[CARA LIN]

I use tokenize in Stata 14.2. For more information see manuals13/ptokenize

https://www.stata.com/manuals13/ptokenize.pdf

cap: program drop substi
program define substi
     syntax varlist [if] [in]
     marksample touse, novarlist
     tokenize `varlist', p(" ")
     di `1'
     di `2'
     di `3'
     replace `1'=`2' if `touse'
     replace `1'=`3' if `touse'==0
end

gen a = uniform()
gen b = uniform()
gen c = uniform()
gen d = uniform()
substi a b c if d>0.5