I believe you are wanting to know the difference between this statement: for i=0,2 do a2[0,i*2]+=a1[i,*] and this: for i=0,2 do a2[i,i*2]=a1[i,*] no? The one critical difference (given your particular data sets, is that the first statement uses a static '0' subscript, where the second uses a dynamic 'i'. By the way, if you think you may be sharing code with others, it is good programming practise to avoid using the '&' command concatenator operator. That really makes code hard to read. Regards, James Jones