Rem By Patrik Jansson 1/9 1996 FreeWare

Rem Out going mess 
Print "Enter an EAN13-cods first 12 digits:"

Rem We need two... 
Dim SUM(2)

Rem  Input 12 numbers  
Do 
   Input NUM$

   Rem 12?
   Exit If Len(NUM$)=12

   Rem Sorry... 
   Print "ERROR!"

Loop 

Rem lets add those odd and even numbers together!
Do 
   For E=1 To 2
      Inc D
      Add SUM(E),Val(Mid$(NUM$,D,1))
      If D=>Len(NUM$) Then Exit 2
   Next 
Loop 

Rem the secret is even*3+odd and what ever remains to the next 10 is the 
Rem checksum 
CHSUM=SUM(1)+SUM(2)*3
CHSUM=10-Val(Right$(Str$(CHSUM),1))

Rem reveal the secret
Print "Checksum digit is:";CHSUM