Re: st: Evaluate if a dataset is empty

[Nick Cox <njcoxstata@gmail.com>]

Good answer to the second. Some rough code sketches (warning: zero checking)

local bad

qui forval i = 1/50000 {
           describe using file`i'
           if r(N) == 0 local bad `bad' `i'
}

<something using bad>

or use -postfile-

tempname handle
postfile `handle' bad using mybadfiles

qui forval i = 1/50000 {
           describe using file`i'
           if r(N) == 0 post `handle' (`i')
}

postclose `handle'

use mybadfiles

gen zero = 0
expand <whatever>



Nick
njcoxstata@gmail.com


On 27 March 2014 11:54, Seliger  Florian <seliger@kof.ethz.ch> wrote:
> Nick:
>> The second is that you are planning to create about 10,000 datasets
>> with not much in them. I would instead create a list of names of empty
>> datasets in a local macro, and then with that create a composite
>> dataset with the same stuff, all just once.
>
> Thanks too.
> I have already implemented Maarten's suggestion, but could you give exemplary code?
>
>> An overarching question is why you need to pretend that empty datasets
>> are populated, but I guess there is a good reason.
>
> I do not pretend that they are populated, the values are in fact all zero.
> In a first stage I calculated counts, but if Stata did not find anything to count it created an empty dataset.
>
> Best,
> Florian
>
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>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Maarten Buis
> Sent: Donnerstag, 27. März 2014 12:04
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: Evaluate if a dataset is empty
>
> On Thu, Mar 27, 2014 at 11:51 AM, Seliger  Florian wrote:
>> I want Stata to go through 20,000 small datasets (with a forvalues loop, datasets are indexed accordingly) and evaluate if the dataset is empty or not. About 50% of the datasets are empty meaning that there are no values for any variable.
>
> capture assert _N == 0
> if _rc == 0 {
>     set obs 18
>     ...
> }
>
> The logic is that _N is the number of observations in the dataset, so _N == 0 corresponds to an empty dataset. -assert- gives an error message when the statement is values, but -capture- captures these error messages and leaves the return code behind in _rc. No error message corresponds with an return code 0, so _rc == 0 corresponds with an empty dataset.
>
> Hope this helps,
> Maarten
>
> ---------------------------------
> Maarten L. Buis
> WZB
> Reichpietschufer 50
> 10785 Berlin
> Germany
>
> http://www.maartenbuis.nl
> ---------------------------------
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