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# Re: st: How to check if observation of a particular variable value exists? If not, create that observation.

 From Nick Cox To statalist@hsphsun2.harvard.edu Subject Re: st: How to check if observation of a particular variable value exists? If not, create that observation. Date Tue, 29 May 2012 10:20:07 +0100

```Should be

replace B = `j' in L

On Tue, May 29, 2012 at 10:19 AM, Nick Cox <njcoxstata@gmail.com> wrote:
> If it is really a case of whether you have four records (Stata
> terminology is "observations"), you don't need a program, but just to
> work in the Editor. But I guess your real problem is more substantial.
>
> Look at the help for -fillin-.
>
> This version of your code should work
>
> qui forvalues i = 1/2 {
>             forvalues j = 1/2 {
>                     count if A == `i' & B == `j'
>                     if r(N) == 0 {
>                             set obs `=_N + 1'
>                             replace A = `i' in L
>                             replace B = `i' in L
>                     }
>        }
> }
>
> It is best not to use "missing" to mean "not present". Missing in
> Stata usually means that a value is missing.
>
> Nick
>
> On Tue, May 29, 2012 at 10:09 AM, Ran Gu <lionup@gmail.com> wrote:
>
>> Suppose I have a data with 2 variables, say A and B, both with values
>> 1 and 2. The data are sorted by A and B. Then there should be 4
>> records: (A=1 B=1) (A=1 B=2) (A=2 B=1) (A=2 B=2)
>>
>> Suppose I don't know whether the 4 observations are all exist or not.
>> How could I check that in a do file?
>> Then, If a record is missing, say (A=1 B=1), I would like to create it.
>>
>> for example:
>> forvalues i = 1/2 {
>>  forvalues j = 1/2 {
>>    if obs with A == `i' and B == `j' is missing {
>>      local new = _N+1
>>      set obs `new'
>>      replace A == `i' if _n==`new'
>>      replace B == `j' if _n==`new'
>>    }
>>  }
>> }
>>

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