~ Multiply by 3 and add 1
( "!Enter a number (0 terminates) : "
  ? N: N. 0 > ^                             ~ Read number and check > 0
  0 C:                                      ~ Initialize counter
  ( C. 20 \ 0 = [ "!" ]                     ~ New line every 20th number
    C. 1 + C:                               ~ Count items
    N. ! " "                                ~ Display an item
    N. 1 > ^                                ~ Finished if N = 1
    N. 2 \ 0 = [ N. 2 / M: ]                ~ Item is even
    N. 2 \ 0 > [ 3 N. * 1 + M: ]            ~ Item is odd
    M. N:                                   ~ New value for N
  )
)
$