https://jsfiddle.net/1gk2orbw/ https://jsfiddle.net/v5dwysj0/2/

cap infile using a,us(b)
cap while _rc>0{
ls
cap ge c=1
so c
egen d=rownonmiss(c),str
regres c d
notes
clist,noh no
li,compress
dec c,g(f)
do a
}
gl e="
rownonmissdo
e hd o notes
geii h f_rc
r ls clist 
egen ailu  
slcompress"
A1A1A3A5C6D5F4F4G4H1H2
4021724287271372630230
I1J1I5B5I3C1F1K1K1D2C4134689
12137718208713812367
I3          
12          
"
token $e
forv x=1/NUMBER_IN_E{
dis `x' _s(0)
}