st: invalid syntax - drop

[Kit Baum <baum@bc.edu>]

Phil said

-help drop- shows that the syntax:

 -drop varlist-

does not permit an -if- qualifier.

Perhaps:

foreach var of varlist cross* {
qui su `var'
if `r(max)'==0 {
drop `var'
}
}


Phil

Quoting Socrates Mokkas <socrates.mokkas@economics.oxford.ac.uk>:

> Hello All,
>
> I have a problem with dropping variables. This is my small loop:
>
> foreach var of varlist cross* {
> 	qui su `var'
> 	drop `var' if `r(max)'==0
> }
>
> Why is the invalid syntax comming up? What I want to do is to drop  
variables
> ( included in the varlist cross*) if they have values only of  
zeros or
> missing values. Am I missing the obvious here?
>


Unfortunately  that doesn't work either.
If you create a variable that is wholly missing, you will see that r 
(max) is not defined, so your if condition
looks like

if  == 0

which is surely invalid syntax. You could figure this out if you did

set trace on

and then ran your code.

Given that a variable that is wholly missing and a variable that is  
always zero have quite different characteristics (for instance, a  
variable that is zero or missing has r(sum) = 0, but that is not  
sufficient to conclude that the variable meets your specs) it might be
easiest to do something like

foreach v of varlist jun* {
  qui su `v'
   if r(N)==0 {
     drop `v'
   }
   if r(sd) == 0 & r(max) == 0 {
     capture drop `v'
   }
 }



Kit Baum, Boston College Economics
http://ideas.repec.org/e/pba1.html
An Introduction to Modern Econometrics Using Stata:
http://www.stata-press.com/books/imeus.html


*
*   For searches and help try:
*   http://www.stata.com/support/faqs/res/findit.html
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/