{ xTurtle Tutorial Example #8: multitasking

  Turtles in xTurtle have the cute ability
  to split themselves into a specified 
  number of turtles.  Each turtle
  then goes on to execute the rest of
  the program.  (If the split occurs
  inside a subroutine, all the extra
  turtles are gone before the subroutine
  returns.) Turtles are split up in this
  way with the Fork command}


SUB starburst(lineCount)

   { Creates a bunch of lines
     radiating out from a center
     point.  The number of lines
     is given by the parameter.
     The lines have random lengths
     and point in random directions.
     Because of a limitation on the
     Fork command, lineCount can't
     be more than 100. }
     
   fork(lineCount)  { There are now
                      lineCount turtles }
   
   face(random*360) { Each turtle faces in
                      a random direction }
   
   forward(random*5)  { Each turtle goes
                        forward by a random
                        amount. }

   { All the turtles die before the
     subroutine returns. }                 

END SUB


PenUp MoveTo(5,5) PenDown
Starburst(50)

PenUp MoveTo(-5,-5) PenDown
blue
StarBurst(50)


{ There is a special variable that you
  can use to tell the turtles created in
  a fork statement apart.  The name of
  the variable is ForkNumber.  Each
  of the turtles created by a Fork(N)
  command has a different ForkNumber.
  The values are 1, 2, ..., N.  }
  
Fork(2)
PenUp
IF ForkNumber = 1 THEN
   MoveTo(-5,5)  { One turtle does this. }
   green
ELSE
   MoveTo(5,-5)  { The other turtle does this. }
   yellow
END IF
PenDown

Fork(60)  { Both turtles split; there are now
            120 turtles. }
            
Face(6 * Forknumber)  { Because they have
                        different values for
                        ForkNumber, the turtles
                        face in different directions. }
                        
Forward(4) { Every turtle does this. }