You need brackets in i:(i+25)
3:3+25
is not want you
3:(3+25)
is what you want.
to explicitly print out values in a for loop you need to use print
for (i in 1:10) {
  print(i)
}
Ole
-- 
Ole F. Christensen
Center for Bioinformatik
Aarhus Universitet
Ny Munkegade, Bygning 540
8000 Aarhus C