doc{
A couple of helper functions to make the rest of the
code more readable.
}doc

: rand  ( -n )  random 20 mod ;
: .  ( n- )  putn space ;

doc{
One approach is to use a simple repeat/again loop, and
a conditional exit. For instance:
}doc

: foo   ( - )
  repeat rand dup . 10 = if; rand . again ;

doc{
The other approach uses a structured while loop with the
second printing handled by a conditional clause.
}doc

[ rand dup . 10 <> [ [ rand . ] ifTrue ] sip ] while