st: Adding variables containing empty values

["Ziad El-Khatib" <Ziad.El-Khatib@ki.se>]

Dear STATA lister,
 Any tips how to add (summation) 3 variables when one has empty value
 (example below)
 I did not want to use replace var=0 if var==. because i want to know
 when all the 3 variables (ndrug1,ndrug2 and ndrug3 have empty values
 at the same time)


 id      visitn      ndrug1      ndrug2      ndrug3      total
 -----------------------------------------------------------------------------
 1      2              .                2             1             3
 1      4              2                .             3             5
 1      8              2                3            3             8
 1      12            .                 .            .              .
 2      2              .                 .            2             2
 2      4              1                1            .              2

 etc..




 Any tips?
 Thank you in advance and best regards
 ziad
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