st: Converting a SAS datastep to Stata

[Daniel Feenberg <feenberg@nber.org>]

I have done programs to calculate income tax liability in SAS and fortran. 
Both those languages allow tax parameters that vary across years and 
filing status to be held in initialized arrays. For example, in SAS one 
could declare:

   array exmp(1993:2010) _temporary_;
   retain exmp 2350 2450 2500 2550 2650 2700 2750 2800 2900 3000 3050 3100
               3200 3300 3400 3500;

and then assigning the correct value of the personal exemption to every 
individual record is just:

   exemption = exmp(fldpyr);

where flpdyr is a variable in the data with the filing year. I am at a bit 
of a loss as to how to do this in Stata. I don't like:

   gen exemption = (flpdyr==1993)*2350 + (flpdyr==1994)*2450...(for 18 subexpressions in all)

or

   gen     exemption = 2350, if flpdyr==1993
   replace exemption = 2450, if flpdyr==1994
   ...(for 18 lines in all)...

because these require (and execute) so much repetitive code.

Another possibility is to create a dataset of parameters by year and 
filing status, then sort the tax return data by year and filing status, 
and finally merge the parameters onto the tax return data. But that 
requires a sort and a lot of I/O, which could be slow with potentially 
millions of returns. The additional memory required is probably not a big 
issue.

I don't actually know Mata, but I think I could define a rowvector:

    exmp =  ( 2350 2450 2500 2550 2650 2700 2750 2800 2900 3000 3050 3100
               3200 3300 3400 3500);

and then loop over all the tax returns executing:

    exemption[i] = exmp[flpdyr[i]-1992];

for each return (where i indexes returns). That seems to mean that every 
variable is going to have to carry around a [i] subscript and there will 
be a 1,000 lines of Mata code executed for each return (rather than the 
preferred 1,000 lines of code for all the returns together). That is much 
less attractive than leaving the observation number implicit, as the 
regular Stata language does. Brief study of [M-2]subscripts doesn't 
suggest any "matrixy" way of coding this.

I expect I am missing something obvious, can someone point me in the right 
direction?

Thanks

Daniel Feenberg
NBER
Cambridge MA
feenberg@nber.org


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