.706
>          95       1        0  -0.662
>          98       0        0  -0.678
>         101       0        0  -0.574
>         104       0        0  -0.532
>         110       0        0  -0.448
>         118       0        0  -0.444
> end
>
> bysort group (time): generate time_seq = _n
> tsset group time_seq
> by group: generate observ = RESULT * l.RESULT * l2.RESULT * l3.RESULT
> * end code
>
> which produces
>
> . list, clean
>
>       time   group   var   RESULT   time_seq     observ
>  1.     57       0     0    -.916          1          .
>  2.     58       0     0    -.834          2          .
>  3.     67       0     0    -.874          3          .
>  4.     74       0     0    -.792          4   .5288082
>  5.     79       0     0     -.75          5   .4329761
>  6.     89       0     0    -.706          6   .3665241
>  7.     98       0     0    -.678          7   .2843288
>  8.    101       0     0    -.574          8   .2060666
>  9.    104       0     0    -.532          9   .1461699
>  10.    110       0     0    -.448         10   .0927537
>  11.    118       0     0    -.444         11   .0607414
>  12.     30       1     0    -.958          1          .
>  13.     79       1     1     .125          2          .
>  14.     82       1     1     .125          3          .
>  15.     95       1     0    -.662          4   .0099093
>
> .
>
> On Thu, Sep 29, 2011 at 09:22, George Bouliotis <[email protected]> wrote:
>
>> Although an old Stata user, currently I am doing my first steps in programming.
>>
>> One of the parts in my programme tries (unsuccessfully) to replicate the column RESULT below. The difficulty is in how to loop a sequential product as, for instance: observ4= obs4 X obs3 (lag1) X obs2 (lag2)  X obs1 (lag1).
>>
>> I tried some loops with "forvalue" but none was successful. I would appreciate any help with this.
>
> [...]
>
>>
>> #####################################
>> set more off
>> clear
>> input    time   group   var  RESULT
>>           30       1        0  -0.958
>>           57       0        0  -0.916
>>           58       0        0  -0.834
>>           67       0        0  -0.874
>>           74       0        0  -0.792
>>           79       0        0  -0.750
>>           79       1        1   0.125
>>           82       1        1   0.125
>>           89       0        0  -0.706
>>           95       1        0  -0.662
>>           98       0        0  -0.678
>>          101       0        0  -0.574
>>          104       0        0  -0.532
>>          110       0        0  -0.448
>>          118       0        0  -0.444
>> end
>>
>>
>> list , clean
>>
>> //Generating Ssize variable
>> egen ssize=seq() if var==0,from(47) to(1)
>> replace ssize=ssize[_n-1] if ssize==.
>> list, noobs clean
>>
>>
>> //Generating product variable
>> gen  product= (ssize/(ssize+1))
>>
>>
>> //Generating Score variable (PRODUCT)
>> gen  score= 1-(2*product) in 1/1 if var==0
>> // for the first observation only
>>
>> //**REPLACEMENT A: when var==0
>> replace  score= 1-(2*(product*product[_n-1]))  if var==0 & score==.
>> // fine for the second obs only  (correct formula for when var=0)
>>
>> //**REPLACEMENT B: when var==1
>> replace  i1= 1-1*(score*score[_n-1])  if var==1
>> // fine for the second observ only (correct formula for when var=1)
>> // but instead of [_n-1] I need a loop for [_n-`n(lagged)'] with "forvalue" command?
>>
>> list, clean noobs
>
> *
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>

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```