hello,
I have a little problem with my script:

I select a subset from a data frame
 
for (i in 1:20){
T$pippo[T$pluto==i]
}

but sometime it is void because T$pluto==5 does not exists, the R answer is:

T$pippo[T$pluto==5]
> numeric(0)

how to exclude this null subset? 
and what is numeric(0), similar to NA?
I try an if statement, but how to write if T$pippo[T$pluto==i] is numeric(0)???

Thanks a lot.
Maxi


-------------
Ing.  Massimiliano Cannata
Istituto di Scienze della Terra - SUPSI
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Tel +41 91 /935 12 25 - Fax +41 91 /935 12 09
eMail: massimiliano.cannata@supsi.ch
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