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Re: st: Loop to replace values of a variable


From   Nick Cox <njcoxstata@gmail.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: Loop to replace values of a variable
Date   Wed, 18 Dec 2013 16:22:12 +0000

In principle, yes. In practice, this means repeated addition of 0.1;
but 0.1 is a good example of a decimal that looks simple but can't be
held exactly as a binary. Do something like this

set obs 100
gen y1 = 0
replace y1 = y[_n-1] + 0.1 in 2/L
gen y2 = (_n-1)/10
edit y?

to convince yourself that the second method gives a better
approximation than the first.

More discussion at

SJ-10-1 pr0051  . . . . . . . . . . . . Stata tip 85: Looping over nonintegers
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N. J. Cox
        Q1/10   SJ 10(1):160--163                                (no commands)
        tip on using forvalues to loop over numbers

http://www.stata-journal.com/sjpdf.html?articlenum=pr0051

Nick
njcoxstata@gmail.com


On 18 December 2013 14:34, Jeph Herrin <stata@spandrel.net> wrote:
> A solution that doesn't require knowing the value of x[1] in advance:
>
>  sort x
>  replace x=x[_n-1]+0.1 if _n>1
>
> hth,
> Jeph
>
>
>
> On 12/18/2013 9:04 AM, emanuele mazzini wrote:
>>
>> Yes, the way Marteen suggested worked greatly.
>> Thank you!!
>>
>> 2013/12/18 Fabrizio Colella - 1348010 <colella.fabrizio@studbocconi.it>:
>>>
>>> you generated x as Marteen suggested, now you just have to subtract:
>>>
>>> gen x = _n*.1
>>> gen y = -2.6+x
>>>
>>> Fabrizio
>>>
>>>
>>> ----- Messaggio originale -----
>>> Da: "Nick Cox" <njcoxstata@gmail.com>
>>> A: statalist@hsphsun2.harvard.edu
>>> Sent: Mercoledì, 18 dicembre 2013 14:49:34
>>> Oggetto: Re: st: Loop to replace values of a variable
>>>
>>> For -2.5(0.1)2.1 or -2.5(0.1)2.5, you can still divide integers by 10.
>>> No loop needed as _n always gives you integers in sequence.
>>>
>>>
>>> Nick
>>> njcoxstata@gmail.com
>>>
>>>
>>> On 18 December 2013 13:44, emanuele mazzini <madsoenistata@gmail.com>
>>> wrote:
>>>>
>>>> Oh, yes, I see. Thank you very much for your answers, but I may have
>>>> oversimplified, since what you said would fit in this case, as x is
>>>> exactly = _n/10, but my problem is slightly more complex.
>>>>
>>>> What if the values I want to replace go from -2.5 to 2.15, still with
>>>> a 0.1 interval? In this case I'd still need a loop, right?
>>>>
>>>> 2013/12/18 Maarten Buis <maartenlbuis@gmail.com>:
>>>>>
>>>>> no need to loop:
>>>>>
>>>>> gen x = _n*.1
>>>>>
>>>>> Hope this helps,
>>>>> Maarten
>>>>>
>>>>>
>>>>> On Wed, Dec 18, 2013 at 2:33 PM, emanuele mazzini
>>>>> <madsoenistata@gmail.com> wrote:
>>>>>>
>>>>>> Dear all statalist users,
>>>>>>
>>>>>> I would like to program a (relatively) simple loop to replace the
>>>>>> values of a variable I created, but I cannot  figure it out how to
>>>>>> write the syntax.
>>>>>>
>>>>>> The structure of my data is very simple:
>>>>>> I have 300 observations and a variable, say x, that in the first cell
>>>>>> (i.e. for n=1) is equal to 0.1, while it is missing for all other 299
>>>>>> cells. Data look like:
>>>>>>
>>>>>> n     x
>>>>>> 1    0.1
>>>>>> 2     .
>>>>>> 3     .
>>>>>> 4     .
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> 299  .
>>>>>> 300  .
>>>>>>
>>>>>> I would like the loop to replace the missing values of my variable by
>>>>>> going from cell 2 to cell 300, and to have a replace that would be at
>>>>>> a 0.1 intervals, so as to obtain data that look like:
>>>>>>
>>>>>> n     x
>>>>>> 1    0.1
>>>>>> 2    0.2
>>>>>> 3    0.3
>>>>>> 4    0.4
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> .
>>>>>> 299  29.9
>>>>>> 300  30
>>>>>>
>>>>>> I can obviously do it by hand, but a loop would be naturally much more
>>>>>> efficient!
>>>>>> Does anyone know how to make it?

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