st: Programming a slightly complex list of independent variables

["Nic" <3817629@hotmail.ca>]

Hi statalist

I am attempting to create a .do file which will run a number of OLS 
regressions containing a single continuous x continuous interaction term.

My ultimate question is: how can I program my regression command so that the 
s* and f* variables at the end of the command refer to all "s" and "f" 
variables EXCEPT for the two specific "s" and "f" (`x' and `z') variables 
referenced at the beginning of the equation?

Here is the applicable code of what I have so far:

-------------------------------------------------------------------------
foreach y of varlist d* {
local laby : variable label `y'
   foreach x of varlist s*  {
   local labx : variable label `x'
   local prex = substr("`x'",1,3)
       foreach z of varlist f* {
       local labz : variable label `z'
       local prez = substr("`z'",1,2)

           regress `y' `x' `z' i`prex'`prez' g* c* e* s* f*
--------------------------------------------------------------------------

As you can see, the inclusion of s* and f* at the end of the equation will 
result in two variables being repeated in the equation: `x' and `z'. The 
consequence is that one instance of the repeated variables is omitted 
because of collinearity.

I would assume that the second instance (s* or f*) of the repeated variable 
in the equation would be the one that is omitted, but this is not always so. 
Sometimes it is the first instance (`x' or `z'). Apparently this is normal 
("Which variable it omits is somewhat arbitrary") according to the Stata 
FAQ, "Why do estimation commands sometimes omit variables?" located at 
www.stata.com/support/faqs/stat/drop.html.

The consequence of the above is that the location of the values in the e(b) 
and e(V) matrices is unpredictable. This is a problem for me because the 
next step in my .do file is to call upon the first and second independent 
variables listed in the regression command as well as their interaction term 
(to ultimately create a graph):

----------------------
matrix b=e(b)
matrix V=e(V)

scalar b1=b[1,1]
scalar b2=b[1,2]
scalar b3=b[1,3]


scalar varb1=V[1,1]
scalar varb2=V[2,2]
scalar varb3=V[3,3]

scalar covb1b3=V[1,3]
scalar covb2b3=V[2,3]
-----------------------

As you can see, when the second instance of the repeated variables is 
omitted, b1/b2/b3 etc refer to a valid cell in the matrix. But when the 
first instance is "somewhat arbitrarily" omitted instead, b1/b2/b3 etc no 
longer refer to the intended cells in the matrix.

So my ultimate question is: how can I program my regression command so that 
the s* and f* variables at the end of the command refer to all "s" and "f" 
variables EXCEPT for the two specific "s" and "f" (`x' and `z') variables 
referenced at the beginning of the equation? Logic tells me that this is 
surely possible but I am still so new to Stata and programming in particular 
that I simply have not been able to suss it out.

With gratitude,
Nic 

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