{ xTurtle Tutorial Example #6:  Subroutines

  It is possible to define subroutines 
  and functions that can be used just
  like built-in subroutines, such as
  forward(x), and built-in functions,
  such as sqrt(x).  A subroutine is
  just a list of statements to be
  executed.  A function is similar, 
  except it returns a value to be
  used in further computation. }
  
SUB Triangle   { Begin definition of
                 a subroutine named
                 "Triangle". }
  forward(3)
  turn(120)
  forward(3)  { Statements to define }
  turn(120)   {    what the subroutine }
  forward(3)  {    does. }
  turn(120)

END SUB  { Marks the end of the subroutine. }

{ Defining this subroutine has not
  actually drawn anything.  When the
  subroutine is called by using its
  name as a statement, the commands
  inside the subroutine are exeuted. }
  
Triangle  { Call the subroutine to
            draw a triangle. }
            
SUB Square(length)  { Define a subroutine
                      that has a parameter called
                      "length".  A value for the
                      parameter will be supplied
                      when the subroutine is called. }
  forward(length)
  turn(90)
  forward(length)  { Statements to define }
  turn(90)         {    what the subroutine }
  forward(length)  {    does. }
  turn(90)
  forward(length)
  turn(90)

END SUB  { Marks the end of the subroutine. }

PenUp
MoveTo(-6,-6)
PenDown
blue

Square(5)  { Draw a 5-by-5 square }
PenUp MoveTo(-5,-5) PenDown
Square(3)  { Draw a 3-by-3 square }
PenUp MoveTo(-4,-4) PenDown
Square(1)  { Draw a 1-by-1 square }


FUNCTION Area(length,width)
    { Begin definition of a function
      named "Area", with two parameters
      named "length" and "width" }
      
   DECLARE val  { This is a local variable,
                  for use inside this
                  function only. }
                  
   val := length * width  { Compute a value. }
   
   RETURN val  { Specify the value to be
                 returned by the function. }

END FUNCTION

DECLARE answer
answer := Area(5,7)  { Call the function Area
                       with parameter values
                       5 and 7. }
                       
PenUp MoveTo(-7,7) Pendown
green

DrawText("A 5-by-7 rectangle has area #answer.")