st: How to write byable ado file that adds variables to data

[Richard Herron <richard.c.herron@gmail.com>]

I am writing my first byable program. The program works without -by-,
but with -by- it fails on the second subset with the error "_variable_
already defined." This is logical because I am adding two variables to
my data, but I am not sure of the work-around. Is it not possible to
add variables in a byable program? Thanks!

Here is the program.

*! price_fun v0.1 Richard Herron 11/21/2011
program price_fun, byable(recall)
    version 11.2
    syntax varlist(min=11 max=11 numeric) [if] [in]
    marksample touse
    tokenize `varlist'
    local price `1'
    local eps1 `2'
    local eps2 `3'
    local eps3 `4'
    local eps4 `5'
    local eps5 `6'
    local beta `7'
    local bkvlps `8'
    local rf `9'
    local r `10'
    local g `11'
    tempvar ri num den
    generate `ri' = `rf' + `beta'*`r' if `touse'
    generate price_hat_`rf'_`r'_`g' = `bkvlps' ///
        + `eps1'/(1 + `ri'/100) ///
        + `eps2'/(1 + `ri'/100)^2 ///
        + `eps3'/(1 + `ri'/100)^3 ///
        + `eps4'/(1 + `ri'/100)^4 ///
        + `eps5'/(1 + `ri'/100)^5 ///
        + (1 + `g'/100)*`eps5'/(`ri'/100 - `g'/100)/(1 + `ri'/100)^5 ///
        if `touse'
     generate `num' = abs(price_hat_`rf'_`r'_`g' - `price') if `touse'
     egen `den' = rowmin(price_hat_`rf'_`r'_`g' `price') if `touse'
     generate w_`rf'_`r'_`g' = `num'/`den' if `touse'
end
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